Organic Chemistry

and can also be dehydrated under basic

conditions, so heating the aldol addition product in either acid or base leads to dehy-

dration. The product of dehydration is called an enone—“ene”for the double bond and

“one”for the carbonyl group.

Dehydration sometimes occurs under the conditions in which the aldol addition is

carried out, without additional heating. In such cases, the com-

pound is an intermediate and the enone is the final product of the reaction. For exam-

ple, the formed from the aldol addition of acetophenone loses water

as soon as it is formed, because the double bond formed by loss of water is conjugated

not only with the carbonyl group, but also with the benzene ring. Conjugation stabi-

lizes the dehydrated product and therefore makes it relatively easy to form.

PROBLEM 20 SOLVED

How could you prepare the following compounds using a starting material containing no

more than three carbon atoms?

a. b. c.

SOLUTION TO 20a A compound with the correct four-carbon skeleton can be obtained

if a two-carbon aldehyde undergoes an aldol addition. Dehydration of the addition product

forms an aldehyde. Catalytic hydrogenation forms an aldehyde. Some of

the aldehyde might be reduced to an alcohol, but that’s all right because

both the aldehyde and the alcohol can be oxidized by an acidic solution of CrO 3 to the tar-

get compound (Section 20.2).

19.13 The Mixed Aldol Addition

If two different carbonyl compounds are used in an aldol addition, four products can

be formed because each enolate can react both with another molecule of the carbonyl

compound from which the enolate was formed and with the other carbonyl compound.

In the following example, both carbonyl compound A and carbonyl compound B can

H 2 Pt

O

CH 3 CH

HO−

major minor

OH O

CH 3 CHCH 2 CH

O

CH 3 CH

O

CH 3 CH 2 CH 2 COH

CrO 3

H 2 SO 4

O

CH 3 CH 2 CH 2 CH CH 3 CH 2 CH 2 CH 2 OH

CHCH

H 3 O+or HO−

∆

+

a,b-unsaturated

a,b-unsaturated

CH 3 CH 2 CH 2 CCH 3

O

CH 3 CH 2 CHCHCH O

CH 3

Br

CH 3 CH 2 CH 2 COH

O

b-hydroxyketone

b-hydroxycarbonyl

OOHO

HO−

2 CH 3 CCH 3 CH 3 CCH 2 CCH 3

a -hydroxyketone -unsaturated ketone

an enone

an ,

HO−

CH 3

O

CH 3 C CHCCH 3 H 2 O

CH 3

∆ +

b-Hydroxyaldehydes b-hydroxyketones

808 CHAPTER 19 Carbonyl Compounds III

O

2CHC 3

OH O

C CH 2 C

CH 3 CH 3

H 2 O

O

C CH C

HO−

+