Organic Chemistry

SOLUTION TO 13a The starting material for the synthesis of the two-carbon com-

pound must be formaldehyde. Addition of hydrogen cyanide followed by addition of to

the triple bond of the cyanohydrin forms the desired compound.

SOLUTION TO 13b The starting material for the synthesis of the three-carbon -hy-

droxycarboxylic acid must be ethanal. Addition of hydrogen cyanide, followed by hydrolysis

of the cyanohydrin, forms the target molecule.

18.5 Reactions of Carbonyl Compounds

with Hydride Ion

Addition of hydride ion to an aldehyde or ketone forms an alkoxide ion. Subsequent
protonation by an acid produces an alcohol. The overall reaction adds to the car-
bonyl group. Recall that the addition of hydrogen to an organic compound is a
reduction reaction(Section 4.8).

Aldehydes and ketones are generally reduced using sodium borohydride as the
source of hydride ion. Aldehydes are reduced to primary alcohols, and ketones are
reduced to secondary alcohols. Notice that the acid is not added to the reaction mixture
until the reaction with the hydride donor is complete.

PROBLEM 14

What alcohols are obtained from the reduction of the following compounds with sodium

borohydride?

a. 2-methylpropanal c. benzaldehyde

b. cyclohexanone d. acetophenone

1. NaBH 4

butanal

an aldehyde

CH 3 CH 2 CH 2 H 1-butanol

a primary alcohol

CH 3 CH 2 CH 2 CH 2 OH

1. NaBH 4
   2.H 3 O+

2.H 3 O+

OH

2-pentanone

a ketone

CH 3 CH 2 CH 2 CH 3 2-pentanol

a secondary alcohol

CH 3 CH 2 CH 2 CHCH 3

C

O

C

O

(NaBH 4 )

RR'

+

H

RC

O−

R'

H

OH

H− RCR'

B

HB+

C

O

H 2

OH OH

O

CH 3 CHC N CH 3 CHCOH

NaC

HCl

N

∆

HCl, H 2 O

CH 3 CH

O

a

O

HCH HOCH 2 C N HOCH 2 CH 2 NH 2

NaC

HCl

N H 2

Pt

H 2

Section 18.5 Reactions of Carbonyl Compounds with Hydride Ion 743