Organic Chemistry

If there is only enough electrophilic reagent to add to one of the double bonds, it
will add preferentially to the more reactive double bond. For example, in the reaction
of 2-methyl-1,5-hexadiene with HCl, addition of HCl to the double bond on the left
forms a secondary carbocation, whereas addition of HCl to the double bond on the
right forms a tertiary carbocation. Because the transition state leading to formation of
a tertiary carbocation is more stable than that leading to a secondary carbocation, the
tertiary carbocation is formed faster (Section 4.4). So in the presence of a limited
amount of HCl, the major product of the reaction will be 5-chloro-5-methyl-1-hexene.

PROBLEM 4

Which of the double bonds in zingiberene (whose structure is given on the first page of this

chapter) is the most reactive in an electrophilic addition reaction?

PROBLEM 5

What stereoisomers are obtained from the two reactions in Section 8.5? (Hint:Review

Section 5.19.)

PROBLEM 6

Give the major product of each of the following reactions, and show the stereoisomers that

would be obtained (equivalent amounts of reagents are used in each case):

a. c.

b. d.

8.6 Electrophilic Addition Reactions

of Conjugated Dienes

If a conjugated diene, such as 1,3-butadiene, reacts with a limited amount of elec-
trophilic reagent so that addition can occur at only one of the double bonds, two addi-
tion products are formed. One is a 1,2-addition product, which is a result of addition at
the 1- and 2-positions. The other is a 1,4-addition product, the result of addition at the
1- and 4-positions.

CH 2 CHCH 2 CH 2 C CH 2

CH 3

HBr

peroxide

HC CCH 2 CH 2 CH CH 2

Cl 2

CH 2 CHCH 2 CH 2 CH CCH 3

CH 3

HCl HBr

CH 3

CH 2 CHCH 2 CH 2 C CH 2 + HCl CH 2 CHCH 2 CH 2 CCH 3

Cl

2-methyl-1,5-hexadiene

1 mol

5-chloro-5-methyl-1-hexene

major product

1 mol

CH 3 CH 3

Section 8.6 Electrophilic Addition Reactions of Conjugated Dienes 305

CH 2 CH CH CH 2 + Cl 2
1,3-butadiene
1 mol
3,4-dichloro-1-butene
1,2-addition product

1,4-dichloro-2-butene

1,4-addition product

1 mol

CH 2 CH CH CH 2 + CH 2 CH CH CH 2

Cl

CH 2 CH CH CH 2 + HBr

1,3-butadiene

1 mol

1 mol

CH 3 CH CH CH 2 + CH 3 CH CH CH 2

Br Br

3-bromo-1-butene

1,2-addition product

1-bromo-2-butene

1,4-addition product

Cl Cl Cl