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O R G A N I C C H E M I S T RY
Aldehydes and
Ketones
LEARNING GOALS
1 ▶ Draw the structures and discuss the physical
2 ▶
3 ▶
4 ▶
5 ▶
6 ▶
7 ▶
8 ▶
9 ▶
properties of aldehydes and ketones.
From the structures, write the common and
I.U.P.A.C. names of aldehydes and ketones.
List several aldehydes and ketones that are of
natural, commercial, health, and environmental
interest and describe their significance.
Write equations for the preparation of
aldehydes and ketones by the oxidation of
alcohols.
Write equations representing the oxidation of
carbonyl compounds.
Write equations representing the reduction of
carbonyl compounds.
Write equations for the preparation of
hemiacetals, hemiketals, acetals, and ketals.
Draw the keto and enol forms of aldehydes
and ketones.
Write equations showing the aldol
condensation.
OUTLINE
Introduction 422
13.1 Structure and Physical Properties 423
13.2 I.U.P.A.C. Nomenclature and Common
Names 425
Naming Aldehydes 425
Naming Ketones 427
13.3 Important Aldehydes and Ketones 430
13.4 Reactions Involving Aldehydes and Ketones 432
Preparation of Aldehydes and Ketones 432
Oxidation Reactions 432
A Medical Perspective: Formaldehyde and Methanol
Poisoning 433
A Human Perspective: Alcohol Abuse and
Antabuse 436
Reduction Reactions 436
A Medical Perspective: That Golden Tan Without the
Fear of Skin Cancer 438
Addition Reactions 439
Keto-Enol Tautomers 442
Aldol Condensation 443
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INTRODUCTION
Cinnamon tree blossoms.
For centuries we have used bloodhounds to locate missing persons or criminals. This
works because of the amazing ability of bloodhounds to detect scent molecules, and
because individuals have characteristic odor prints that are as unique as their fingerprints or DNA. Forensic scientists are now making the next step in developing this
technology for more accurate detection of people associated with a crime scene. The
first step is to identify the components of the odor print and an appropriate source
of the sample. Some scientists suggest that odor molecules should be collected from
the hands, since this is the part of the body that would handle a gun, bomb, or other
materials at a crime scene. When samples are collected from hands, complex mixtures
of compounds are collected. Among the molecules identified as prominent in these
mixtures are some large and complex members of the two groups of compounds
we will study in this chapter, the aldehydes and ketones. Among these are the aldehydes nonanal, decanal, and undecanal and the ketones 6-methyl-5-hepten-2-one
and 6,10-dimethyl-5,9-undecadien-2-one. The next step in using this information to
identify individuals will be the development of an instrument to detect, identify, and
quantify these and other components of the human odor print samples found at the
scene of a crime so that that pattern can be compared with suspects in the case.
O
C
Nonanal
H
O
C
H
Decanal
O
C
Undecanal
H
O
C
6-Methyl-5-hepten-2-one
O
C
6,10-Dimethyl-5,9-undecadien-2-one
The aldehydes and ketones are characterized by the presence of the carbonyl
group, a functional group made up of a carbon atom bonded to an oxygen atom by
a double bond.
a
O
+
C
d
Carbonyl group
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13.1
423
Structure and Physical Properties
Compounds containing a carbonyl group are called carbonyl compounds. These
include the aldehydes and ketones covered in this chapter, as well as the carboxylic
acids and amides discussed in Chapters 14 and 15.
0
-
0
-
Aldehyde
-
R
R
R
Ketone
0
H
O
+
C
-
R
O
+
C
OH
R
Carboxylic Acid
O
+
C
0
O
+
C
NR2
Amide
13.1 Structure and Physical Properties
Aldehydes and ketones are carbonyl group containing compounds distinguished
by the location of the carbonyl group within the carbon chain. In aldehydes the
carbonyl group is always located at the end of the carbon chain (carbon-1). In
ketones the carbonyl group is located within the carbon chain of the molecule.
Thus, in ketones the carbonyl carbon is attached to two other carbon atoms. However, in aldehydes the carbonyl carbon is attached to at least one hydrogen atom;
the second atom attached to the carbonyl carbon of an aldehyde may be another
hydrogen or a carbon atom (Figure 13.1). Aldehydes and ketones are polar compounds because of the polar carbonyl group.
LEARNING GOAL
1▶
Draw the structures and discuss
the physical properties of aldehydes and ketones.
2
Od
1
Cd
# !
Because of the dipole-dipole attractions between molecules, they boil at higher temperatures than hydrocarbons or ethers that have the same number of carbon atoms
or are of equivalent molecular mass. Because they cannot form intermolecular
hydrogen bonds, their boiling points are lower than those of alcohols of comparable
molecular mass. These trends are clearly demonstrated in the following examples:
d2O
d1C
[ Y1
Cd
2
Od
Y[
Dipole-dipole attraction
CH3CH2CH2CH3
CH3—O—CH2CH3
CH3CH2CH2 —OH
O
CH3CH2 —C—H
Butane
(butane)
M.M. 58
b.p. 0.5 C
Methoxyethane
(ethyl methyl ether)
M.M. 60
b.p. 7.0 C
1-Propanol
(propyl alcohol)
M.M. 60
b.p. 97.2 C
Propanal
(propionaldehyde)
M.M. 58
b.p. 49 C
O
CH3—C—CH3
Propanone
(acetone)
M.M. 58
b.p. 56 C
Aldehydes and ketones can form intermolecular hydrogen bonds with water
(Figure 13.2). As a result, the smaller members of the two families (five or fewer
carbon atoms) are reasonably soluble in water. However, as the carbon chain
R
O
O
C
C
R
H
Aldehyde
R H, R, or Ar
An aldehyde
Propanal
(a)
R
Figure 13.1 The structures of alde-
Ketone
R R or Ar
A ketone
Propanone
(b)
hydes and ketones. (a) The general
structure of an aldehyde and a balland-stick model of the aldehyde propanal. (b) The general structure of a
ketone and a ball-and-stick model of
the ketone propanone.
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Chapter 13 Aldehydes and Ketones
O
R
H
H
O
O
C
H
R
R
between the carbonyl group of an
aldehyde or ketone and water.
(b) Polar interactions between carbonyl groups of aldehydes or ketones.
C
R
(a)
O
H
C
O
R
O
C
O
Figure 13.2 (a) Hydrogen bonding
R
R
R
R
C
R
(b)
length increases, the compounds become less polar and more hydrocarbonlike.
These larger compounds are soluble in nonpolar organic solvents.
Question 13.1 Which member in each of the following pairs will be more
water-soluble?
O
a. CH3(CH2)2CH3
or
CH3CCH3
b. CH3CCH 2CH2CH3
or
CH3CHCH 2CH2CH3
|
O
OH
Question 13.2 Which member in each of the following pairs will be more
water-soluble?
CH3
or
H*C+O
a.
b. HOCH 2 CH2 OH
or
C H
HCC
OO
Question 13.3 Which member in each of the following pairs would have a
higher boiling point?
a. CH3CH2COH
or
CH3CH2CH
O
O
O
b. CH3COH
or
CH3CCH3
O
Question 13.4 Which member in each of the following pairs would have a
higher boiling point?
O
a. CH3CH2OH
or
CH3CH
b. CH3(CH2)6CH3
or
CH3(CH2)5CH
O
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13.2 I.U.P.A.C. Nomenclature and Common Names
13.2 I.U.P.A.C. Nomenclature and Common Names
Naming Aldehydes
In the I.U.P.A.C. system, aldehydes are named according to the following set of
rules:
• Determine the parent compound, that is, the longest continuous carbon chain
containing the carbonyl group.
• Replace the final -e of the parent alkane with -al.
• Number the chain beginning with the carbonyl carbon (or aldehyde group) as
carbon-1.
• Number and name all substituents as usual. No number is used for the position of the carbonyl group because it is always at the end of the parent chain.
Therefore, it must be carbon-1.
LEARNING GOAL
2▶
From the structures, write the
common and I.U.P.A.C. names of
aldehydes and ketones.
Several examples are provided here with common names given in parentheses:
O
1+
H8C8H
O
2
1+
CH38C8H
O
3 2
1+
CH3CH28C8H
Methanal
(formaldehyde)
Ethanal
(acetaldehyde)
Propanal
(propionaldehyde)
O
5 4
3 2 1+
CH3CH2CH2CH8C8H
*
CH3
2-Methylpentanal
EXAMPLE 13.1
Using the I.U.P.A.C. Nomenclature System
to Name Aldehydes
For many years, scientists were puzzled about what compounds give the nutty
flavor to expensive, aged cheddar cheeses. In 2004, Dr. Mary Anne Drake of
North Carolina State University solved this puzzle. She identified a group of
aldehydes that impart this desirable flavor. Use the I.U.P.A.C. Nomenclature
System to name two of these aldehydes shown below.
LEARNING GOAL
2▶
From the structures, write the
common and I.U.P.A.C. names of
aldehydes and ketones.
Solution
O
+
CH3CHC8H
3 2* 1
CH3
O
+
CH3CHCH2C8H
4 *3 2 1
CH3
Parent compound:
propane
(becomes propanal)
butane
(becomes butanal)
Position of carbonyl group:
carbon-1 (must be!)
carbon-1 (must be!)
