Organic Chemistry

754 CHAPTER 18 Carbonyl Compounds II

The extent to which an aldehyde or a ketone is hydrated in an aqueous solution de-

pends on the aldehyde or ketone. For example, only 0.2% of acetone is hydrated at

equilibrium, but 99.9% of formaldehyde is hydrated. Why is there such a difference?

The equilibrium constant for a reaction depends on the relative stabilities of the re-

actants and products. The equilibrium constant for hydrate formation, therefore, de-

pends on the relative stabilities of the carbonyl compound and the hydrate. We have

seen that electron-donating alkyl groups make a carbonyl compound more stable(less

reactive) (Section 18.2).

In contrast, alkyl groups make the hydrate less stablebecause of steric interactions

between the alkyl groups.

The electron clouds of the alkyl substituents do not interfere with each other in the

carbonyl compound because the bond angles are 120°. However, the bond angles in

the tetrahedral hydrate are 109.5°, so the alkyl groups are closer to one another.

Alkyl groups, therefore, shift the equilibrium to the left—toward reactants—

because they stabilize the carbonyl compound and destabilize the hydrate. As a result,

less acetone than formaldehyde is hydrated at equilibrium.

In conclusion, the percentage of hydrate present in solution at equilibrium depends

on both electronic and steric effects. Electron donation and bulky substituents

decreasethe percentage of hydrate present at equilibrium, whereas electron withdraw-

al and small substituents increaseit.

CH 3 CH 3

C

O

CH 3

CH 3

HO

C

OH

120 ° 109.5°

CH 3 C CH 3 CH 3 C H H C H

OH

<<

OH

OH

OH

OH

OH

least

stable

CH 3 CH 3

C

O

>>

H

O

H

O

C

CH 3

CC

H

CC

moststable CH 3

CH 3 CH 3

O

H

C

O

H

O

acetone

99.8%

+ H 2 OCHCH 3 C 3 2 × 10 −^3

OH

OH

formaldehyde

0.1%

+ H 2 O HCH 2.3 × 103

OH

OH

acetaldehyde

42%

+ H 2 O CH 3 C H 1.4

OH

OH

Keq

0.2%

99.9%

58%

C

CH 3

C

H

CC

3-D Molecules:

Acetone;

Acetone hydrate