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CHAPTER
13 Aldehvdesand Ketones
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Ardehvde
Figure
r5.2
Afdehydes
andketonescan
form hydrogenbondswith water.
@^re
Ketone
ffik!/
W
or six carbons, solubility of both aldehydesand ketones is very low. All aldehydes and ketones are soluble in nonpolar solvents.
PRACTICE
EXERCISE
I5.4
Arrange the following substances in order of increasing solubility in
water:
(a) acetone
(b) butanal
(c) pentanol
(d) benzophenone
(e) benzaldehyde
ll,!
Redoxreoctionsof organiccompounds
AIMS: To describethe processesof oxidation ond reductionin
organic chemistryin termsof the lossor goin of oxygen,
hydrogen,or electrons.Torelote the energy contentof o
moleculeto its degreeof oxidotion or reduction.
Focus
Manyreactions of organic
compounds involve oxidation
or reduction.
Now that we ale familiar with functional groups in organic chemistry we
can examine oxidation-reduction reactions of organic molecules. We are
interestedin redox reactionsof organic moleculesbecausethey are important in energy production in living organisms. Oxidation reactions are
energy-releasing,and the more reduced a carbon compound is, the more
energy the compound can release upon its complete oxidation. You may
recall the principles of redox reactions from Section 6.4: Oxidation reactions
involve a gain of oxygen,a loss of hydrogen, or a loss of electrons; reduction
reactionsinvolve a loss of oxygen,a gain of hydrogen, or a gain of electrons.
Oxidation and reduction reactionsmust be coupled; if a compound is oxidized in a reaction,some other compound in the reactionmust be reduced.
We will discuss electron transfers in redox reactions in living organisms in
Chapter 23. Our concern with redox reactions in this chapter will focus on
those reactions that involve oxygen and hydrogen. For example,methane, a
15.5 Redox Reactionsof OrganicCompounds
401
saturated hydrocarbon, czrnbe oxidized in steps to carbon dioxide by alter
nately gaining oxygensand losing hydrogens. Methane can be oxidized to
methanol, then to formaldehyde, then to formic acid, and finally to carbon
dioxide.
H
I
oH
ualn or
|
.
uloarlon
I
U[oallon
I
Gain
of
I
Lossol
H-i-n -+#ii n-C-n +#ffi
H-A-u #*;
Loss
of
I
H-A-on#tri
o:c:o
HH
Methane
Most energetic
molecule
Methanol
(methyl
alcohol)
Many fat molecules have long
hydrocarbon chains as part of
their structure. As such, they are
highly reduced molecules and
have a relatively high energy content compared with other nutrient
molecules.
Least oxidized
(most reduced)
HH
\./
H-C-C-H
./\
HH
Ethane
Methanal
(formaldehyde)
Methanoic
acid
(formic acid)
Carbon dioxide
Leastenergetic
molecule
In any series consisting of an alkane, alcohol, aldehyde (or ketone), carboxylic acid, and carbon dioxide, the alkane is the least oxidized (most
reduced) conxpound and the carbon dioxide is the most oxidized (least
reduced) conxpound.In biological systems, fats are similar in molecular
structure to alkanes,and sugarsare similar to alcohols.The more reduced a
compound is, the more energyit can releaseupon its complete oxidation to
carbon dioxide. Therefore,the complete oxidation of a carbon atom in a fat
can produce more energy than the complete oxidation of a carbon atom in
a sugar.The oxidation of carbon compounds to carbon dioxide can be
reversed.For example, in photoslmthesis, green plants can extract carbon
dioxide from the atmosphere and produce sugars by a series of reduction
reactions(seeSec.23.2).
We can use the loss and gain of hydrogen to find the relative degree of
oxidation of organic molecules that contain carbon-carbon double bonds
and carbon-carbon triple bonds. For example, ethane (an alkane) can lose
hydrogen and go to ethene (an alkene) and then to ethyne (an al\me).
H
Loss ofhydrogen
(dehydrogenation).