Substituents:
2-methyl
3-methyl
Name:
2-Methylpropanal
3-Methylbutanal
Notice that the position of the carbonyl group is not indicated by a number. By
definition, the carbonyl group is located at the end of the carbon chain of an
Continued—
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Chapter 13 Aldehydes and Ketones
aldehyde. The carbonyl carbon is defined to be carbon-1; thus, it is not
necessary to include the position of the carbonyl group in the name of the
compound.
Practice Problem 13.1
Many molecules contribute to the complex flavors of olive oils. Among these
are hexanal and trans-2-hexenal, which have flavors described as “green,
grassy” and “green, bitter,” respectively. Draw the structures of these two
compounds.
Another aldehyde associated with
the nutty flavor of cheddar cheese is
2-methylbutanal. Draw the condensed
formula of this molecule.
Carboxylic acid nomenclature is described
in Section 14.1.
▶ For Further Practice: Questions 13.5, 13.29, and 13.36a, b, and c.
The common names of the aldehydes are derived from the same Latin roots as
the corresponding carboxylic acids. The common names of the first five aldehydes
are presented in Table 13.1.
In the common system of nomenclature, substituted aldehydes are named as
derivatives of the straight-chain parent compound (see Table 13.1). Greek letters
are used to indicate the position of the substituents. The carbon atom bonded to
the carbonyl group is the ␣-carbon, the next is the ␤-carbon, and so on.
d g b a O
*C*C*C*C*C*H
Consider the following examples:
O
CH3CH2CH2CH*C*H
CH3
O
CH3CH2CHCH2*C*H
CH3
2-Methylpentanal
(a-methylvaleraldehyde)
3-Methylpentanal
(b-methylvaleraldehyde)
d
g
b
a
d
g
b a
Table 13.1 I.U.P.A.C. and Common Names and Formulas
for Several Aldehydes
I.U.P.A.C. Name
Methanal
Ethanal
Propanal
Butanal
Pentanal
Common Name
Formula
Formaldehyde
O
+
H—C—H
Acetaldehyde
O
+
CH3C—H
Propionaldehyde
O
+
CH3CH2C—H
Butyraldehyde
O
+
CH3CH2CH2C—H
Valeraldehyde
O
+
CH3CH2CH2CH2C—H
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13.2 I.U.P.A.C. Nomenclature and Common Names
EXAMPLE 13.2
Using the Common Nomenclature System
to Name Aldehydes
Name the aldehydes represented by the following condensed formulas.
LEARNING GOAL
2▶
Solution
O
d g b a CH3CHCH2CH2C*H
Br
O
g b a CH3CHCH2C*H
CH3
From the structures, write the
common and I.U.P.A.C. names of
aldehydes and ketones.
Parent compound:
pentane
butane
(becomes valeraldehyde) (becomes butyraldehyde)
Position of carbonyl group: carbon-1 (must be!)
carbon-1 (must be!)
Substituents:
␥-bromo
␤-methyl
Name:
␥-Bromovaleraldehyde ␤-Methylbutyraldehyde
Notice that the substituents are designated by Greek letters, rather than by Arabic
numbers. In the common system of nomenclature for aldehydes, the carbon atom
bonded to the carbonyl group is called the ␣-carbon, the next is the ␤-carbon, etc.
Remember to use these Greek letters to indicate the position of the substituents
when naming aldehydes using the common system of nomenclature.
Also remember that by definition, the carbonyl group is located at the
beginning of the carbon chain of an aldehyde. Thus, it is not necessary to
include the position of the carbonyl group in the name of the compound.
Practice Problem 13.2
Use the common nomenclature system to name each of the following
compounds.
O
O
CH3
|
b. CH3CH2CH2CHCH
a. CH3CHCHCH2CH
|
|
CH3
CH2CH3
O
c. CH 3CHCH
|
Cl
O
d. CH3CHCH2CH
|
OH
▶ For Further Practice: Questions 13.6, 13.41c and d, 13.42b and d.
Naming Ketones
The rules for naming ketones in the I.U.P.A.C. Nomenclature System are directly
analogous to those for naming aldehydes. In ketones, however, the -e ending of
the parent alkane is replaced with the -one suffix of the ketone family, and the location of the carbonyl carbon is indicated with a number. The longest carbon chain is
numbered to give the carbonyl carbon the lowest possible number. For example,
O
+
CH38C8CH3
1
2 3
O
+
CH3CH28C8CH3
4
3
2 1
Propanone
Butanone
(no number necessary) (no number necessary)
(acetone)
(methyl ethyl ketone)
LEARNING GOAL
2▶
From the structures, write the
common and I.U.P.A.C. names of
aldehydes and ketones.
O
+
CH3CH2CH2CH28C8CH2CH2CH3
8 7 6
5
4 3
2 1
4-Octanone
(not 5-octanone)
(butyl propyl ketone)
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Chapter 13 Aldehydes and Ketones
EXAMPLE 13.3
LEARNING GOAL
2▶
From the structures, write the
common and I.U.P.A.C. names of
aldehydes and ketones.
Using the I.U.P.A.C. Nomenclature System
to Name Ketones
Two molecules associated with the aroma of blue cheese are the ketones shown
below. Name these ketones using the I.U.P.A.C. Nomenclature System.
Solution
O
O
+
+
CH3CH2CH2CH2CH2CCH3
CH3CCH2CH2CH2CH2CH2CH2CH3
7
6 5 4 3 2 1
1 2 3 4
5 6 7 8 9
Parent compound:
heptane
nonane
(becomes heptanone)
(becomes nonanone)
Position of carbonyl group: carbon-2 (not carbon-6) carbon-2 (not carbon-8)
Name:
2-Heptanone
2-Nonanone
Practice Problem 13.3
Provide the I.U.P.A.C. name for each of the following ketones.
Two other molecules that contribute to the aroma of blue cheese are
hexanoic acid and butanoic acid, two
carboxylic acids we will study in Chapter 14. Refer to Table 10.2 for the
structure of carboxylic acids and draw
the condensed structural formula for
each of these molecules.
O
+
a. CH3CH2CHCH2CH2CH2CCH3
|
CH2CH3
O
+
b. CH3CH2CH2CCHCH3
|
CH3
O
+
c. CH3CHCCHCH3
|
|
CH3 CH3
▶ For Further Practice: Questions 13.7, 13.41a, b, and e, and 13.42a and c.
The common names of ketones are derived by naming the alkyl groups that are
bonded to the carbonyl carbon. These are used as prefixes followed by the word ketone.
The alkyl groups may be arranged alphabetically or by size (smaller to larger).
EXAMPLE 13.4
LEARNING GOAL
2▶
From the structures, write the
common and I.U.P.A.C. names of
aldehydes and ketones.
Using the Common Nomenclature System
to Name Ketones
Name the ketones represented by the following condensed formulas.
Solution
Identify the alkyl groups that are bonded to the carbonyl carbon.
O
CH3CH2CH2*C*CH3
Alkyl groups:
Name:
O
CH3CH2*C*CH2CH2CH2CH2CH3
propyl and methyl
Methyl propyl ketone
ethyl and pentyl
Ethyl pentyl ketone
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13.2 I.U.P.A.C. Nomenclature and Common Names
429
Practice Problem 13.4
Provide the common names for each of the following ketones:
O
O
a. CH3CH2CCH2CH3
b. CH3CH2CH2CH2CCH2CH3
O
d. CH3CHCCHCH3
O
c. CH3CH2CH2CH2CH2CH2CCH3
|
|
CH3 CH3
▶ For Further Practice: Questions 13.41a, b, and e and 13.44.
Because the two groups bonded to the carbonyl carbon are named, a ketone is
actually one carbon longer than an aldehyde with a similar common name. For
example, methyl butyl ketone has six carbons, but ␤-methylbutyraldehyde has
only five.
O
CH3*C*CH2CH2CH2CH3
1
2 3 4
5 6
Methyl butyl ketone
O
4 3 2
CH3CHCH2*C*H
|
1
CH3
5
b-Methylbutyraldehyde
This is because the aldehyde carbonyl carbon is included in the name of the parent chain, butyraldehyde. The carbonyl carbon of the ketone is not included in
the common name. It is treated only as the carbon to which the two alkyl or aryl
groups are attached.
Question 13.5 From the I.U.P.A.C. names, draw the structural formula for
each of the following aldehydes.
a.
b.
c.
d.
e.
2,3-Dichloropentanal
2-Bromobutanal
4-Methylhexanal
Butanal
2,4-Dimethylpentanal
Question 13.6 Write the condensed formula for each of the following
compounds.
a.
b.
c.
d.
3-Methylnonanal
␤-Bromovaleraldehyde
4-Fluorohexanal
␣,␤-Dimethylbutyraldehyde
Question 13.7 Use the I.U.P.A.C. Nomenclature System to name each of the
following compounds.
O
+
a. CH3CHCCH3
*
I
O
+
b. CH3CHCH2CCH3
*
CH2CH2CH2CH3
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Chapter 13 Aldehydes and Ketones
O
+
c . CH3CHCCH3
*
CH3
e.
O
+
d. CH3CHCCH2CH3
*
CH3
O
+
CH3CHCCH2CH3
*
F
Question 13.8 Write the condensed formula for each of the following
compounds.
a. Methyl isopropyl ketone (What is the I.U.P.A.C. name for this
compound?)
b. 4-Heptanone
c. 2-Fluorocyclohexanone
d. Hexachloroacetone (What is the I.U.P.A.C. name of this compound?)
13.3 Important Aldehydes and Ketones
LEARNING GOAL
List several aldehydes and
ketones that are of natural, commercial, health, and environmental interest and describe their
significance.
