Download Chapter Sixteen Aldehydes and Ketones

Fundamentals of General,
Organic, and Biological Chemistry
5th Edition
Chapter Sixteen
Aldehydes and Ketones
James E. Mayhugh
Oklahoma City University
2007 Prentice Hall, Inc.
Outline
► 16.1 The Carbonyl Group
► 16.2 Naming Aldehydes and Ketones
► 16.3 Properties of Aldehydes and Ketones
► 16.4 Some Common Aldehydes and Ketones
► 16.5 Oxidation of Aldehydes
► 16.6 Reduction of Aldehydes and Ketones
► 16.7 Addition of Alcohols: Hemiacetals and Acetals
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16.1 The Carbonyl Group
► Carbonyl compound: Any compound that contains
a carbonyl group, C=O.
► Carbonyl group: A functional group that has a C
atom joined to an O atom by a double bond.
► The bond angles between the three substituents on
the carbonyl carbon atom are 120°, or close to it.
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► Aldehyde: A compound that has a carbonyl group
bonded to at least one hydrogen.
► Ketone: A compound that has a carbonyl group
bonded to two carbons in organic groups that can be
the same or different.
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16.2 Naming Aldehydes and Ketones
► The simplest aldehydes are known by their common
names, which end in -aldehyde, for example,
formaldehyde, acetaldehyde, and benzaldehyde.
► In the IUPAC system, the -e ending of the alkane
with the same number of C’s is replaced by -al.
► When substituents are present, the chain is numbered
beginning with 1 for the carbonyl carbon. For
example, 3-methylbutanal.
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► Common names for ketones give the names of the
two alkyl groups bonded to the carbonyl carbon
followed by the word ketone.
► Ketones are named systematically by replacing the
final -e of the alkane name with -one. The
numbering of the chain begins at the end nearest the
carbonyl group. The location of the carbonyl group
is indicated by placing the number of the carbonyl
carbon in front of the name. Using this nomenclature
scheme, acetone would be named 2-propanone.
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16.3 Properties of Aldehydes and
Ketones
► Aldehydes and ketones cannot hydrogen-bond with
one another, so they are lower boiling than alcohols.
► Aldehydes and ketones are higher boiling than
alkanes because of the polarity of the carbonyl
group. Common aldehydes and ketones are liquids.
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► Simple aldehydes and ketones are water-soluble due
to hydrogen bonding with water molecules, and
ketones are good solvents.
► Simple ketones are less toxic than simple aldehydes.
► Many aldehydes and ketones have distinctive odors.
►A perfumer sits at a
mixing table testing
new combinations of
fragrances, many of
which are aldehydes
and ketones.
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16.4 Some Common Aldehydes and
Ketones
► Formaldehyde is a colorless gas. Low
concentrations in air cause eye, throat,
and bronchial irritation, and high
concentrations trigger asthma attacks.
Skin contact produces dermatitis.
CH2O is very toxic by ingestion,
causing serious kidney damage, coma,
and sometimes even death.
► CH2O kills viruses, fungi, and bacteria
by reaction with the groups in proteins,
allowing for its use in disinfecting and
sterilizing equipment and as a
preservative for biological specimens.
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►The first successful synthetic plastic was a polymer of
phenol and formaldehyde known as Bakelite, once
widely used for such items as pot handles, fountain
pens, and cameras. Urea–formaldehyde polymers are
now more widely used than Bakelite.
►CH2O polymers
release CH2O into
the air. Concerns
over the toxicity
and possible
carcinogenicity of
CH2O limits most
household
applications.
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► Acetaldehyde is a sweet-smelling, flammable liquid
present in ripe fruits, and produced in the normal
breakdown of carbohydrates.
► Acetaldehyde is a general narcotic, and large doses
can cause respiratory failure. Chronic exposure
produces symptoms like those of alcoholism.
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► Acetone is one of the most widely used of all organic
solvents. It dissolves most organic compounds and is
also miscible with water. Acetone is the solvent in
many varnishes, lacquers, and nail polish removers.
► When the metabolism of fats and carbohydrates is
out of balance (for example, in starvation or diabetes
mellitus), acetone is produced in the liver, a
condition known as ketosis that in severe cases
leaves the odor of acetone on a patient’s breath.
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16.5 Oxidation of Aldehydes
► In aldehyde oxidation, the hydrogen bonded to the
carbonyl carbon is replaced by an –OH group.
