Organic Chemistry

Section 8.6 Electrophilic Addition Reactions of Conjugated Dienes 307

Let’s review by comparing addition to an isolated diene with addition to a conjugated
diene. The carbocation formed by addition of an electrophile to an isolated diene is not
stabilized by resonance. The positive charge is localized on a single carbon, so only di-
rect (1,2) addition occurs.

The carbocation formed by addition of an electrophile to a conjugated diene, in contrast,
is stabilized by resonance. The positive charge is shared by two carbons, and as a result,
both direct (1,2-) and conjugate (1,4-) addition occur.

If the conjugated diene is not symmetrical, the major products of the reaction are
those obtained by adding the electrophile to whichever terminal carbon results in for-
mation of the more stable carbocation. For example, in the reaction of 2-methyl-1,3-
butadiene with HBr, the proton adds preferentially to C-1 because the positive charge on
the resulting carbocation is shared by a tertiary allylic and a primary allylic carbon.
Adding the proton to C-4 would form a carbocation with the positive charge shared by a
secondary allylic and a primary allylic carbon. Because addition to C-1 forms the more
stable carbocation, 3-bromo-3-methyl-1-butene and 1-bromo-3-methyl-2-butene are the
major products of the reaction.

CH 2 CH CH 2

2-methyl-1,3-butadiene

CH 3

CH 3 C CH 2 CH 3

3-bromo-3-methyl-

1-butene

1-bromo-3-methyl-

2-butene

CH 3 CH 3

Br Br

++HBr C CH CH 2

14

CH 3 CCHCH 2

carbocation formed by adding H+ to C-1

CH 3

CH 3 C CH

CH 3

++CH^2

CH 2 C CHCH 3

carbocation formed by adding H+ to C-4

CH 3

CH 2 C

CH 3

++CHCH^3

C CH

sp^2

Br

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

HBr

+ Br−

+

CH 2

1,5-hexadiene

5-bromo-1-hexene

addition to an isolated diene

addition of

the electrophile

addition of

the nucleophile

HBr + +

+Br–

+Br–

2,4-hexadiene

2-bromo-3-hexene

1,4-addition product

Br

4-bromo-2-hexene

1,2-addition product

addition to a conjugated diene

CH 3 CH CH CH CHCH 3

CH 3 CH 2 CH CH CHCH 3

Br

CH 3 CH 2 CH CH CHCH 3

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

the carbocation is

stabilized by electron

delocalization

addition of

the electrophile addition of the nucleophile

An isolated diene undergoes

only 1,2-addition.

A conjugated diene

undergoes both

1,2- and 1,4-addition.