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13.5 Properties of Aldehydes and Ketones

Chapter 13 Organic Compounds with oxygen and Sulfur

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Physical Properties

• The polar carbonyl group provides dipole-dipole interactions.

+ - + -

C=O C=O

• Without an H on the oxygen, aldehydes and ketones 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

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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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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 soluble O ||

C. CH3—CH2—CH2—C—H soluble

O ||

D. CH3—C—CH3 soluble

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Tollens’ Test

In 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,

• Benedict’s reagent, which contains Cu2+,

reacts with aldehydes that have an adjacent OH group.

• An aldehyde is oxidized to a carboxylic acid, while Cu2+ is reduced to give red Cu2O(s).

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Learning Check

Write the structure and name of the oxidized productwhen 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—OH butanoic acid

B. acetaldehyde O ||

CH3—C—OH acetic acid

C. ethyl methyl ketone None. Ketones are not oxidized by Tollen’s reagent.