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16.1 Structure and Bonding16.2 Naming Aldehydes and Ketones16.3 Some Important Aldehydes and Ketones

Chapter 16 Aldehydes, Ketones, and Chiral Molecules

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Aldehydes and Ketones

The carbonyl group is found in aldehydesand ketones.In an aldehyde, an H atom is attached to a carbonyl group.In a ketone, two carbon groups are attached to a carbonyl group.

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The Polar Carbonyl GroupThe 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 aldehydesand ketones.

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In the IUPAC names for aldehydes, the -e in the corresponding alkane is replaced by –al.Common names use a prefix with aldehyde:form (1C), acet (2C), propion (3), and butry (4C)

16.2 Naming Aldehydes

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Aldehydes in Flavorings Several naturally occurring aldehydes are used as flavorings for foods and fragrances.

CH=CH C

O

H

C

O

H

Benzaldehyde (almonds)

Cinnamaldehyde (cinnamon)

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In the IUPAC name, the -e in the alkane name is replaced with –one.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—CH3Propanone 2-Butanone

(Dimethyl ketone; (Ethyl methyl ketone)Acetone)

Naming Ketones

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Ketones in Common Use

Propanone, Dimethylketone, Acetone

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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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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 D. 2|

CH3

Solution

O

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Name the following compounds.O||

A. CH3—CH2—C—CH3 B.

CH3 O| ||

C. CH3—C—CH2—C—H|CH3

O

Learning Check

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O||

A. CH3—CH2—C—CH3 B.2-Butanone; ethyl methyl ketone

CH3 O| || Cyclohexanone

C. CH3—C—CH2—C—H|CH3

3,3-Dimethylbutanal

SolutionO

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Draw the structural formulas for each:

A. 3-Methylpentanal

B. 2,3-Dichloropropanal

C. 3-Methyl-2-butanone

Learning Check

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A. 3-Methylpentanal CH3 O | ||

CH3—CH2—CH—CH2—C—H

Cl O| ||

B. 2,3-Dichloropropanal Cl—CH2—CH—C—HO||

C. 3-Methyl-2-butanone CH3—CH—C—CH3|

CH3

Solution

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Physical PropertiesThe 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.

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Comparison of Boiling PointsAldehydes and ketones have higher boiling points than alkanes and ethers of similar mass, but lower boiling points than alcohols.

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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

B.

C. CH3—CH2—OH or CH3—O—CH3

or

O

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Solution

Select the compound in each pair that would have the higher boiling point.A. CH3—CH2—OH

B.

C. CH3—CH2—OH

O

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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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Solution

Indicate if each is soluble or insoluble in water.A. CH3—CH2—CH3 insoluble

B. CH3—CH2—OH solubleO||

C. CH3—CH2—CH2—C—H solubleO||

D. CH3—C—CH3 soluble

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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

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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 TestBenedict’s reagent, which contains Cu2+,reacts with aldehydesthat have an adjacent OH group.When an aldehyde is oxidized to a carboxylic acid, 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.A. Butanal

B. Acetaldehyde

C. Ethyl methyl ketone

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Solution

A. Butanal O||

CH3—CH2—CH2—C—O-

Salt of Butanoic acidB. Acetaldehyde O

||CH3—C—O-

Salt of Acetic acidC. Ethyl methyl ketone None. Ketones are not

oxidized by Tollen’s reagent.

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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.

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16.7 Addition of AlcoholsPolar 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+ δ- δ+ δ- |—C=O + X—Y — C—O—X

|Y

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Hemiacetal FormationThe addition of one alcohol to an aldehyde or ketoneforms an intermediate called a hemiacetal.Usually, hemiacetals are unstable and difficult to isolate.

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Acetal FormationAlcohols add to the carbonyl group of aldehydesand ketones.The addition of two alcohols forms acetals.

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Cyclic HemiacetalsStable hemiacetals form when the C=O group and the —OH are both part of five- or six-atom carbon compounds.

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Learning Check

Identify each as a 1) hemiacetal or 2) acetal.OCH3|

A. CH3—CH2—C—H|OHOCH2CH3|

B. CH3—CH2—C—CH2—CH3|OCH2CH3

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SolutionIdentify each as a 1) hemiacetal or 2) acetal.

OCH3|

A. CH3—CH2—C—H 1) hemiacetal|OHOCH2CH3|

B. CH3—CH2—C—CH2—CH3 2) acetal|OCH2CH3

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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