Organic Chemistry

Section 4.7 Addition of Halogens 157

d. This carbocation will not rearrange because it is a secondary carbocation and a carbo-

cation rearrangement would yield another secondary carbocation.

e. This carbocation will rearrange because a 1,2-hydride shift will convert a secondary

carbocation into a tertiary carbocation.

PROBLEM 14

Give the major product(s) obtained from the reaction of each of the following with HBr:

a. d.

b. e.

c. f.

4.7 Addition of Halogens

The halogens and add to alkenes. This may be surprising because it is not im-
mediately apparent that an electrophile—which is necessary to start an electrophilic
addition reaction—is present.

However, the bond joining the two halogen atoms is relatively weak (see the bond dis-
sociation energies listed in Table 3.1) and, therefore, easily broken. When the elec-
trons of the alkene approach a molecule of or one of the halogen atoms
accepts the electrons and releases the shared electrons to the other halogen atom.
Therefore, in an electrophilic addition reaction, behaves as if it were and
Br-,and Cl 2 behaves as if it were Cl+and Cl-.

Br 2 Br+

Br 2 Cl 2 ,

p

CH 3 CH CH 2 Br 2

Br Br

Cl Cl

+

CH 3 CH CH 2 Cl 2

CH 3 CH CH 2

+ CH 3 CH CH 2

Br 2 Cl 2

CH 3 CHCH 2 CH CH 2 CH 3

CH 3

CH 2 CHCCH 3

CH 3

CH 3

CH 2

CH 3 CHCH CH 2 CH 3

CH 3

CH 3 C

CH 3

H

+

CHCH 3 CH 3 CCH 2 CH 3

CH 3

+

In any reaction that forms a carbocation

intermediate, always check to see if the

carbocation will rearrange.

a bromonium ion

1,2-dibromoethane

a vicinal dibromide

H 2 C CH 2

Br

+ −

Br

Br Br

Br

H 2 C CH 2

+

Br CH 2 CH 2