Oxidation
H
H
C:C
Lossofhydrogen
(dehydlogenation),
H_C:C_H
trxl0arl0n
Most oxidized
(leastreduced)
H
Ethyne
Each of these lossesof hydrogen representsan oxidation of the compound
that losesthe hydrogen. Ethane is the least oxidized compound, and ethlme
is the most oxidized. The fewer the number of hydrogens on a carbon-carbon bond, the more oxidized is the bond. A.rcdox reaction inuoluingthe loss
of hydrogen from an organic molecule is called a dehydrogenation reaction. The loss of hydrogen from ethane to give ethene and the loss of hydrogen from methanol to give methanal are examples of dehydrogenation
reactions. Strongheating and a catalyst are usuallynecessaryto make dehydrogenation reactions occur in the laboratory. In living organisms, dehydrogenation reactions arc catalyzedby enzymes called dehydrogenasesand
occur atverymild conditions. Like other oxidation reactions, dehydrogenation reactions can be reversed.Allcynescan be reduced to alkenes and
alkenes can be reduced to alkanes by addition of hydrogen to the double
bond.
4O2
CHAPTER
li Aldehvdes
and Ketones
EXAMPTE 15.4
ldentifyingthe relativedegreeof oxidation
List the compounds 2-propanol, propane, and propanone in order from
most reducedto most oxidized.
sotuTtoN
Write the structure of each compound:
o
OH
I
cH3cHCH3
2-Propanol
CH3CH2CH3
CH3CCH3
Propane
Propanone
Propane is most reduced; it has the maximum number of hydrogens.
Propanone is the most oxidized. It has the same number of oxygens as
2-propanol but fewer hydrogens. The order is propane, 2-propanol,
and propanone.
PRACTICE
EXERCISE
I5.5
Indicate the most oxidizedcompound in eachpair.
(a) l-buqme and l-butene
(b) propanal and propane
(c) cyclohexaneand cyclohexanol
(d) 3-pentanol and 3-pentanone
15.4 Redoxreoctionsinvolvingoldehydesond ketones
AIMS: To write structaresfor the oldehydeond ketoneproducts
(if any) of the oxidotion of primary,secondory,ond
tertiary olcohols.To write structuresfor the productsof
the reductionof aldehydesond ketones.
Focus
Redoxreactionsareusedto
interconvert aldehydes or
ketones and alcohols.
The transformation of an alcohol to an aldehydeor ketone can be accomplished by an oxidation reaction. Conversely,aldehydesand ketones can be
reducedtoalcohols.
Oxidatron of alcohols
Primary alcohols can be oxidized to aldehydes,and secondary alc..,holscan
be oxidized to ketones.These oxidations are represented as follows:
ij]j
R
-H itgttL
U
R-,-H
I'I
Primary
alcohol
CH
R-{.-R i+.-
O
R-: -R
1j
Aldehyde
Secondary
alcohol
Ketone
15.4RedoxReactions
lnvolvingAldehydes
and Ketones
405
Tertiary alcohols ire not oxidized because there is no hydrogen to remove
from the carbon bearing the hydroxyl group.
Oxidation of the primary alcohols methanol and ethanol by warming
them at about 50'C with acidified potassium dichromate (K2Cr2O) produces formaldehyde and acetaldehyde,respectively:
oHooHo
rlll
t-?-"
--t"'# H-a-H
Methanol
(methyl alcoho|
(bp 65 "C)
Methanal
(formaldehyde)
(bp -21 "C).
cH3-c-H ffi=
cH.-t-H
Ethanol
(ethyl alcohol)
(bp 78 "C)
Ethanal
(acetaldehyde)
(bp 2t .C)
The preparation of an aldehyde by this method is often a problem
becausealdehydesare easily oxidized to carboxylic acids:
n_c_H #*
R_c_oH
Aldehyde
./
Carboxylicacid
Further oxidation is not a problem with aldehydes that have low boiling
points, such as acetaldehyde,becausethe product can be distilled from the
reaction mixture as it is formed.
Oxidation of the secondaryalcohol2-propanol bywarmingwith acidified potassium dichromate produces acetone:
OH
cH.-J-cu. +*.'
"
'
|
H25U4
cH.-[-cu.
H
rc"3,n#il?Lo
nT"?ff;3"
Ketones are resistant to further oxidation. There is no need to remove them
from the reaction mixture during the course of the reaction.
