Transcript Chapter 14

Lecture 4: Aldehydes, Ketones, and Chiral
Molecules
14.1
Aldehydes and Ketones
Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
1
Carbonyl Group in Aldehydes and
Ketones
A carbonyl group (C=O)
 In an aldehyde is attached
to at least one H atom.
 In a ketone is attached to
two carbon groups.
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2
Naming Aldehydes
An aldehyde
 Has an IUPAC name in which the -e in the alkane
name is changed to -al.
 Has a common name for the first four aldehydes that
use the prefixes form (1C), acet (2C), propion (3C),
and butyr (4C), followed by aldehyde.
O
O
O
║
║
║
H−C−H
CH3−C−H
CH3−CH2−C−H
methanal
(formaldehyde)
ethanal
(acetaldehyde)
propanal
(propionaldehyde)
3
Naming Aldehydes
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4
Aldehydes in Flavorings
 Several naturally occurring aldehydes are used
as flavorings for foods and fragrances.
O
H
C
Benzaldehyde
(almonds)
O
CH=CH C H
Cinnamaldehyde (cinnamon)
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5
Naming Ketones
Ketones are named
 In the IUPAC system by replacing the -e in the alkane
name with one. The carbonyl carbon is indicated by
a number.
 With a common name by indicating the alkyl groups
attached to the carbonyl group in alphabetical order
followed by ketone.
O
O
║
║
CH3−C−CH3
CH3−C−CH2−CH3
propanone
2-butanone
6
(dimethyl ketone)
(ethyl methyl ketone)
Ketones in Common Use
Butter
flavoring
Nail polish remover,
Solvent
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
O
D.
8
Learning Check
Classify each as an aldehyde (1), ketone (2), alcohol (3),
or ether (4).
O
║
A. CH3─CH2─C─CH3
B. CH3─O─CH3
CH3
O
│
║
C. CH3─C─CH2─C─H
│
CH3
OH
│
D. CH3─CH─CH3
9
Learning Check
Name each of the following:
O
║
1. CH3─CH2─CH2─CH2─C─H
O
║
2. Cl─CH2─CH2─C─H
O
║
3. CH3─CH2─C─CH3
10
Learning Check
Draw the structural formulas for each:
A. 4-methylpentanal
B. 2,3-dichloropropanal
C. 3-methyl-2-butanone
11
Lecture 4: Aldehydes, Ketones,
and Chiral Molecules
14.2
Physical Properties
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12
Polar Carbonyl Group
The polar carbonyl group
 Provides dipole-dipole interactions.
+ + C=O
C=O
 Does not have H on the oxygen atom.
 Cannot form hydrogen bonds.
13
Boiling Points
Aldehydes and ketones have
 Polar carbonyl groups (C=O).
+ C=O
 Attractions between polar groups.
+ + C=O
C=O
 Higher boiling points than alkanes and ethers of
similar mass.
 Lower boiling points than alcohols of similar mass.
14
Comparison of Boiling Points
58
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15
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
16
Solubility in Water
The electronegative O atom of the carbonyl
group in aldehydes and ketones forms
hydrogen bonds with water.
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17
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
18
Lecture 4: Aldehydes, Ketones and
Chiral Molecules
14.3
Oxidation and Reduction
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19
Oxidation
 Aldehydes are easily oxidized to carboxylic acids.
O
O
||
[O]
||
CH3—C—H
CH3—C—OH
Acetaldehyde
Acetic acid
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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
In the Benedict’s Test,
 Cu2+ reacts with aldehydes
that have an adjacent OH
group.
 Aldehydes are oxidized to
carboxylic acids.
 Cu2+ is reduced to give
Cu2O(s).
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22
Learning Check
Write the structure and name of the oxidized product
when each is mixed with Tollens’ reagent.
1. butanal
2. acetaldehyde
3. ethyl methyl ketone
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Lecture 4: Aldehydes, Ketones and
Chiral Molecules
14.4
Addition Reactions
| + + —C=O + X—Y
|
—C—O—X
|
Y
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Addition Reactions
When a polar molecule adds to the carbonyl group of
an aldehyde or ketone
 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.
| + + —C=O + X—Y
|
— C—O—X
|
Y
25
Acetal Formation
 Alcohols add to the carbonyl group of aldehydes and
ketones.
 The addition of two alcohols forms acetals.
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26
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.
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27
Cyclic Hemiacetals
A stable cyclic hemiacetal forms when the C=O group
and the —OH are both part of a five- or
six-atom carbon compound.
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28
Learning Check
Identify each as a 1) hemiacetal or 2) acetal.
O—CH3
|
A. CH3—CH2—C—H
|
OH
O—CH2CH3
|
B. CH3—CH2—C—CH2—CH3
|
O—CH2CH3
29
Learning Check
Draw the structure of the acetal formed by adding
CH3OH to butanal.
30
Lecture 4: Aldehydes, Ketones,
and Chiral Molecules
15.5
Chiral Molecules
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31
Chiral Compounds
Chiral compounds
 Have the same number of atoms
arranged differently in space.
 Have one or more chiral carbon
atoms bonded to four different
groups.
 Are mirror images like your
hands. Try to superimpose your
thumbs, palms, back of hands,
and little fingers.
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Mirror Images
The mirror images of
 Chiral compounds cannot be superimposed.
 Two compounds of bromochloroiodomethane can align
the H and I atoms, which places the Cl and Br atoms
on opposite sides.
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33
Achiral Structures are
Superimposable
 When the mirror image of an achiral structure is
rotated, the structure can be aligned with the initial
structure. Thus this mirror image is superimposable.
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Learning Check
Identify each as a chiral or achiral compound.
Cl
Cl
H
C
CH3
CH2CH3
A
H
C
Cl
CH3
H
C
H
Br
B
C
CH3
35
Fischer Projections
A Fischer projection
 Is a 2-dimensional representation of a 3-dimensional molecule.
 Places the most oxidized group at the top.
 Uses vertical lines in place of dashes for bonds that go back.
 Uses horizontal lines in place of wedges for bonds that come
forward.
36
Drawing Fischer Projections
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37
D and L Notations
 By convention, the letter L is assigned to the structure
with the —OH on the left.
 The letter D is assigned to the structure with the —OH
on the right.
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38
Learning Check
Indicate whether each pair is a mirror image that
cannot be superimposed.
CH2OH
CH2OH
A.
B.
H
H
C
CH3
and
CH3 C
Br
Br
Cl
Cl
C
H
CH3
and
CH3 C
H
H
H
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