O
+
C
H
Methanal
O
+
C
0 -
CH3
CH3
H
Ethanal
O
+
C
CH3
Propanone
0 -
0 -
H
O
+
C
0 -
3▶
CH3
CH2CH3
Butanone
Methanal (formaldehyde) is a gas (b.p. ⫺21⬚C). It is available commercially as an
aqueous solution called formalin. Formalin has been used as a preservative for tissues and as an embalming fluid. See A Medical Perspective: Formaldehyde and
Methanol Poisoning for more information on methanal.
Ethanal (acetaldehyde) is produced from ethanol in the liver. Ethanol is oxidized in this reaction, which is catalyzed by the liver enzyme alcohol dehydrogenase. The ethanal that is produced in this reaction is responsible for the symptoms
of a hangover.
Propanone (acetone), the simplest ketone, is an important and versatile solvent
for organic compounds. It has the ability to dissolve organic compounds and is
also soluble in water. As a result, it has a number of industrial applications and
is used as a solvent in adhesives, paints, cleaning solvents, nail polish, and nail
polish remover. Propanone is flammable and should therefore be treated with
appropriate care. Butanone, a four-carbon ketone, is also an important industrial
solvent.
Many aldehydes and ketones are produced industrially as food and fragrance chemicals, medicinals, and agricultural chemicals. They are particularly
important to the food industry, in which they are used as artificial and/or natural
additives to food. Vanillin, a principal component of natural vanilla, is shown in
Figure 13.3. Artificial vanilla flavoring is a dilute solution of synthetic vanillin dissolved in ethanol. Figure 13.3 also shows other examples of important aldehydes
and ketones.
Question 13.9 Draw the structure of the aldehyde synthesized from ethanol
in the liver.
Question 13.10 Draw the structure of a ketone that is an important, versatile
solvent for organic compounds.
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13.3 Important Aldehydes and Ketones
CH3O
O
C
O
H Benzaldehyde—almonds
HO
O
CH
CH
C
C
H Vanillin—vanilla beans
CH3
H Cinnamaldehyde—cinnamon
CH3
C
CH3
CH
CH2
CH2
C
O
CH
C
H Citral—lemongrass
O
␣-Demascone—berry flavoring
O
CH3CH2CH2CH2CH2CH2
C
CH3 2-Octanone—mushroom flavoring
Figure 13.3 Important aldehydes and ketones.
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Chapter 13 Aldehydes and Ketones
13.4 Reactions Involving Aldehydes and Ketones
Preparation of Aldehydes and Ketones
LEARNING GOAL
4▶
Write equations for the preparation of aldehydes and ketones by
the oxidation of alcohols.
Aldehydes and ketones are prepared primarily by the oxidation of the corresponding alcohol. As we saw in Chapter 12, the oxidation of methyl alcohol gives
methanal (formaldehyde). The oxidation of a primary alcohol produces an aldehyde, and the oxidation of a secondary alcohol yields a ketone. Tertiary alcohols
do not undergo oxidation under the conditions normally used.
EXAMPLE 13.5
LEARNING GOAL
4▶
Write equations for the preparation of aldehydes and ketones by
the oxidation of alcohols.
Differentiating the Oxidation of Primary,
Secondary, and Tertiary Alcohols
Use specific examples to show the oxidation of a primary, a secondary, and a
tertiary alcohol.
Solution
The oxidation of a primary alcohol to an aldehyde:
H
*
CH3CH2CH28C8OH
*
H
Pyridinium
dichromate
1-Butanol
(butyl alcohol)
O
+
CH3CH2CH28C8H
Butanal
(butyraldehyde)
A mild oxidizing agent must be used in the oxidation of a primary alcohol to an
aldehyde. Otherwise, the aldehyde will be further oxidized to a carboxylic acid.
The oxidation of a secondary alcohol to a ketone:
CH3
*
CH3CH2CH2CH28C8OH
*
H
KMnO4, OH ,
H2O
2-Hexanol
O
+
CH3CH2CH2CH28C8CH3
2-Hexanone
Tertiary alcohols cannot undergo oxidation:
CH3
*
CH3CH2CH28C8OH
*
CH3
H2CrO4
No reaction
2-Methyl-2-pentanol
Practice Problem 13.5
Write equations showing the oxidation of (a) 1-propanol and (b) 2-butanol.
▶ For Further Practice: Questions 13.61 and 13.62.
LEARNING GOAL
5▶
Write equations representing the oxidation of carbonyl
compounds.
Oxidation Reactions
Aldehydes are easily oxidized further to carboxylic acids, whereas ketones do
not generally undergo further oxidation. The reason is that a carbon-hydrogen
bond, present in the aldehyde but not in the ketone, is needed for the reaction
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13.4
433
Reactions Involving Aldehydes and Ketones
A Medical Perspective
Formaldehyde and Methanol Poisoning
Most aldehydes have irritating, unpleasant odors, and formaldehyde is no exception. Formaldehyde is also an extremely
toxic substance. As an aqueous solution, called formalin, it has
been used to preserve biological tissues and for embalming. It
has also been used to disinfect environmental surfaces, body
fluids, and feces. Under no circumstances is it used as an antiseptic on human tissue because of its toxic fumes and the skin
irritations that it causes.
Formaldehyde is used in the production of some killedvirus vaccines. When a potentially deadly virus, such as polio
virus, is treated with heat and formaldehyde, the genetic information (RNA) is damaged beyond repair. The proteins of the
virus also react with formaldehyde. However, the shape of the
proteins, which is critical for a protective immune response
against the virus, is not changed. Thus, when a child is injected
with the Salk killed-virus polio vaccine, the immune system
recognizes viral proteins and produces antibodies that protect
against polio virus infection.
Formaldehyde can also be produced in the body! As we
saw in Chapter 12, methanol can be oxidized to produce formaldehyde. In the body, the liver enzyme alcohol dehydrogenase,
whose function it is to detoxify alcohols by oxidizing them, catalyzes the conversion of methanol to formaldehyde (methanal).
The formaldehyde then reacts with cellular macromolecules,
including proteins, causing severe damage (remember, it is used
as an embalming agent!). As a result, methanol poisoning can
cause blindness, respiratory failure, convulsions, and death.
Clever physicians have devised a treatment for methanol
poisoning that is effective if administered soon enough after
ingestion. Since the same enzyme that oxidizes methanol to
formaldehyde (methanal) also oxidizes ethanol to acetaldehyde (ethanal), doctors reasoned that administering an intravenous solution of ethanol to the patient could protect against
the methanol poisoning. If the ethanol concentration in the
body is higher than the methanol concentration, most of the
alcohol dehydrogenase enzymes of the liver will be carrying
out the oxidation of the ethanol. This is called competitive inhibition because the methanol and ethanol molecules are competing for binding to the enzymes. The molecule that is in the
higher concentration will more frequently bind to the enzyme
and undergo reaction. In this case, the result is that the alcohol
dehydrogenase enzymes are kept busy oxidizing ethanol and
producing the less-toxic (not nontoxic) product, acetaldehyde.
This gives the body time to excrete the methanol before it is
oxidized to the potentially deadly formaldehyde.
For Further Understanding
▶
Acetaldehyde is described as less toxic than formaldehyde.
Do some background research on the effects of these two
aldehydes on biological systems.
▶ You have studied enzymes in previous biology courses.
Using what you learned in those classes with information
from Sections 19.4 and 19.10 in this text, put together an
explanation of the way in which competitive inhibition
works. Can you think of other types of poisoning for which
competitive inhibition might be used to develop an effective
treatment?
to occur. In fact, aldehydes are so easily oxidized that it is often very difficult to
prepare them because they continue to react to give the carboxylic acid rather
than the desired aldehyde. Aldehydes are susceptible to air oxidation, even at
room temperature, and cannot be stored for long periods. The following example shows a general equation for the oxidation of an aldehyde to a carboxylic
acid:
O
+
R8C8H
[O]
Aldehyde
O
+
R8C8OH
Carboxylic acid
Many oxidizing agents can be used. Both basic potassium permanganate and
chromic acid are good oxidizing agents, as the following specific example
shows:
O
CH3*C*H
Ethanal
(acetaldehyde)
KMnO4,
H2O, OH
O
CH3*C*O
Ethanoate anion
(acetate anion)
In basic solution, the product is the
carboxylic acid anion:
O
+
CH3—C—O
The rules for naming carboxylic acid
anions are described in Section 14.1.
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Chapter 13 Aldehydes and Ketones
The oxidation of benzaldehyde to benzoic acid is an example of the conversion of an aromatic aldehyde to the corresponding aromatic carboxylic acid:
O
+
8C8H
H2CrO4
Benzaldehyde
Silver ions are very mild oxidizing agents.
They will oxidize aldehydes but not
alcohols.
O
+
8C8OH
Benzoic acid
Aldehydes and ketones can be distinguished on the basis of differences in
their reactivity. The most common laboratory test for aldehydes is the Tollens’
test. When exposed to the Tollens’ reagent, a basic solution of Ag(NH3)2⫹, an aldehyde undergoes oxidation. The silver ion (Ag⫹) is reduced to silver metal (Ag0) as
the aldehyde is oxidized to a carboxylic acid anion.
