Organic Chemistry

820 CHAPTER 19 Carbonyl Compounds III

O

CH 3 CH 2 CH 2 CH 2 COH + CO 2 + AgBr

1. Ag 2 O


2. Br 2 , ∆ CH^3 CH^2 CH^2 CH^2 Br
   pentanoic acid 1-bromobutane

THE HUNSDIECKER REACTION

Heinz and Clare Hunsdiecker found that a car-

boxylic acid can be decarboxylated if a heavy

metal salt of the carboxylic acid is heated with bromine or io-

dine. The product is an alkyl halide with one less carbon atom

than the starting carboxylic acid. The heavy metal can be silver

ion, mercuric ion, or lead(IV). The reaction is now known as

the Hunsdiecker reaction.

The reaction involves formation of a hypobromite as a result of

precipitation of AgBr. A radical reaction is initiated by ho-

molytic cleavage of the bond of the hypobromite. The

carboxyl radical loses and the alkyl radical thus formed

abstracts a bromine radical from the hypobromite to propagate

the reaction.

a hypobromite

O

RCO−Ag+ + Br 2 Br + AgBr

O

RCO

CO 2 ,

O¬Br

O

RCO

O

RCO

O

Br RCO + Br

O

R CO R+ CO 2

O

RCO Br + R RBr +

initiation step

propagation step

propagation step

In summary, carboxylic acids with a carbonyl group at the 3-position (both

acids and acids) lose when they are heated.

OO

CH 3 CH 2 CH 2 CCH 2 COH + CO 2

+ CO 2

∆

∆

O

CH 3 CH 2 CH 2 CCH 3

OO

CH 3

HOCCHCOH + CO 2

∆

O

CH 3 CH 2 COH

O

COOH

O

3-oxohexanoic acid

2-oxocyclohexane-

carboxylic acid

2-pentanone

-methylmalonic acid

propionic acid

cyclohexanone

b-ketocarboxylic b-dicarboxylic CO 2

PROBLEM 34

Which of the following compounds would be expected to decarboxylate when heated?

a. c.

b. d.

HO

OO OO

O

OH

O

HO

OO

OH

Heinz (1904–1981) andClare
(1903–1995) Hunsdieckerwere both
born in Germany, Heinz in Cologne
and Clare in Kiel. They both received
Ph.D.s from the University of
Cologne and spent their careers
working in a private laboratory in
Cologne.