1 Lecture 4: Aldehydes, Ketones, and Chiral Molecules 14.1 Aldehydes and Ketones Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings.
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Lecture 4: Aldehydes, Ketones, and Chiral Molecules
14.1
Aldehydes and Ketones
Copyright © 2007 by Pearson Education, Inc.Publishing as Benjamin Cummings
2
Carbonyl Group in Aldehydes and KetonesA 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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3
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 ethanal propanal
(formaldehyde) (acetaldehyde) (propionaldehyde)
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Naming Aldehydes
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5
Aldehydes in Flavorings
Several naturally occurring aldehydes are used as flavorings for foods and fragrances.
H
O
C
H
O
CCH=CH
Benzaldehyde(almonds)
Cinnamaldehyde (cinnamon)
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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
(dimethyl ketone) (ethyl methyl ketone)
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Ketones in Common Use
Nail polish remover, Solvent
Propanone, Dimethylketone, Acetone
Butter flavoring
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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 D. | CH3
Learning Check
O
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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 OH
│ ║ │C. CH3─C─CH2─C─H D. CH3─CH─CH3
│ CH3
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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
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Learning Check
Draw the structural formulas for each:
A. 4-methylpentanal
B. 2,3-dichloropropanal
C. 3-methyl-2-butanone
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14.2
Physical Properties
Lecture 4: Aldehydes, Ketones, and Chiral Molecules
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13
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.
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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.
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Comparison of Boiling Points
58
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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
B.
C. CH3—CH2—OH or CH3—O—CH3
or
O
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Solubility in Water
The electronegative O atom of the carbonyl group in aldehydes and ketones forms hydrogen bonds with water.
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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
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Lecture 4: Aldehydes, Ketones and Chiral Molecules
14.3
Oxidation and Reduction
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20
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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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 ofan 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
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Acetal Formation
Alcohols add to the carbonyl group of aldehydes and ketones.
The addition of two alcohols forms acetals.
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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.
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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.
Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings
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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
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Learning Check
Draw the structure of the acetal formed by adding CH3OH to butanal.
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15.5
Chiral Molecules
Lecture 4: Aldehydes, Ketones, and Chiral Molecules
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32
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. Copyright © 2007 by Pearson Education, Inc.
Publishing as Benjamin Cummings
33
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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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.
A
CH2CH3
H CH3
Cl
C
B
H
H CH3
Cl
C
C
Br
H CH3
Cl
C
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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.
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Drawing Fischer Projections
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38
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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39
Learning Check
Indicate whether each pair is a mirror image that
cannot be superimposed.
B.
A.
and
and
Br
CH3 H
CH2OH
C
H
CH3 H
Cl
C
Br
H CH3
CH2OH
C
H
H CH3
Cl
C
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