► Ketones, because they do not have this hydrogen, do
not react with mild oxidizing agents.
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► (a) In the Tollens’ test, colorless silver ion is reduced
to metallic silver.
► (b) In the Benedict’s test, the blue copper(II) ion is
reduced to copper(I) in brick-red copper(I) oxide
► In both tests, the aldehyde is oxidized to the
carboxylic acid.
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16.6 Reduction of Aldehydes and
Ketones
► The reduction of a carbonyl group occurs with the
addition of hydrogen across the double bond to
produce an –OH group, a reaction that is the reverse
of the oxidation of an alcohol.
► Aldehydes are reduced to primary alcohols, and
ketones are reduced to secondary alcohols.
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16.7 Addition of Alcohols: Hemiacetals
and Acetals
The initial product of addition reactions of aldehydes
and ketones with alcohols are known as
hemiacetals. Compounds with both an -OH group
and an -OR group bonded to the same carbon atom.
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► Ethanol forms hemiacetals with acetaldehyde and
acetone.
► Hemiacetals rapidly revert back to aldehydes or
ketones by loss of alcohol and establish an
equilibrium with the aldehyde or ketone.
► When equilibrium is reached, very little hemiacetal
is present.
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► A major exception occurs when the C=O and
-OH functional groups that react are part of the
same molecule. The resulting cyclic hemiacetal is
more stable than a noncyclic hemiacetal.
► Most simple sugars exist mainly in the cyclic
hemiacetal form, as shown below for glucose, rather
than in the open-chain form.
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► If a small amount of acid catalyst is added to the
reaction of an alcohol with an aldehyde or ketone,
the hemiacetal initially formed is converted into an
acetal in a substitution reaction.
► An acetal is a compound that has two -OR groups
bonded to what was once the carbonyl carbon atom.
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The aldehyde or ketone from which an acetal is
formed can be regenerated by reversing the reaction.
Reversal requires an acid catalyst and a large
quantity of water. Hydrolysis: A reaction in which a
bond or bonds are broken and the -H and -OH of
water add to the atoms of the broken bond or bonds.
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Chapter Summary
► The carbonyl group is a C=O. The group is polar, with
a partial (-) charge on O and a partial (+) charge on C.
The O and the two substituents on the carbonyl-group
C atom form a planar triangle.
► The simplest aldehydes and ketones are known by
common names. Aldehydes are named systematically
by replacing the final -e in an alkane name with -al.
► Ketones are named systematically by replacing the
final -e in an alkane name with -one and numbering
starting with 1 at the end nearer the group. The
location of the carbonyl group is indicated by placing
the number of its carbon before the name.
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Chapter Summary Cont.
► Aldehyde and ketone molecules are polar, do not
hydrogen-bond with each other, but can hydrogenbond with water. Small ones are water-soluble.
Aldehydes and ketones are higher boiling than
alkanes but lower boiling than alcohols.
► Aldehydes and ketones are present in many plants,
where they contribute distinctive, pleasant odors.
Such natural aldehydes and ketones are widely used
in perfumes and flavorings.
► Formaldehyde (an irritating and toxic substance) is
used in polymers, is present in smog-laden air, and is
produced biochemically from ingested methanol.
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Chapter Summary Cont.
► Acetone is a widely used solvent and is a by-product
of food breakdown during diabetes and starvation.
Many sugars are aldehydes or ketones.
► Mild oxidizing agents (Tollens’ and Benedict’s
reagents) convert aldehydes to carboxylic acids but
have no effect on ketones.
► With reducing agents, hydride ion adds to the C of
the group in an aldehyde or ketone and hydrogen ion
adds to the O to produce primary or secondary
alcohols, respectively.
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Chapter Summary Cont.
► Aldehydes and ketones establish equilibria with
alcohols to form hemiacetals or acetals.
► Hemiacetals, which have an -OH and an -OR on
what was the carbonyl carbon, result from addition
of one alcohol molecule to the C=O bond.
► The more stable acetals, which have two -OR groups
on what was the carbonyl carbon, form by addition
of a second alcohol molecule to a hemiacetal.
► The aldehyde or ketone can be regenerated from an
acetal by treatment with an acid catalyst and a large
quantity of water, which is an example of a
hydrolysis reaction.
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End of Chapter 16
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