EXAMPTE 15.5
Writingan equationfor oxidatisnof an alcohol
Write an equation for the oxidation of 4-methyl-2-hexanol.
SOLUTION
A secondary alcohol is oxidized to a ketone. The equation is
oH 9r.
tl
? 9",
CH3CHCI{2CHCH,CH3
curicHriHCHrCH3
+#
Sincethe reactant is a secondary alcohol, oxidation to a carboxylic acid will
not occur.
404
l3 Aldehydesand Ketones
CHAPTER
PRACTICE
EXERCISE
I5.6
\Mhat products are expected when the following compounds are oxidized?
(a) CH.CHTCH2CH2OH
(b)
oH
cH3cH2cHCH3
(c)
oH
I
CH"CH,CCH"
"-l
CHt
(d)
OH
T
IT.7
PRACTICE
EXERCISE
the
name
and
structure of the alcohol you must oxidize to make
Give
the following compounds.
(a) CHgCHzCHO
(b)
o
CH3CH2CCH3
(c) cH.cH,cHCHo
"-l
CH.
Oddadon
of alcohols
in the Hver
Ethanol is oxidized in the liver to acetaldehyde.Acetaldehyde is then oxidized to acetic acid and finallv to carbon dioxide and water:
o
o
CH3CH2-OH
Ethanol
=-
CHs-C-fl
Acetaldehyde
+
CH3-C-OH._
CO,
Aceticacid
Carbon
+ HzO
Water
dioxide
Figure15.5
can
Chronic
alcoholconsumption
of the liver.
causecirrhosis
Consumption of large quantities of ethanol causesthe buildup of high concentrations of acetaldehydein the blood. This can lead to a sharp decrease
in blood pressure, a more rapid heartbeat, and a generally uncomfortable
feeling-a hangover. Continued overindulgence in ethanol eventually
leads to yellowing and hardening of the liver, called cirrhosis (Fig. 13.3),
because of the sustained high levels of acetaldehyde. Cirrhosis is an irreversible degeneration of functioning liver cells.
Methanol, sometimes called wood alcohol, is extremely toxic. lVhen
methanol enters the body, it is quickly absorbed into the bloodstream and
passesto the liver, where it is oxidized to formaldehyde. Formaldehyde is a
very reactive compound. It destroys the catalytic power of enzymes and
causesliver tissue to become hard. This is whv formaldehvde solutions are
13.4 RedoxReactionsInvolvingAldehydesand Ketones
405
Fonow-upro rHECnsrrn Porxr:Fetalalcoholsyndrome
Forlunately, Marla was able to follow her physician's
advice, and at the end of a full-term pregnancy, she
gave birlh to a healthy baby girl. The doctor's concern aboul Marla's history of alcoholism resulted
from the knowledge that alcohol consumption can
cause great damage to the fetus. The fetus processes
ethanol more s1ow1ythan the mother, so the injurious
effects of alcohol and ils oxidation product, acetaldehyde, affect the fetus for a longer time. A pregnant
woman who drinks a1coholpasses these effects in a
magnified form to her unborn child. The damagrng
effects of alcoho] on the unbom are called fetal alcohol effects (FAE). The range of FAE can be mild to
severe. Some FAE are so broad and long-lasting that
they can crlpple a child's physical, social, and academic development. Fetal alcohol syndrome (FAS) is
the name given to extremely severe FAE. FAS children can be underweight or below normal height and
have abnormally sma11heads or other abnormal facial
features, crossed eyes, underdeveloped jawbones,
clef[ palates, and dysfunction of the central nervous
system. FAS is the leading cause of mental retardatlon in chjldren, and the retardation may be profound.
The symptoms of FAE/FAS last a tifetime, and new
symptoms often appear as the child grows up. Some
children who do not have visible signs of FAE/FAS at
birth develop lhem over time. For example, an infanl
who appears normal aL birth may display siowed
development in walking or talking as a toddler.
*"iliJ,illJf
J,;; ffi';
"*,H""xi;:TJ:,31ti:
"_
have FAE/FAS. If th
^ii - 111T.T,,.
