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Chapter 15 Aldehydes and Ketones and Chiral Molecules

15.6 Oxidation and Reduction

15.7 Addition Reactions

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Oxidation

Aldehydes are easily oxidized to carboxylic acids.

O O || [O] ||

CH3—C—H CH3—C—OH

Acetaldehyde Acetic acid

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Hydrate formation Hydrate formation is the reason that aldehydes are

oxidized ([O]=-2H=oxidation) to carboxylic acids in aqueous media.

O OH O

|| H2O | [O] ||

RCH ↔ RC – OH → RCOH

|

H

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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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Tollens’ reagent detects an aldehyde Tollens’ reagent (a solution of Ag+ in aqueous NH3) can be

used to detect the presence of the aldehyde group in the unknown sample.

As the oxidation of the aldehyde proceeds, silver metal is deposited on the walls of the reaction flask as a shiny mirrow.

If upon addition of the Tollens’ reagent to the unknown, the test tube becomes silvery, the unknown is an aldehyde.

RCHO + 2Ag(NH3)2

+ +2OH- → RCOO- + NH4+ + 2Ag + 3NH3 + H2O

an Tollens’ salt of a silver

aldehyde reagent carboxylic acid mirrow

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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).

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Test for glucose (Benedict’s test)

CHO

OHH

HHO

OHH

OHH

CH2OH

+ Cu2+

Benedict's (blue)

COOH

OHH

OHH

HHO

OHH

CH2OH

+Cu2O(red)

D-GlucoseD-Gluconic acid

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Addition Reactions 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.

| + - + - | —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

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