O
R*C*H 1
Aldehyde
Ag(NH3)2
Silver
ammonia complex—
Tollens’ reagent
O
R*C*O 1 Ag0
Carboxylate
anion
Silver
metal
mirror
Silver metal precipitates from solution and coats the flask, producing a smooth
silver mirror, as seen in Figure 13.4. The test is therefore often called the Tollens’
silver mirror test. The commercial manufacture of silver mirrors uses a similar
process. Ketones cannot be oxidized to carboxylic acids and do not react with the
Tollens’ reagent.
EXAMPLE 13.6
LEARNING GOAL
5▶
Write equations representing the oxidation of carbonyl
compounds.
Writing Equations for the Reaction of an Aldehyde
and of a Ketone with Tollens’ Reagent
Write equations for the reaction of propanal and 2-pentanone with Tollens’
reagent.
Solution
O
+
CH3CH2C8H
Ag(NH3)2
Propanal
O
+
CH3CH2C8O
Ag0
Propanoate anion
O
+
CH3CH2CH2CCH3
Ag(NH3)2
No reaction
2-Pentanone
Practice Problem 13.6
Write equations for the reaction of (a) ethanal and (b) propanone with Tollens’
reagent.
▶ For Further Practice: Questions 13.66 and 13.71.
Cu(II) is an even milder oxidizing agent
than silver ion.
Another test that is used to distinguish between aldehydes and ketones is
Benedict’s test. Here, a buffered aqueous solution of copper(II) hydroxide and
sodium citrate reacts to oxidize aldehydes but does not generally react with
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13.4
Reactions Involving Aldehydes and Ketones
435
Figure 13.4 The silver precipitate
produced by the Tollens’ reaction is
deposited on glass. The progress of
the reaction is visualized in panels (a)
through (d). Silver mirrors are made in
a similar process.
(a)
(b)
(c)
(d)
ketones. Cu2⫹ is reduced to Cu⫹ in the process. Cu2⫹ is soluble and gives a blue
solution, whereas the Cu⫹ precipitates as the red solid copper(I) oxide, Cu2O.
All simple sugars (monosaccharides) are either aldehydes or ketones. Glucose
is an aldehyde sugar that is commonly called blood sugar because it is the sugar
found transported in the blood and used for energy by many cells. In uncontrolled
diabetes, glucose may be found in the urine. One early method used to determine
the amount of glucose in the urine was to observe the color change of the Benedict’s test. The amount of precipitate formed is directly proportional to the amount
of glucose in the urine (Figure 13.5). The reaction of glucose with the Benedict’s
reagent is represented in the following equation:
O
H
6 C
*
H8C8OH
*
HO8C8H
*
H8C8OH
*
H8C8OH
*
CH2OH
2Cu2
OH
O
O
6 C
*
H8C8OH
*
HO8C8H
*
H8C8OH
*
H8C8OH
*
CH2OH
Cu2O
Glucose
We should also note that when the carbonyl group of a ketone is bonded to
a OCH2OH group, the molecule will give a positive Benedict’s test. This occurs
because such ketones are converted to aldehydes under basic conditions. In Chapter 16 we will see that this applies to the ketone sugars, as well. They are converted
to aldehyde sugars and react with Benedict’s reagent.
Figure 13.5 The amount of precipitate formed and thus the color change
observed in the Benedict’s test are
directly proportional to the amount of
reducing sugar in the sample.
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Chapter 13 Aldehydes and Ketones
A Human Perspective
Alcohol Abuse and Antabuse
According to a recent study carried out by the Centers for Disease Control and Prevention,1 more than 75,000 Americans die
each year as a result of alcohol abuse. Of these, nearly 35,000
people died of cirrhosis of the liver, cancer, or other drinkingrelated diseases. The remaining nearly 41,000 died in alcoholrelated automobile accidents. Of those who died, 72% were men
and 6% were under the age of twenty-one. In fact, a separate
study has estimated that 1400 college-age students die each
year of alcohol-related causes.
These numbers are striking. Alcohol abuse is now the third
leading cause of preventable death in the United States, outranked only by tobacco use and poor diet and exercise habits.
As the study concluded, “These results emphasize the importance of adopting effective strategies to reduce excessive drinking, including increasing alcohol excise taxes and screening for
alcohol misuse in clinical settings.”
H3C
CH2
H2C
H3C
N
C
H 3C
CH2
N
S
S
CH3
S
Tetraethylthiuram disulfide
(disulfiram)
Antabuse
C
C
H2
S
One approach to treatment of alcohol abuse, the drug tetraethylthiuram disulfide or disulfiram, has been used since 1951.
The activity of this drug, generally known by the trade name
Antabuse, was discovered accidentally by a group of Danish
researchers who were testing it for antiparasitic properties.
They made the observation that those who had taken disulfiram
became violently ill after consuming any alcoholic beverage.
Further research revealed that this compound inhibits one of
the liver enzymes in the pathway for the oxidation of alcohols.
In Chapter 12 we saw that ethanol is oxidized to ethanal
(acetaldehyde) in the liver. This reaction is catalyzed by the
enzyme alcohol dehydrogenase. Acetaldehyde, which is more
toxic than ethanol, is responsible for many of the symptoms of
a hangover. The enzyme acetaldehyde dehydrogenase oxidizes
acetaldehyde into ethanoic acid (acetic acid), which then is used
in biochemical pathways that harvest energy for cellular work
or that synthesize fats.
Antabuse inhibits acetaldehyde dehydrogenase. This inhibition occurs within 1 to 2 hours (h) of taking the drug and continues up to 14 days. When a person who has taken Antabuse
drinks an alcoholic beverage, the level of acetaldehyde quickly
reaches levels that are five to ten times higher than would normally occur after a drink. Within just a few minutes, the symptoms of a severe hangover are experienced and may continue
for several hours.
Experts in drug and alcohol abuse have learned that drugs
such as Antabuse are generally not effective on their own. However, when used in combination with support groups and/or
psychotherapy to solve underlying behavioral or psychological
problems, Antabuse is an effective deterrent to alcohol abuse.
1. Alcohol-Attributable Deaths and Years of Potential Life Lost—United States,
2001, Morbidity and Mortality Weekly Report, 53 (37): 866–870, September 24,
2004, also available at http://www.cdc.gov/mmwr/preview/mmwrhtml/
mm5337a2.htm.
For Further Understanding
▶
Antabuse alone is not a cure for alcoholism. Consider some
of the reasons why this is so.
▶ Write equations showing the oxidation of ethanal to ethanoic
acid as a pathway with the product of the first reaction serving
as the reactant for the second. Explain the physiological effects
of Antabuse in terms of these chemical reactions.
Reduction Reactions
LEARNING GOAL
6 ▶ Write equations representing the
reduction of carbonyl compounds.
One way to recognize reduction,
particularly in organic chemistry, is the
gain of hydrogen. Oxidation and reduction
are discussed in Section 12.6.
Hydrogenation was first discussed in
Section 11.5 for the hydrogenation of
alkenes.
Aldehydes and ketones are both readily reduced to the corresponding alcohol by
a variety of reducing agents. Throughout the text the symbol [H] over the reaction
arrow represents a reducing agent.
The classical method of aldehyde or ketone reduction is hydrogenation. The
carbonyl compound is reacted with hydrogen gas and a catalyst (nickel, platinum,
or palladium metal) in a pressurized reaction vessel. Heating may also be necessary.
The carbon-oxygen double bond (the carbonyl group) is reduced to a carbon-oxygen
single bond. This is similar to the reduction of an alkene to an alkane (the reduction of
a carbon-carbon double bond to a carbon-carbon single bond). The addition of hydrogen to a carbon-oxygen double bond is shown in the following general equation:
O
H
+
C *
- 0 2
H
R1
R
Aldehyde
or ketone
Hydrogen
Pt
OH
*
R 8C8H
*
R2
1
Alcohol
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437
Reactions Involving Aldehydes and Ketones
The hydrogenation (reduction) of a ketone produces a secondary alcohol, as seen
in the following equation showing the reduction of the ketone, 3-octanone:
O
CH3CH2CCH2CH2CH2CH2CH3
Hydrogen
3-Octanone
(A ketone)
EXAMPLE 13.7
H2
Ni
OH
CH3CH2CCH2CH2CH2CH2CH3
H
3-Octanol
(A secondary alcohol)
Writing an Equation Representing
the Hydrogenation of a Ketone
Write an equation showing the hydrogenation of 3-pentanone.
LEARNING GOAL
6▶
Solution
The product of the reduction of a ketone is a secondary alcohol, in this case,
3-pentanol.
O
CH3CH2 CCH2CH3
H2
Pt
3-Pentanone
Write equations representing the reduction of carbonyl
compounds.
OH
CH3CH2 CCH2CH3
H
3-Pentanol
Practice Problem 13.7
Write an equation for the hydrogenation of (a) propanone and (b) butanone.
▶ For Further Practice: Question 13.68.
The hydrogenation of an aldehyde results in the production of a primary alcohol,
as seen in the following equation showing the reduction of the aldehyde, butanal:
O
CH3CH2CH2CH
Butanal
(An aldehyde)
EXAMPLE 13.8
H2
Pt
Hydrogen
OH
CH3CH2CH2 C H
H
1-Butanol
(A primary alcohol)
Writing an Equation Representing
the Hydrogenation of an Aldehyde
Write an equation showing the hydrogenation of 3-methylbutanal.
LEARNING GOAL
6▶
Solution
Recall that the reduction of an aldehyde results in the production of a primary
alcohol, in this case, 3-methyl-1-butanol.