11I
chances ofFAE
/FAs'"-^1"^r:^*
'l"l;
inLensined
FAE/FAS
lllll?lL,Tl":n#ff
women abstain from aicohol consumption and smoking during pregnancy. Fathers are not relieved of
responslbitity in preventing FAE/FAS. One study
shows the birth weight of babies born to fathers who
drink regularly averages 181 g less lhan children born
to fathers who drink only occasronally.
used to preservebiological specimens.When methanol is ingested,temporary or permanent blindnessmay occur.
The consumption of even moderate amounts of ethanol by pregnant
women can have devastating effects on the unborn fetus. In the Case in
Point for this chapter, you may recall that Marla, a pregnant woman with a
history of alcoholism,was strongly advisedby her doctor to avoid alcohol
during her pregnancy.In the Follow-up to the Casein Point, above,we will
learn more about the consequencesof using alcohol during pregnancy.
Reduction
of aldehydes
and ketones
Aldehydesand ketonescan be reducedto alcoholsby the addition ofhydrogen, H-H, to the -C:O
double bond. Reductionsof an aldehydeand a
ketone can be representedas follows:
o
A
tt-\--rl
OH
Reduction.
+rH'------
I
R-C-H
I
ooH
t . ,D. , ---;n-Kle O U C T I O n - D
A
R-c-R
n-f - *
n
H
Aidehyde
Primary
alcohol
H
Ketone
Secondary
alcohol
As we can see from these reactions,the reduction of aldehydesproduces
primary alcohols,and the reduction of ketonesproduces secondaryalcohols.A variety of reagentsare availablefor the reduction of -C:O
groups
406
l3 Aldehydesand Ketones
CHAPTER
An important bioiogical example
of reduction of an aldehydeoccurs in fermentation. At the last
step in the metabolism of glucose,
yeastand other organismsreduce
acetaldehvdeto ethanol.
groups.For example,hydrogen gascan be used with a platto -CH-OH
inum or palladium catalyst.
o
a->
tl
Pt,
+ H-H
>T
tl
\-,.-
\-.-
Cyclohexanol
Cyclohexanone
The hydrogenation of C:O bonds is similar to the hydrogenation of C-C
bonds discussed previously. Other reducing reagents for C:O bonds
include compounds called hydrides. Lithium aluminum hydride (LiAlH4)
and sodium tetrahydroborate (NaBHa) are often used.
o
OH
II
cH.cH2-c-H
"
T |\ITJ,
'
I
CH3CH2-C-H
I
H
l-Propanol
(propyl alcohol)
Propanal
o
ilo
;\-i-7\
ll
I
\2
ll
\.2
N^or /-\J
I'I\i.'J",ll
Diphenylmethanone
henzophenone)
\'2
||
H
Diohenvlmethanol
Reductions of aldehydes and ketones in living organisms are catalyzed by
dehydrogenase enzyrnes. Enzymes work reversibly, and dehydrogenases
catalyzeboth the oxidation of alcohols and the reduction of aldehydesand
ketones.
Il.5 Aldehydedetection
or o
o Benedict's,
AIM: To describethe resultsof o Tollens',
Fehling'stest on on aldehyde,o ketone,ond on olpho'
hydroxy ketone.
Focus
Aldehydes can be distinguished
from ketones byusing mild
oxidizing agents.
Aldehydes and ketones react with a wide variety of compounds. In general,
however, aldehydes are more reactive than ketones. Chemists have taken
advantage of the easewith which an aldehyde can be oxidized to develop
severalvisual tests for their detection. The most widely used tests for aldehyde detection are Tollens',Benedict's,and Fehling's.
Tbllens'test
The mild oxidizing agent used in this test, Tollens'reagent, is an alkaline
solution of siluer nitrate.It is clear and colorless.To prevent the silver ions
from precipitating as silver oxide (Ag2O)at the high pH, a few drops of an
13.5 Aldehyde Detection
407
ammonia solution are added.The ammonia forms a water-soluble complex
with silver ions:
llg* (aq) + 2NH3@4) -
+
[Ag(NH3)2] (dq)
When an aldehyde is oxidized with Tollens' reagent,the corresponding
carboxylic acid is formed, and simultaneously, silver ions are reduced to
metallic silver. For example, acetaldehyde goes to acetic acid. The silver is
usually deposited as a mirror on the inside surface of the reaction vessel.
