Download Aldehydes and Ketones

Chapter 16 Aldehydes, Ketones,
and Chiral Molecules
16.1 Structure and Bonding
16.2 Naming Aldehydes and Ketones
16.3 Some Important Aldehydes and Ketones
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
1
Aldehydes and Ketones
ƒ The carbonyl group
is found in aldehydes
and ketones.
ƒ In an aldehyde, an H
atom is attached to a
carbonyl group.
ƒ In a ketone, two
carbon groups are
attached to a
carbonyl group.
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
2
The Polar Carbonyl Group
The carbonyl group
(C=O):
„ Consists of a sigma and
pi bond.
„ Has a partially negative
O and a partially
positive C.
„ Has polarity that
influences the
properties of aldehydes
and ketones.
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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16.2 Naming Aldehydes
ƒ In the IUPAC names for aldehydes, the -e in
the corresponding alkane is replaced by –al.
ƒ Common names use a prefix with aldehyde:
form (1C), acet (2C), propion (3), and butry (4C)
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Aldehydes in Flavorings
„
Several naturally occurring aldehydes are
used as flavorings for foods and fragrances.
O
C H
Benzaldehyde (almonds)
O
CH=CH C H
Cinnamaldehyde (cinnamon)
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Naming Ketones
ƒ In the IUPAC name, the -e in the alkane name is
replaced with –one.
ƒ In the common name, add the word ketone after
naming the alkyl groups attached to the carbonyl
group.
O
O
||
||
CH3—-C—CH3
CH3—C—CH2—CH3
Propanone
(Dimethyl ketone;
Acetone)
2-Butanone
(Ethyl methyl ketone)
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Ketones in Common Use
Propanone,
Dimethylketone,
Acetone
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Learning Check
Classify each as 1) aldehyde or 2) ketone.
O
O
||
||
A. CH3—CH2—C—CH3
B. CH3—C—H
CH3
O
|
||
C. CH3—C—CH2—C—H
|
CH3
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
O
D.
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Solution
Classify as an 1) aldehyde or 2) ketone.
O
O
||
||
A. 2 CH3—CH2—C—CH3
B. 1 CH3—C—H
CH3
O
|
||
C. 1 CH3—C—CH2—C—H
|
CH3
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
O
D. 2
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Learning Check
Name the following compounds.
O
||
A. CH3—CH2—C—CH3
B.
O
O
CH3
|
||
C. CH3—C—CH2—C—H
|
CH3
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Solution
O
||
A. CH3—CH2—C—CH3
B.
2-Butanone; ethyl methyl ketone
O
CH3
|
||
C. CH3—C—CH2—C—H
|
CH3
3,3-Dimethylbutanal
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O
Cyclohexanone
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Learning Check
Draw the structural formulas for each:
A. 3-Methylpentanal
B. 2,3-Dichloropropanal
C. 3-Methyl-2-butanone
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Solution
A. 3-Methylpentanal
CH3
O
|
||
CH3—CH2—CH—CH2—C—H
Cl O
|
||
B. 2,3-Dichloropropanal Cl—CH2—CH—C—H
O
||
C. 3-Methyl-2-butanone
CH3—CH—C—CH3
|
CH3
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Physical Properties
„
The polar carbonyl group provides
dipole-dipole interactions.
δ+ δ -
„
δ+ δ-
C=O
C=O
Without a H on the oxygen atom,
aldehydes and ketones cannot hydrogen
bond.
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Comparison of Boiling Points
„
Aldehydes and ketones have higher boiling
points than alkanes and ethers of similar
mass, but lower boiling points than alcohols.
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15
Solubility in Water
„
The electronegative O atom of the carbonyl
group of aldehydes and ketones forms
hydrogen bonds with water.
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Comparison of Physical Properties
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Learning Check
Select the compound in each pair that would
have the higher boiling point.
A. CH3—CH2—CH3 or CH3—CH2—OH
O
B.
or
C. CH3—CH2—OH or CH3—O—CH3
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Solution
Select the compound in each pair that would
have the higher boiling point.