O
OH
Pt
CH3CHCH2 C H
CH3CHCH2 C H H2
H
CH3
CH3
3-Methylbutanal
Write equations representing the reduction of carbonyl
compounds.
3-Methyl-1-butanol
Continued—
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Chapter 13 Aldehydes and Ketones
A Medical Perspective
That Golden Tan Without the Fear of Skin Cancer
Self-tanning lotions have become very popular in recent years.
This seems to be the result of our growing understanding of
the link between exposure to the sun and skin cancer and to
improvements in the quality of the tan produced by these selftanners.
The active ingredient in most self-tanners is dihydroxyacetone (DHA).
CH2OH
|
C+O
|
CH2OH
Dihydroxyacetone
DHA is a ketone, but because it also has hydroxyl groups, DHA
is a sugar, more precisely, a keto sugar.
A researcher by the name of Eva Wittgenstein discovered
the tanning reaction while she was studying a human genetic
disorder in children. These children were unable to store glycogen, a polysaccharide, or sugar polymer, which is our major
energy storage molecule in the liver. She was trying to treat the
disease by feeding large doses of DHA to the children. Sometimes, however, the children spit up some of the sickeningly
sweet solution, which ended up on their clothes and skin. Dr.
Wittgenstein noticed that the skin darkened at the site of these
spills and decided to investigate the observation.
DHA works because of a reaction between its carbonyl
group and a free amino group (ONH3⫹) of several amino acids
in the skin protein keratin. Amino acids are the building blocks
of the biological polymers called proteins (Chapter 18); keratin
is just one such protein. The DHA produces brown compounds
called melanoids when it bonds to the keratins. These polymeric melanoids are chemically linked to cells of the stratum
corneum, the dead, outermost layer of the skin. DHA does not
penetrate this outer layer, so the chemical reaction that causes
tanning only affects the stratum corneum. As this dead skin
sloughs off, so does your tan!
Over the years research has improved the quality of the tan
that is produced. Early self-tanning lotions produced an orange
tan; the tans from today’s lotions are much more natural. The
DHA used today is in a much purer form and the other components of the lotion have been redesigned to promote greater
penetration. Research has also taught us that the tanning reaction works best at acid pH; so newer formulations are buffered
to pH 5. All of these changes have resulted in self-tanners that
produce a longer lasting tan with a more natural, golden color.
We have also learned that it is important to exfoliate before
using a self-tanner. Anywhere that the dead skin layer is thicker,
there will be more keratin. From our study of chemistry, we have
learned that when we begin with more reactant, we often get
more product. The greater the amount of product in this case,
the darker the color! The resulting tan often looked splotchy or
streaky. By gently removing some of the stratum corneum by a
gentle exfoliation process, the surface of the skin, and hence the
tanning reaction, becomes more uniform.
People often ask whether the tan from a bottle can protect against burn, much as a natural tan does. The melanoids
do absorb light of the same wavelengths absorbed by melanin
(the substance formed by suntanning), so you might expect
some protection against sunburn. However, the protection is
minimal, rated at a sun protection factor (SPF) of only 2 or 3.
Self-tanners offer an excellent substitute for a suntan, producing
the same golden tan without the danger of overexposure to the
sun’s harmful ultraviolet rays.
For Further Understanding
▶
Explain in terms of a chemical reaction why using a selftanner daily results in an increasingly darker tan.
▶ The incidence of skin cancer in men and women has
risen dramatically in recent years. Using the Internet and
Chapter 20 in this book, develop a hypothesis to explain
this observation.
Practice Problem 13.8
Write equations for the hydrogenation of 3,4-dimethylhexanal and
2-chloropentanal.
▶ For Further Practice: Questions 13.69 and 13.70.
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A biological example of the reduction of a ketone occurs in the body, particularly during strenuous exercise when the lungs and circulatory system may
not be able to provide enough oxygen to the muscles. Under these circumstances,
the lactate fermentation begins. In this reaction, the enzyme lactate dehydrogenase
reduces pyruvate, the product of glycolysis (a pathway for the breakdown of glucose) into lactate. The source of hydrogen ions for this reaction is nicotinamide
adenine dinucleotide (NADH), which is oxidized in the course of the reaction.
O O
Lactate dehydrogenase
CH3*C*C*O2
NADH
NAD+
a.
b.
c.
d.
e.
The role of the lactate fermentation in
exercise is discussed in greater detail in
Section 21.4.
OH O
CH3* C*C*O2
H
Pyruvate
Question
reaction.
439
Reactions Involving Aldehydes and Ketones
Lactate
13.11 Label each of the following as an oxidation or a reduction
Ethanal to ethanol
Benzoic acid to benzaldehyde
Cyclohexanone to cyclohexanol
2-Propanol to propanone
2,3-Butanedione (found in butter) to 2,3-butanediol
Question 13.12 Write an equation for each of the reactions in Question 13.11.
Addition Reactions
The principal reaction of the carbonyl group is the addition reaction across the polar
carbon-oxygen double bond. This reaction is very similar to some that we have
already studied, addition across the carbon-carbon double bond of alkenes. Such
reactions require that a catalytic amount of acid be present in solution, as shown by
the H⫹ over the arrow, for the reactions shown in the following examples.
An example of an addition reaction is the reaction of aldehydes with alcohols in the presence of catalytic amounts of acid. In this reaction, the hydrogen of
the alcohol adds to the carbonyl oxygen. The alkoxyl group of the alcohol (OOR)
adds to the carbonyl carbon. The predicted product is a hemiacetal.
LEARNING GOAL
7▶
Write equations for the preparation of hemiacetals, hemiketals,
acetals, and ketals.
Addition reactions of alkenes are described
in detail in Section 11.5.
OH
*
R8C8OR
*
H
General structure of a hemiacetal
However, this is not the product typically isolated from this reaction. Hemiacetals are quite reactive. In the presence of acid and excess alcohol, they undergo
a substitution reaction in which the OOH group of the hemiacetal is exchanged
for another OOR group from the alcohol. The product of this reaction is an acetal.
Acetal formation is a reversible reaction, as the general equation shows:
O
+
C
- 0
R1
H
H
*
OR2
Aldehyde Alcohol
H
OH
*
1
R 8C8OR2
*
H
Hemiacetal
H
*
OR2
H
OR2
*
R18C8OR2
*
H
H2O
Acetal
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Chapter 13 Aldehydes and Ketones
Consider the acid-catalyzed reaction between propanal and methanol:
O
+
CH3CH28C8H
CH3OH
Propanal
Methanol
OH
*
CH3CH28C8OCH3
*
H
H
OCH3
*
CH3CH28C8OCH3
*
H
H
CH3OH
Hemiacetal
H2O
Propanal dimethyl acetal
Addition reactions will also occur between a ketone and an alcohol. In this
case the more reactive intermediate is called a hemiketal and the product is called
a ketal. The general equation for ketal formation is shown here:
O
+
C
- 0 2
R1
R
H
*
OR3
Ketone
Alcohol
H
OH
*
R18C8OR3
*
R2
H
*
OR3
H
OR3
*
R18C8OR3
*
R2
Hemiketal
H2O
Ketal
A simple scheme is helpful in recognizing these four types of compounds.
Begin by drawing a carbon atom with four bonds and follow the flowchart as
additional groups are added that will identify the molecules.
*
8C8
*
Add an alkyl group
and an H atom.
Add two
alkyl groups.
R
*
H8C8
*
R
*
R8C8
*
Add a hydroxyl
group and an
alkoxyl group.
Add a hydroxyl
group and an
alkoxyl group.
Add two alkoxyl groups.
R
*
H8C8 OH
*
OR
R
*
H8C8 OR
*
OR
R
*
R8C8 OR
*
OR
R
*
R8C8 OH
*
OR
Hemiacetal
Acetal
Ketal
Hemiketal
A ketal is the final product in the reaction between propanone and ethanol, seen in
the following equation:
O
CH3*C*CH3
CH3CH2OH
Propanone
Ethanol
H
OH
CH3*C*OCH2CH3
CH3
Hemiketal
CH3CH2OH
H
OCH2CH3
CH3*C*OCH2CH3
CH3
H2O
Ketal
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13.4
Reactions Involving Aldehydes and Ketones
441
Question 13.13 Identify each of the following structures as a hemiacetal, acetal,
hemiketal, or ketal.
CH3
|
a. H—C—OH
|
OCH3
CH3
|
b. H3C—C—OCH3
|
OCH3
CH3
|
c. H—C—OCH3
|
OCH3
CH3
|
d. H3C—C—OH
|
OCH3
Question 13.14 Identify each of the following structures as a hemiacetal, acetal,
hemiketal, or ketal.
CH2CH3
|
a. H—C—OH
|
OCH3
CH3
|
b. CH3CH2 —C—OH
|
OCH3
CH3
|
c. H—C—OCH2CH3
|
OCH3
CH3
|
d. H3C—C—OCH3
|
OCH2CH3
Hemiacetals and hemiketals are readily formed in carbohydrates. Monosaccharides contain several hydroxyl groups and one carbonyl group. The linear form
of a monosaccharide quickly undergoes an intramolecular reaction in solution to
give a cyclic hemiacetal or hemiketal.
Earlier we noted that hemiacetals and hemiketals formed in intermolecular
reactions were unstable and continued to react, forming acetals and ketals. This
is not the case with the intramolecular reactions involving five- or six-carbon sugars. In these reactions the cyclic or ring form of the molecule is more stable than
the linear form. This reaction is shown for the sugar glucose (blood sugar) in
Figure 13.6, and is discussed in detail in Section 16.2.