The appearance of a silver mirror is a positive test for an aldehyde. If
acetaldehydeis treated with Tollens' reagent' the reaction is
o
+ 2tAg(NHg)21++ 2o}l- ------ cHs-c-o-NH4+
CH3-C-H
Acetaldehyde
+ 2Ag(s) + 3NH3 + H2o
Silver
(mirror)
Aceticacid
(asammoniumsalt)
Tollens'
reagent
The aldehyde,acetaldehyde,is oxidized to a carboxylic acid, acetic acid; it is
a reducing agent.The silver ions are reduced to metallic silver; they are oxidizing agents.Mirrors (Fig. f 3.4) are often silvered byusingTollens' reagent.
The commercial process uses glucose or formaldehyde as the reducing
agent.
PRACTICE
EXERCISE
T5.8
.
Figute15.4
The reflectivecoatingon a dentist's mirror is producedon the
back of a sheet of glasswhen
formaldehydereducessilverions
in solutionto silvermetal.The
formaldehydeis oxidizedto
formic acid in the process.
You have two unlabeled test tubes, one containing pentanal and the
other containing 2-pentanone. \Mhat simple test could you do to find
.:, out which tube contains pentanal and which tube contains 2-pen" tanone?
'"
Benedict's
and Fehling's
tests
Benedict's andFeliingb reagents are deepblue alkaline solutions of copper
sulfate of slightly differing compositions. lVhen an aldehyde is oxidized
with Benedict's or Fehling's reagents, a brick red precipitate of copper(I)
oxide (CuzO)is obtained. The reaction with acetaldehydeis
o
CH3-C-H
Acetaldehyde
+
o
tl
2ctt*2
+ 5OH- ------ CHa-C-O
Copper(Il)
ion complex
(blue solution)
Acetic acid
(asacetate
ion)
+
CuzO(s)
+ 3H2O
Copper(I)
oxide
(red precipitate)
The acetaldehydeis oxidized to acetic acid; copper(Il) ions (Cu2*) are
reducedto copper(I) ions (Cu+).
Alpha-hydrory
ketones
Ketones are not usually oxidized by mild oxidizing agents such as Tollens'
and Benedict's solutions. However, ketones that contain a carbonyl group
attached to a carbon that bears a hydroxyl group give positive testswith Tol-
408
13 Aldehydesand Ketones
CHAPTER
Personsin good health do not have
sugar in their urine. Clinitest
tablets are used to screenfor sugar
in urine. The reaction ofthe tablet
with the urine givescoloredproducts corresponding to differing
levelsof sugar.The reaction chemistry is that of a Benedict'stest.
lens',Benedict's,and Fehling'sreagents.Thesecompoundsarecalledalphahydrory ketones,whichhauethisgeneralformula:
oHo
til
R-C-C-R
I
H
Alpha-hydroxy ketone
PRACTICEEXERCISEI5.9
Determine which of the following substancesgive a positive test (red
precipitate) with Benedict'sreagent.
(a)
(d)
o
(\'r-
CH3CCH3
\/
(b) CH3CH2CHO
(c)
(e)
OH
\r'
HOO
ltl
I
CH3CHCHO
CH3CHCCH2CH3
Il.6 Additionsto the corhonylgrouP
AIM: To illustrotewith equotionsthe formotion of o hydrote,o
hemiocetaland on ocetol,ond o hemiketolond a ketol'
Focus
Reactions of aldehydes and
ketones are reactions ofthe
carbonyl group.
The most characteristic reactions of both aldehydes and ketones are additionreactions. A wide variety of compounds add to the carbon-oxygen double bond of the carbonyl group. We have already seen additions to carboncarbon double bonds in SectionsL2.2 and I2.4, and additions to carbonoxygen double bonds are quite similar. Our main concern here is with the
addition of water and alcohols to the carbonyl group.
Addition
of water
Water will add to most aldehydes and ketones to form hydrates-compounds that haue two hydroxyl groups on the same carbon.The reaction is
reversible.with few exceptions, the equilibrium lies to the left, in favor of
the starting materials.
ooH
,,1
R-C-H(R)
+ H
OH :
R-C-H(R)
OH
Carbonyl group
ofaldehyde
or ketone
Water
Hydrate