Organic Chemistry

736 CHAPTER 18 Carbonyl Compounds II

3-D Molecules:

Formaldehyde;

Acetaldehyde;

Acetone

3-D Molecules:

3-Methyl-2-butanone;

2,4-Dimethyl-3-pentanone

Steric factors also contribute to the greater reactivity of an aldehyde. The carbonyl car-

bon of an aldehyde is more accessible to the nucleophile than is the carbonyl carbon of

a ketone because the hydrogen attached to the carbonyl carbon of an aldehyde is

smaller than the alkyl group attached to the carbonyl carbon of a ketone. Steric factors

also become important in the tetrahedral transition state because the bond angles are

109.5°; therefore, the alkyl groups are closer to one another than they are in the car-

bonyl compound, in which the bond angles are 120°. Ketones have greater steric

crowding in their transition states, so they have less stable transition states than alde-

hydes have.

For the same reason, ketones with small alkyl groups bonded to the carbonyl carbon

are more reactive than ketones with large alkyl groups.

PROBLEM 4

Which ketone is more reactive?

a. 2-heptanone or 4-heptanone

b. p-nitroacetophenone or p-methoxyacetophenone

How does the reactivity of an aldehyde or a ketone toward nucleophiles com-

pare with the reactivity of the carbonyl compounds whose reactions you studied in

Chapter 17? Aldehydes and ketones are right in the middle—less reactive than

acyl halides and acid anhydrides, but more reactive than esters, carboxylic acids,

and amides.

The carbonyl compounds discussed in Chapter 17 have a lone pair on an atom at-

tached to the carbonyl group that can be shared with the carbonyl carbon by reso-

nance, which makes the carbonyl carbon less electron deficient. We have seen that the

reactivity of these carbonyl compounds is related to the basicity of (Section 17.5).

The weaker the basicity of the more reactive is the carbonyl group because weak

bases are less able to donate electrons by resonance to the carbonyl carbon and are bet-

ter able to withdraw electrons inductively from the carbonyl carbon.

C

O

R

C

Y R

O

Y+

−

Y-,

Y-

relative reactivities

C

O

C

O

C

O

most reactive >>least reactive

H 3 CHCH 3 3 CCH 3 CH

CH 3 CH 3

CHCH 3

CH 3

CHCH 3

relative reactivities

>>

HH RR'

formaldehyde an aldehyde a ketone

C

O

RH

C

O

C

O

most reactive least reactive

Aldehydes and ketones are less reactive
than acyl chlorides and acid anhydrides,
but are more reactive than esters and
amides.

relative reactivities of carbonyl compounds toward nucleophiles

most reactive least reactive

acyl halide > acid anhydride > aldehyde > ketone > ester ∼ carboxylic acid > amide > carboxylate ion