Organic Chemistry

746 CHAPTER 18 Carbonyl Compounds II

of the amide. Overall, the reaction converts a carbonyl group into a methylene group.

(Notice that rather than is used in the second step of the reaction. The prod-

uct, therefore, is an amine rather than an ammonium ion.)

The mechanism of the reaction shows why the product of the reaction is an amine.

Take a minute to note the similarities between the mechanisms for the reaction of

hydride ion with an N-substituted amide and with a carboxylic acid.

The mechanisms for the reaction of with unsubstituted and N,N-disubstituted

amines are somewhat different, but have the same result: the conversion of a carbonyl

group into a methylene group.

PROBLEM 15

What amides would you treat with in order to prepare the following amines?

a. benzylmethylamine c. diethylamine

b. ethylamine d. triethylamine

PROBLEM 16

Starting with N-benzylbenzamide, how would you make the following compounds?

a. dibenzylamine c. benzaldehyde

b. benzoic acid d. benzyl alcohol

LiAlH 4

LiAlH 4

N O

CH 3

N-methyl-γ-butyrolactam N-methylpyrrolidine

a tertiary amine

1. LiAlH 4


2. H 2 O N

CH 3

benzamide benzylamine

a primary amine

1. LiAlH 4
   NH 2. H^2 O
   2

CH 2 NH 2

N-methylacetamide

ethylmethylamine

a secondary amine

1. LiAlH 4
   CH 2. H^2 O
   3 NHCH 3

CH 3 CH 2 NHCH 3

C

O

C

O

H 2 O H 3 O+

H 2 O

O

+

+ +

an amide

mechanism for the reaction of an N-substituted amide with hydride ion

an amine

CH 3 C NCH 3 H

H

NCH 3 NCH 3 NCH 3

+ H 2

AlH 3

AlH 3

O

CH 3 C

OAlH 2

CH 3 C CH 3 CH

NCH 3

−

−

HO− CH 3 CH 2 NHCH 3 CH 3 CH 2 NCH 3 CH 3 CH AlH 2 O−

− HAlH− 3

−

O H

H H

Al

−

hydride ion

removes a proton

new hydride

donor

second addition

of hydride ion