When the hemiacetal or hemiketal of one monosaccharide reacts with a
hydroxyl group of another monosaccharide, the product is an acetal or a ketal.
A sugar molecule made up of two monosaccharides is called a disaccharide. The
Figure 13.6 Hemiacetal formation in
CH2OH
6
5
H
H
O
C
OH
6
2
HO
C
H
H
C
OH
C
C
OH
HO
3
H
4
H
1
HO
1
H
4
5
CH2OH
6
D-Glucose
(open-chain form)
H
H
4
C
OH
CH2OH
H
H
OH
H
C
C
3
H
H
OH
2
3
H
OH
5C
sugars, shown for the intramolecular
reaction of D-glucose.
O
OH
␣-D-Glucose
C
1
O
CH2OH
2
6
5
OH
H
O
OH
H
4
HO
1
OH
3
H
H
H
2
OH
␤-D-Glucose
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Chapter 13 Aldehydes and Ketones
Glycosidic bond
6
CH2OH
5
H
4
1
H
⫹
OH
2
3
5
O
H
HO
H
CH2OH
CH2OH
O
H
OH
6
1
OH
␣-Glucose
2
H
H
HO
H
5
4
CH2OH
6
HO
3
4
H
OH
CH2OH
O
H
1
H
3
H
2
O
HO
H
OH
H
␤-Fructose
1
O
2
H
HO
3
OH
5
⫹ H2O
CH2OH
6
4
OH
H
Sucrose
Figure 13.7 Acetal formation, demonstrated in the formation of the disaccharide sucrose, common table sugar. The reaction
between the hydroxyl groups of the monosaccharides glucose and fructose produces the acetal sucrose. The bond between the two
sugars is a glycosidic bond.
COOOC bond between the two monosaccharides is called a glycosidic bond
(Figure 13.7).
LEARNING GOAL
8▶
Draw the keto and enol forms of
aldehydes and ketones.
Keto-Enol Tautomers
Many aldehydes and ketones may exist in an equilibrium mixture of two constitutional or structural isomers called tautomers. Tautomers differ from one another in
the placement of a hydrogen atom and a double bond. One tautomer is the keto form
(on the left in the following equation). The keto form has the structure typical of an
aldehyde or ketone. The other form is called the enol form (on the right in the following equation). The enol form has a structure containing a carbon-carbon double
bond (en) and a hydroxyl group, the functional group characteristic of alcohols (ol).
H O
OH
R1
* +
0
1
3
(R1, R2, and R3
R 8C8C8R
C9C
0
*
H or alkyl group)
R2
R3
R2
Keto form
Enol form
Because the keto form of most simple aldehydes and ketones is more stable, they
exist mainly in that form.
EXAMPLE 13.9
LEARNING GOAL
8▶
Draw the keto and enol forms of
aldehydes and ketones.
Writing an Equation Representing the Equilibrium
Between the Keto and Enol Forms of a Simple
Aldehyde
Draw the keto form of ethanal and write an equation representing the
equilibrium between the keto and enol forms of this molecule.
Solution
H O
H*C*C*H
H
Ethanal
Keto form
More stable
O*H
H*C+C*H
H
Enol form
Less stable
Practice Problem 13.9
Draw the keto and enol forms of (a) propanal and (b) 3-pentanone.
▶ For Further Practice: Questions 13.89 and 13.90.
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443
Reactions Involving Aldehydes and Ketones
Phosphoenolpyruvate is a biologically important enol. In fact, it is the highest
energy phosphorylated compound in living systems.
O
*
O
C9O
+
*
O8P8O C
+
*
CH2
O
Phosphoenolpyruvate
Phosphoenolpyruvate is produced in the next-to-last step in the metabolic pathway called glycolysis, which is the first stage of carbohydrate breakdown. In the
final reaction of glycolysis, the phosphoryl group from phosphoenolpyruvate
is transferred to adenosine diphosphate (ADP). The reaction produces ATP, the
major energy currency of the cell.
The glycolysis pathway is discussed in
detail in Chapter 21.
Aldol Condensation
The aldol condensation is a reaction in which aldehydes or ketones react to form
larger molecules. A new carbon-carbon bond is formed in the process:
O
R1*CH2*C*R
R
O
R2*CH2*C*R
OH2or
enzyme
H, alkyl, or aryl group
Aldehyde
or
Ketone
Aldehyde
or
Ketone
LEARNING GOAL
9▶
Write equations showing the
aldol condensation.
OH
O
R1*CH2*C*CH*C*R
R R2
Aldol
This is actually a very complex reaction that occurs in multiple steps. Here we
focus on the end results of the reaction, using the example of the reaction between
two molecules of ethanal. As shown in the equation below, the ␣-carbon (carbon-2) of one aldehyde forms a bond with the carbonyl carbon of a second aldehyde (shown in blue). A bond also forms between a hydrogen atom on that same
␣-carbon and the carbonyl oxygen (shown in red).
H O
H* C*C*H
H
Ethanal
H O
H*C*C*H
H
OH
H OH
O
H*C— C— CH2*C*H
H H
3-Hydroxybutanal
(b-hydroxybutyraldehyde)
Ethanal
The result is similar when two ketones react:
O
CH3*C*CH3
H O
H*C*C*CH3
H
Propanone
Propanone
OH
OH
O
CH3*C*CH2*C*CH3
CH3
4-Hydroxy-4-methyl-2-pentanone
In the laboratory, the aldol condensation is catalyzed by dilute base. But the same
reaction occurs in our cells, where it is catalyzed by an enzyme. This reaction is one
of many in a pathway that makes the sugar glucose from smaller molecules. This
pathway is called gluconeogenesis (gluco- [sugar], neo- [new], genesis [beginnings]),
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Chapter 13 Aldehydes and Ketones
Gluconeogenesis is described in
Chapter 21.
which simply means origin of new sugar. This pathway is critical during starvation or following strenuous exercise. Under those conditions, blood glucose concentrations may fall dangerously low. Because the brain can use only glucose as an
energy source, it is essential that the body be able to produce it quickly.
One of the steps in the pathway is an aldol condensation between the ketone
dihydroxyacetone phosphate and the aldehyde glyceraldehyde-3-phosphate.
O
0 3
C
*
H8C8OH
*
CH2OPO32
CH2OPO32
*
C9O
*
HO8C8H
*
H8C8OH
*
H8C8OH
*
CH2OPO32
H
CH2OPO32
*
C9O
*
H—COH
*
H
Dihydroxyacetone phosphate is a phosphorylated form of dihydroxyacetone
(DHA), the active ingredient in self-tanning
lotions. See A Medical Perspective: That
Golden Tan Without the Fear of Skin
Cancer on page 438.
Dihydroxyacetone
phosphate
Aldolase
Glyceraldehyde3-phosphate
Fructose1,6-bisphosphate
The speed and specificity of this reaction are ensured by the enzyme aldolase. The
product is the sugar fructose-1,6-bisphosphate, which is converted by another
enzyme into glucose-1,6-bisphosphate. Removal of the two phosphoryl groups
results in a new molecule of glucose for use by the body as an energy source.
Gluconeogenesis occurs under starvation conditions to provide a supply of
blood glucose to nourish the brain. However, when glucose is plentiful, it is broken
down to provide ATP energy for the cell. The pathway for glucose degradation is
called glycolysis. In glycolysis, the reaction just shown is reversed. In general, aldol
condensation reactions are reversible. These reactions are called reverse aldols.
ATP, the universal energy currency, is
discussed in Section 21.1.
Question 13.15 Write an equation for the aldol condensation of two molecules
of propanal.
Question 13.16 Write an equation for the aldol condensation of two molecules
of butanal.
SUMMARY OF REACTIONS
Aldehydes and Ketones
Reduction of Aldehydes and Ketones
Oxidation of an Aldehyde
O
+
R8C8H
[O]
Aldehyde
O
H
+
C *
- 0 2
H
R1
R
O
+
R8C8OH
Carboxylic acid
Aldehyde
or Ketone
Pt
Hydrogen
OH
*
R 8C8H
*
R2
1
Alcohol
Addition Reactions
Addition of an alcohol to a ketone—ketal formation:
O
+
C
- 0 2
R1
R
H
*
OR3
Ketone
Alcohol
H
OH
*
1
R 8C8OR3
*
R2
Hemiketal
H
*
OR3
H
OR3
*
R18C8OR3
*
R2
H2O
Ketal
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Summary
Addition of an alcohol to an aldehyde—acetal formation:
O
+
C
- 0
R1
H
H
*
OR2
Aldehyde Alcohol
H
OH
*
R18C8OR2
*
H
H
*
OR2
Hemiacetal
Aldol Condensation
H O
R1*C*C*R3
R2
O
R1*CH2*C*R
Keto form
Enol form
O
R2*CH2*C*R
Aldehyde or ketone
R H, alkyl,
or aryl group
SUMMARY
13.1 Structure and Physical Properties
The carbonyl group (1
. C9O) is characteristic of the aldehydes
and ketones. The carbonyl group and the two groups attached
to it are coplanar. In ketones the carbonyl carbon is attached to
two carbon-containing groups, whereas in aldehydes the carbonyl carbon is attached to at least one hydrogen; the second
group attached to the carbonyl carbon in aldehydes may be
another hydrogen or a carbon atom. Owing to the polar carbonyl group, aldehydes and ketones are polar compounds. Their
boiling points are higher than those of comparable hydrocarbons but lower than those of comparable alcohols. Small aldehydes and ketones are reasonably soluble in water because of
the hydrogen bonding between the carbonyl group and water
molecules. Larger carbonyl-containing compounds are less
polar and thus are more soluble in nonpolar organic solvents.