A. CH3—CH2—OH
B.
O
C. CH3—CH2—OH
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Learning Check
Indicate if each is soluble or insoluble in water.
A. CH3—CH2—CH3
B. CH3—CH2—OH
O
||
C. CH3—CH2—CH2—C—H
O
||
D. CH3—C—CH3
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Solution
Indicate if each is soluble or insoluble in water.
insoluble
A. CH3—CH2—CH3
B. CH3—CH2—OH
soluble
O
||
C. CH3—CH2—CH2—C—H soluble
O
||
D. CH3—C—CH3
soluble
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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16.5 Oxidation
„
Aldehydes are easily oxidized to carboxylic
acids.
O
O
||
[O]
||
CH3—C—H
CH3—C—OH
Acetaldehyde
Acetic acid
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Tollens’ Test
„
„
Tollens’ reagent,
which contains
Ag+, oxidizes
aldehydes, but
not ketones.
Ag+ is reduced to
metallic Ag,
which appears as
a “mirror” in the
test tube.
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Benedict’s Test
„
„
Benedict’s reagent,
which contains Cu2+,
reacts with aldehydes
that have an adjacent
OH group.
When an aldehyde is
oxidized to a carboxylic
acid, Cu2+ is reduced to
give Cu2O(s).
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Learning Check
Write the structure and name of the
oxidized product when each is mixed with
Tollens’ reagent.
A. Butanal
B. Acetaldehyde
C. Ethyl methyl ketone
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Solution
A. Butanal
O
||
CH3—CH2—CH2—C—OSalt of Butanoic acid
B. Acetaldehyde
O
||
CH3—C—OSalt of Acetic acid
C. Ethyl methyl ketone None. Ketones are not
oxidized by Tollen’s reagent.
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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16.6 Reduction of Aldehydes and
Ketones
„
„
„
Reduction is the reverse of oxidation reaction.
Reduction of a carbonyl group is the addition of
hydrogen across the double bond to produce an –OH
group.
Aldehydes are reduced to primary alcohols, and
ketones are reduced to secondary alcohols.
Reduction of carbonyl group occurs by formation of
a bond to the carbonyl carbon by a hydride, H:-, ion
accompanied by bonding of a H+ ion to the carbonyl
oxygen atom.
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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16.7 Addition of Alcohols
„
„
„
Polar molecules can add to the carbonyl groups of
aldehydes and ketones.
The negative part of the added molecule bonds to
the positive carbonyl carbon.
The positive part of the added molecule bonds to
the negative carbonyl oxygen.
| δ+ δδ+ δ—C=O + X—Y
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|
— C—O—X
|
Y
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Hemiacetal Formation
„
„
The addition of one alcohol to an aldehyde or ketone
forms an intermediate called a hemiacetal.
Usually, hemiacetals are unstable and difficult to
isolate.
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Acetal Formation
„
„
Alcohols add to the carbonyl group of aldehydes
and ketones.
The addition of two alcohols forms acetals.
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Cyclic Hemiacetals
„
Stable hemiacetals form when the C=O group
and the —OH are both part of five- or six-atom
carbon compounds.
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Learning Check
Identify each as a 1) hemiacetal or 2) acetal.
OCH3
|
A. CH3—CH2—C—H
|
OH
OCH2CH3
|
B. CH3—CH2—C—CH2—CH3
|
OCH2CH3
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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Solution
Identify each as a 1) hemiacetal or 2) acetal.
OCH3
|
A. CH3—CH2—C—H
1) hemiacetal
|
OH
OCH2CH3
|
B. CH3—CH2—C—CH2—CH3 2) acetal
|
OCH2CH3
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
33
Learning Check
Draw the structure of the acetal formed
by adding CH3OH to butanal.
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Solution
Draw the structure of the acetal formed by
adding CH3OH to butanal.
OCH3
|
CH3—CH2—CH2—C—H
|
OCH3
Copyright © 2004 Pearson Education Inc., publishing as Benjamin Cummings.
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