Organic Chemistry

Section 19.12 Dehydration of Aldol Addition Products: Formation of a,b-Unsaturated Aldehydes and Ketones 807

Ketones are less susceptible than aldehydes to attack by nucleophiles (Section 18.2),
so aldol additions occur more slowly with ketones. The relatively high reactivity of
aldehydes in competing aldol addition reactions is what causes them to give low yields
of products (Section 19.8).

Because an aldol addition reaction occurs between two molecules of the same
carbonyl compound, the product has twice as many carbons as the reacting aldehyde
or ketone.

PROBLEM 17

Show the aldol addition product that would be formed from each of the following compounds:

PROBLEM 18

For each of the following compounds, indicate the aldehyde or ketone from which it would

be formed by an aldol addition:

a. 2-ethyl-3-hydroxyhexanal c. 2,4-dicyclohexyl-3-hydroxybutanal

b. 4-hydroxy-4-methyl-2-pentanone d. 5-ethyl-5-hydroxy-4-methyl-3-heptanone

PROBLEM 19

An aldol addition can be catalyzed by acids as well as by bases. Propose a mechanism for

the acid-catalyzed aldol addition of propanal.

19.12 Dehydration of Aldol Addition Products:

Formation of -Unsaturated

Aldehydes and Ketones

You have seen that alcohols are dehydrated when they are heated with acid (Sec-
tion 12.5). The and products of aldol addition
reactions are easier to dehydrate than many other alcohols because the double bond
formed as the result of dehydration is conjugated with a carbonyl group. Conjugation
increases the stability of the product (Section 8.3) and, therefore, makes it easier to
form. If the product of an aldol addition is dehydrated, the overall reaction is called an
aldol condensation. A condensation reactionis a reaction that combines two mole-
cules while removing a small molecule (usually water or an alcohol).

OOHO

HO−

2 CH 3 CH 2 CH CH 3 CH 2 CH CHCH

a -hydroxyaldehyde an , -unsaturated aldehyde

H 3 O+

CH 3

O

CH 3 CH 2 CH CCH H 2 O

CH 3

∆ +

b-hydroxyaldehyde b-hydroxyketone

A,B

d.

O

b. CH 3 CHCH 2 CH 2 CH

CH 3

O

c. CH 3 CH 2 CCH 2 CH 3

O

a. CH 3 CH 2 CH 2 CH 2 CH

O

a-alkylation

a -hydroxyketone

OO O− O

HO− CH 3 CCH 3

O

CH 3 CCH 3 CH 2 CCH 3

CH 3

CH 3 CCH 2 CCH 3

OH O

CH 3

CH 3 CCH 2 CCH 3

− HO−

H 2 O

3-D Molecule:

2-Methyl-2-pentenal

(an -unsaturated aldehyde)a,b

An aldol addition product loses water to

form an aldol condensation product.