13.2 I.U.P.A.C. Nomenclature and Common Names
In the I.U.P.A.C. Nomenclature System, aldehydes are named
by determining the parent compound and replacing the final
-e of the parent alkane with -al. The chain is numbered beginning with the carbonyl carbon as carbon-1. Ketones are named
by determining the parent compound and replacing the -e
ending of the parent alkane with the -one suffix of the ketone
family. The longest carbon chain is numbered to give the carbonyl carbon the lowest possible number. In the common
system of nomenclature, substituted aldehydes are named
as derivatives of the parent compound. Greek letters indicate
the position of substituents. Common names of ketones are
derived by naming the alkyl groups bonded to the carbonyl
carbon. These names are followed by the word ketone.
H2O
Acetal
Keto-enol Tautomerization
OH
R1
#
!
C+C
!
# 3
R2
R
H
OR2
*
R18C8OR2
*
H
Aldehyde
or ketone
OH2or
enzyme
OH
O
R1*CH2*C*CH*C*R
R R2
Aldol
13.3 Important Aldehydes and Ketones
Many members of the aldehyde and ketone families are
important as food and fragrance chemicals, medicinals,
and agricultural chemicals. Methanal (formaldehyde) is
used to preserve tissue. Ethanal causes the symptoms of a
hangover and is oxidized to produce acetic acid commercially. Propanone is a useful and versatile solvent for organic
compounds.
13.4 Reactions Involving Aldehydes and Ketones
In the laboratory, aldehydes and ketones are prepared by the
oxidation of alcohols. Oxidation of a primary alcohol produces
an aldehyde; oxidation of a secondary alcohol yields a ketone.
Tertiary alcohols do not react under these conditions. Aldehydes and ketones can be distinguished from one another on
the basis of their ability to undergo oxidation reactions. The
Tollens’ test and Benedict’s test are the most common such tests.
Aldehydes are easily oxidized to carboxylic acids. Ketones
do not undergo further oxidation reactions. Aldehydes and
ketones are readily reduced to alcohols by hydrogenation. The
most common reaction of the carbonyl group is addition across
the highly polar carbon-oxygen double bond. The addition of
an alcohol to an aldehyde produces a hemiacetal. The hemiacetal reacts with a second alcohol molecule to form an acetal.
The reaction of a ketone with an alcohol produces a hemiketal.
A hemiketal reacts with a second alcohol molecule to form a
ketal. Hemiacetals and hemiketals are readily formed in carbohydrates. Aldol condensation is a reaction in which aldehydes and ketones form larger molecules. Aldehydes and
ketones may exist as an equilibrium mixture of keto and enol
tautomers.
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Chapter 13 Aldehydes and Ketones
13.31
acetal (13.4)
addition reaction (13.4)
aldehyde (13.1)
aldol condensation (13.4)
Benedict’s test (13.4)
carbonyl group (Intro)
hemiacetal (13.4)
hemiketal (13.4)
hydrogenation (13.4)
ketal (13.4)
ketone (13.1)
oxidation (13.4)
Tollens’ test (13.4)
13.32
13.33
QUESTIONS AND PROBLEMS
*
Structure and Physical Properties
Foundations
Explain the relationship between carbon chain length and
water solubility of aldehydes or ketones.
Explain the dipole-dipole interactions that occur between
molecules containing carbonyl groups.
Applications
13.21
13.22
13.23
13.24
13.35
13.36
Foundations
13.26
13.27
13.28
Briefly describe the rules of the I.U.P.A.C. Nomenclature
System for naming aldehydes.
Briefly describe the rules of the I.U.P.A.C. Nomenclature
System for naming ketones.
Briefly describe how to determine the common name of an
aldehyde.
Briefly describe how to determine the common name of a
ketone.
13.37
13.30
Draw each of the following using complete formulas and
line formulas:
a. Ethanal
b. 3,4-Dimethylpentanal
c. 2-Ethylheptanal
d. 5,7-Dichloroheptanal
Draw each of the following using condensed formulas and
line formulas:
a. 2-Nonanone
b. 4-Methyl-2-heptanone
c. 4,6-Diethyl-3-octanone
d. 5-Bromo-4-octanone
CH3
O
+
c. CH3CHCH2CH
*
Br
CH3 O
+
*
d. CH3CCH2CCH2CH2CH3
*
Cl
The molecule shown below has a lovely aroma of lily-of-thevalley. Discovered in 1908, it has been used in hundreds of
perfumes. What is the I.U.P.A.C. name of this molecule?
O
OH
+
*
CH3CCH2CH2CH2CHCH2CAH
*
*
CH3
CH3
Applications
13.29
HO
OH
Name each of the following using the I.U.P.A.C.
Nomenclature System:
O
Br
O
|
a. CH3CH2CH2CH
b. CH3CCH2CH2CH
|
Nomenclature
13.25
Cl
Name each of the following using the I.U.P.A.C.
Nomenclature System:
O
NO2
a.
b.
+
O
+
H8C8
-
13.20
Simple ketones (for example, acetone) are often used as
industrial solvents for many organically based products
such as adhesives and paints. They are often considered
“universal solvents,” because they dissolve so many diverse
materials. Why are these chemicals such good solvents?
Explain briefly why simple (containing fewer than five
carbon atoms) aldehydes and ketones exhibit appreciable
solubility in water.
Draw intermolecular hydrogen bonding between ethanal
and water.
Draw the polar interactions that occur between acetone
molecules.
Why do alcohols have higher boiling points than aldehydes
or ketones of comparable molecular mass?
Why do hydrocarbons have lower boiling points than
aldehydes or ketones of comparable molecular mass?
-
13.19
CH2CH2CH2CH3
Name each of the following using the I.U.P.A.C.
Nomenclature System:
b.
O
Cl O
* +
a. ClACACACH3
*
Cl
-
13.18
13.34
+
13.17
Draw each of the following using condensed formulas and
line formulas:
a. Ethyl isopropyl ketone
b. Ethyl propyl ketone
c. Dibutyl ketone
d. Heptyl hexyl ketone
Draw each of the following using condensed formulas and
line formulas:
a. ␤-Methylbutyraldehyde
b. ␣-Hydroxypropionaldehyde
c. ␣,␤-Dimethylvaleraldehyde
d. ␥-Chlorovaleraldehyde
Use the I.U.P.A.C. Nomenclature System to name each of the
following compounds:
O
O
+
+
b. HCCHCH2CH3
a. CH3CCH2CH3
0
KEY TERMS
13.38
Give the I.U.P.A.C. name for each of the following compounds:
O
CH2CH3
*
+
a. CH3CCH2CCH2CH3
*
CH2CH3
O
+
b. CH3CCH2CHCH2CH3
*
Cl
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Questions and Problems
13.39
Give the I.U.P.A.C. name for each of the following
compounds:
O
+
a. CH3CHCH2CHCCH2CH3
*
*
CH3
CH3
b. CH3
13.49
13.50
O
3
0
CH3 -
13.51
Give the I.U.P.A.C. name for each of the following
compounds:
O
CH3
+
*
a. CH3CH2CHCH2CH
b.
O
Cl
8Cl
-
13.40
Reactions Involving Aldehydes and Ketones
Foundations
13.52
13.53
13.54
13.55
+
13.56
13.41
13.42
13.43
13.44
Give the common name for each of the following
compounds:
O
O
+
+
b. CH3CH2CCH3
a. CH3CCH3
O
O
+
+
c. CH3CH
d. CH3CH2CH
O
+
e. CH3CHCCH3
*
CH3
Give the common name for each of the following
compounds:
O
+
a. CH3CH2CCH2CH3
O
+
b. CH3CH2CH2CHCH
*
CH3
O
+
c. CH3CCH2CH2CH3
O
+
d. CH3CH2CH2CH2CH2CH
Draw the structure of each of the following compounds:
a. 3-Hydroxybutanal
b. 2-Methylpentanal
c. 4-Bromohexanal
d. 3-Iodopentanal
e. 2-Hydroxy-3-methylheptanal
Draw the structure of each of the following compounds:
a. Dimethyl ketone
b. Methyl propyl ketone
c. Ethyl butyl ketone
d. Diisopropyl ketone
Important Aldehydes and Ketones
13.45
13.46
13.47
13.48
Why is acetone a good solvent for many organic
compounds?
List several uses for formaldehyde.
Ethanal is produced by the oxidation of ethanol. Where does
this reaction occur in the body?
List several aldehydes and ketones that are used as food or
fragrance chemicals.
13.57
13.58
13.59
13.60
Explain what is meant by oxidation in organic molecules
and provide an example of an oxidation reaction involving
an aldehyde.
Explain what is meant by reduction in organic reactions and
provide an example of a reduction reaction involving an
aldehyde or ketone.
Define the term addition reaction. Provide an example of an
addition reaction involving an aldehyde or ketone.
Define the term aldol condensation. Provide an example of an
aldol condensation using an aldehyde or ketone.
Write a general equation representing the oxidation of an
aldehyde. What is the product of this reaction?
