Organic Chemistry

PROBLEM 23

Hydration of an aldehyde can also be catalyzed by hydroxide ion. Propose a mechanism

for hydroxide-ion-catalyzed hydration.

Section 18.7 Reactions of Aldehydes and Ketones with Oxygen Nucleophiles 753

18.7 Reactions of Aldehydes and Ketones

with Oxygen Nucleophiles

Addition of Water
Water adds to an aldehyde or a ketone to form a hydrate. A hydrateis a molecule
with two OH groups on the same carbon. Hydrates are also called gem-diols(gem
comes from geminus,Latin for “twin”). Hydrates of aldehydes or ketones are gen-
erally too unstable to be isolated because the tetrahedral carbon is attached to two
oxygen atoms.

Water is a poor nucleophile and therefore adds relatively slowly to a carbonyl
group. The rate of the reaction can be increased by an acid catalyst (Figure 18.3).
Keep in mind that a catalyst has no effect on the position of the equilibrium. A cat-
alyst affects the rateat which the equilibrium is achieved. In other words, the
catalyst affects the rate at which an aldehyde or a ketone is converted to a hydrate;
it has no effect on the amountof aldehyde or ketone converted to hydrate (Sec-
tion 24.0).

+ H 2 O

R R (H)

C

O

RCR (H)

OH

OH

an aldehyde or

a ketone

a gem-diol

a hydrate

Most hydrates are too

unstable to be isolated.

CH 3 H

O

C

CH 3 H

C

+OH

>Figure 18.3

The electrostatic potential maps

show that the carbonyl carbon of

the protonated aldehyde is more

susceptible to nucleophilic attack

(the blue is more intense) than the

carbonyl carbon of the

unprotonated aldehyde.

CH 3 H

C

O

+ H 2 O

mechanism for acid-catalyzed hydrate formation

H

H

CH 3 C

OH

OH

H OH

H

OH

H

+ +OH OH

CH 3 H

H 3 O+

C

OH

42%

58%

CH 3 H

C

+