Organic Chemistry

Section 19.4 Halogenation of the -Carbon of Aldehydes and Ketonesa 795

PROBLEM 6

Explain why the aldehydic hydrogen (the one attached to the carbonyl carbon) is not

exchanged with deuterium.

19.4 Halogenation of the -Carbon

of Aldehydes and Ketones

Acid-Catalyzed Halogenation
When or is added to an acidicsolution of an aldehyde or a ketone, a halogen
replaces oneof the of the carbonyl compound.

In the first step of this acid-catalyzed reaction, the carbonyl oxygen is protonated.
Water is the base that removes a proton from the forming an enol that reacts
with an electrophilic halogen.

Base-Promoted Halogenation
When excess or is added to a basicsolution of an aldehyde or a ketone,
the halogen replaces allthe

RBrCH 2 + 2

O

RRC2 BrC + −

O

HO−

C R

Br

Br

excess

a-hydrogens.

Br 2 ,Cl 2 , I 2

a-carbon,

+ HCl

H 3 O+

H 3 O+

O

+ Cl 2

O

+ HI

O

CCHCH 3

+ I 2

O

CCH 2 CH 3

Cl

I

a-hydrogens

Br 2 ,Cl 2 , I 2

A

CH 3 CH

O

CD 3 CH

O

−OD

D 2 O

Under acidic conditions, one

is substituted for a halogen.

A-hydrogen

O

CH 3 C

+ H 3 O+

+ H 3 O+

OH

+

CH 2 C

+OH

CH 2 C

H Br Br Br + Br−

OH

CH 2 C

H 2 O

H 2 O

O

CH 2 C

Br

HO

H

+ H

acid-catalyzed halogenation

3-D Molecule:

a-Bromoacetone

Under basic conditions, all the

are substituted for

halogens.

A-hydrogens