Write a general equation representing the reduction of an
aldehyde. What is the product of this reaction?
Write a general equation representing the oxidation of a
ketone. What is the product of this reaction?
Write a general equation representing the addition of one
alcohol molecule to an aldehyde.
Write a general equation representing the addition of two
alcohol molecules to an aldehyde.
Write a general equation representing the addition of one
alcohol molecule to a ketone.
Write a general equation representing the addition of two
alcohol molecules to a ketone.
Write a general equation for an aldol condensation.
Applications
13.61
13.62
13.63
13.64
13.65
13.66
Draw the structure of each of the following alcohols. Then
draw and name the product that you would expect to
produce by the oxidation of each.
a. 4-Methyl-2-heptanol
b. 3,4-Dimethyl-1-pentanol
c. 4-Ethyl-2-heptanol
d. 5,7-Dichloro-3-heptanol
Draw the structure of each of the following alcohols. Then
draw and name the product that you would expect to
produce by the oxidation of each.
a. 1-Nonanol
b. 4-Methyl-1-heptanol
c. 4,6-Diethyl-3-methyl-3-octanol
d. 5-Bromo-4-octanol
Draw the generalized equation for the oxidation of a
primary alcohol.
Draw the generalized equation for the oxidation of a
secondary alcohol.
Draw the structures of the reactants and products for each
of the following reactions. Label each as an oxidation or a
reduction reaction:
a. Ethanal to ethanol
c. 2-Propanol to propanone
b. Cyclohexanone to cyclohexanol
An unknown has been determined to be one of the following
three compounds:
O
O
+
+
CH3CH2CH2CH2CH
CH3CH2CCH2CH3
3-Pentanone
Pentanal
CH3CH2CH2CH2CH3
Pentane
The unknown is fairly soluble in water and produces a silver
mirror when treated with the silver ammonia complex. A
red precipitate appears when it is treated with the Benedict’s
reagent. Which of the compounds is the correct structure for
the unknown? Explain your reasoning.
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13.68
13.69
13.70
13.71
13.72
13.73
13.74
13.75
13.76
13.77
13.78
Write a balanced equation for the hydrogenation of each of
the following aldehydes:
a. ␤-Methylvaleraldehyde
b. ␣-Hydroxypropionaldehyde
c. ␣,␤-Dimethylvaleraldehyde
d. ␥-Chlorovaleraldehyde
Write a balanced equation for the hydrogenation of each of
the following ketones:
a. Ethyl isopropyl ketone c. Dibutyl ketone
b. Ethyl propyl ketone
d. Heptyl hexyl ketone
Write a balanced equation for the hydrogenation of each of
the following aldehydes:
a. Butanal
b. 3-Methylpentanal
c. 2-Methylpropanal
Write a balanced equation for the hydrogenation of each of
the following aldehydes:
a. ␤-Methylbutyraldehyde
b. ␥-Bromovaleraldehyde
c. Propionaldehyde
Which of the following compounds would be expected to
give a positive Tollens’ test?
a. 3-Pentanone
d. Cyclopentanol
b. Cyclohexanone
e. 2,2-Dimethyl-1-pentanol
c. 3-Methylbutanal
f. Acetaldehyde
Write an equation representing the reaction of glucose with
the Benedict’s reagent. How was this test used in medicine?
What is the general name for the product that is formed
when an aldehyde reacts with one molecule of alcohol?
Write an equation for the addition of one ethanol molecule
to each of the following aldehydes:
O
O
+
+
b. CH3CH
a. CH3 CH2CH
What is the general name of the product that is formed
when a ketone reacts with one molecule of alcohol? Write an
equation for the addition of one ethanol molecule to each of
the following ketones:
O
O
+
+
b. CH3CCH2CH2CH3
a. CH3 CCH3
What is the general name for the product that is formed
when an aldehyde reacts with two molecules of alcohol?
Write an equation for the addition of two methanol
molecules to each of the following aldehydes:
O
O
+
+
b. CH3CH
a. CH3 CH2CH
What is the general name of the product that is formed when
a ketone reacts with two molecules of alcohol? Write an
equation for the addition of two methanol molecules to each
of the following ketones:
O
O
+
+
b. CH3CCH2CH2CH3
a. CH3 CCH3
An aldehyde can be oxidized to produce a carboxylic acid.
Draw the carboxylic acid that would be produced by the
oxidation of each of the following aldehydes:
a. Pentanal
c. Heptanal
b. Hexanal
d. Octanal
An aldehyde can be oxidized to produce a carboxylic acid.
Draw the carboxylic acid that would be produced by the
oxidation of each of the following aldehydes:
a. 3-Methylpentanal
b. 2,3-Dichlorobutanal
c. 2,4-Diethylhexanal
d. 2-Methylpropanal
13.79
13.80
13.81
13.82
13.83
13.84
13.85
13.86
13.87
13.88
13.89
An alcohol can be oxidized to produce an aldehyde or
a ketone. What aldehyde or ketone is produced by the
oxidation of each of the following alcohols?
a. Methanol
b. 1-Propanol
An alcohol can be oxidized to produce an aldehyde or
a ketone. What aldehyde or ketone is produced by the
oxidation of each of the following alcohols?
a. 3-Pentanol
b. 2-Methyl-2-butanol
Indicate whether each of the following statements is true or
false.
a. Aldehydes and ketones can be oxidized to produce
carboxylic acids.
b. Oxidation of a primary alcohol produces an aldehyde.
c. Oxidation of a tertiary alcohol produces a ketone.
d. Alcohols can be produced by the oxidation of an
aldehyde or ketone.
Indicate whether each of the following statements is true or
false.
a. Ketones, but not aldehydes, react in the Tollens’ silver
mirror test.
b. Addition of one alcohol molecule to an aldehyde results
in formation of a hemiacetal.
c. The cyclic forms of monosaccharides are intramolecular
hemiacetals or intramolecular hemiketals.
d. Disaccharides (sugars composed of two covalently joined
monosaccharides) are acetals, ketals, or both.
Write an equation for the aldol condensation of two
molecules of ethanal.
Write an equation for the aldol condensation of two
molecules of hexanal.
Write an equation for the aldol condensation of two
molecules of acetone.
Write an equation for the aldol condensation of two
molecules of 2-pentanone.
Draw the keto and enol forms of propanone.
Draw the keto and enol forms of 2-butanone.
Draw the hemiacetal or hemiketal that results from the reaction
of each of the following aldehydes or ketones with ethanol:
O
+
a. CH3CH2CH2CCH3
O
+
13.67
Chapter 13 Aldehydes and Ketones
b. CH3C8
c.
9O
13.90
Identify each of the following compounds as a hemiacetal,
hemiketal, acetal, or ketal:
a.
d.
OH
O
O
!
*OCH3
CH3
b.
OH
!
#
OCH2CH3
OH
c. CH3CCH3
OCH2CH3
#
CH3
OCH3
e. CH3CCH3
OCH2CH3
OCH3
f. CH3CH+CHCCH3
OH
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Critical Thinking Problems
13.91
Complete the following synthesis by supplying the missing
reactant(s), reagent(s), or product(s) indicated by the
question marks:
O
+
CH3CCH3
?(1)
?(2)
13.92
CH2OH
OCH2CH3
*
CH3CCH3
*
OCH2CH3
O
O
CH3
O
a. CH3CHCH2CCH3
O
O
b. HCCH2CH2CH
O
O
d. HCCH2CCH3
O
*CH2CH
CH3
O
e. CH3CCH2CH2CH
CH3
f. O+
+O
O
HO H
H
OH H
H
OH
H H
CH2OH
H2SO4
CH3CHCH3
?(3)
Heat
*
OH
Which alcohol would you oxidize to produce each of the
following compounds?
c.
5. Lactose is the major sugar found in mammalian milk. It is a
dissacharide composed of the monosaccharides glucose and
galactose:
H
OH H
H
H
OH
OH
Is lactose a hemiacetal, hemiketal, acetal, or ketal? Explain your
choice or choices.
6. The following are the keto and enol tautomers of phenol:
OAH
*
O
+
-H
0H
Enol form
Keto form
of phenol
of phenol
We have seen that most simple aldehydes and ketones exist
mainly in the keto form because it is more stable. Phenol is an
exception, existing primarily in the enol form. Propose a
hypothesis to explain this.
CRITICAL THINKING PROBLEMS
1. Review the material on the chemistry of vision found on the
Web at www.mhhe.com/denniston and, with respect to the
isomers of retinal, discuss the changes in structure that occur as
the nerve impulses (that result in vision) are produced. Provide
complete structural formulas of the retinal isomers that you
discuss.
2. Classify the structure of ␤-d-fructose as a hemiacetal,
hemiketal, acetal, or ketal. Explain your choice.
CH2OH
O
H
H HO
OH
CH2OH
OH H
3. Design a synthesis for each of the following compounds, using
any inorganic reagent of your choice and any hydrocarbon or
alkyl halide of your choice:
a. Octanal
b. Cyclohexanone
c. 2-Phenylethanoic acid
4. When alkenes react with ozone, O3, the double bond is cleaved,
and an aldehyde and/or a ketone is produced. The reaction,
called ozonolysis, is shown in general as:
!
!
#
#
C+O 1 O+C
C+C 1 O3
!
!
#
#
Predict the ozonolysis products that are formed when each of
the following alkenes is reacted with ozone:
a. 1-Butene
b. 2-Hexene
c. cis-3,6-Dimethyl-3-heptene
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