Organic Chemistry

306 CHAPTER 8 Reactions of Dienes • Ultraviolet and Visible Spectroscopy

Addition at the 1- and 2-positions is called 1,2-additionor direct addition. Addition

at the 1- and 4-positions is called 1,4-additionor conjugate addition. On the basis of

our knowledge of how electrophilic reagents add to double bonds, we expect the

1,2-addition product to form. That the 1,4-addition product also forms may be surpris-

ing because not only did the reagent not add to adjacent carbons, but a double bond

has changed its position. The double bond in the 1,4-product is between the 2- and

3-positions, while the reactant had a single bond in this position.

When we talk about addition at the 1- and 2-positions or at the 1- and 4-positions,

the numbers refer to the four carbons of the conjugated system. Thus, the carbon in the

1-position is one of the carbons at the end of the conjugated system—it is not nec-

essarily the first carbon in the molecule.

To understand why both 1,2-addition and 1,4-addition products are obtained from

the reaction of a conjugated diene with a limited amount of electrophilic reagent, we

must look at the mechanism of the reaction. In the first step of the addition of HBr to

1,3-butadiene, the electrophilic proton adds to C-1, forming an allylic cation. (Recall

that an allylic cation has a positive charge on a carbon that is next to a double-bonded

carbon.) The electrons of the allylic cation are delocalized—the positive charge is

shared by two carbons. Notice that because 1,3-butadiene is symmetrical, adding to

C-1 is the same as adding to C-4. The proton does not add to C-2 or C-3 because doing

so would form a primary carbocation. The electrons of a primary carbocation are lo-

calized; thus, it is not as stable as the delocalized allylic cation.

The contributing resonance structures of the allylic cation show that the positive charge

on the carbocation is not localized on C-2, but is shared by C-2 and C-4. Consequently,

in the second step of the reaction, the bromide ion can attack either C-2 (direct addition)

or C-4 (conjugate addition) to form the 1,2-addition product or the 1,4-addition product,

respectively.

As we look at more examples, notice that the first step in all electrophilic additions to

conjugated dienes is addition of the electrophile to one of the carbons at the end of

the conjugated system. This is the only way to obtain a carbocation that is stabilized by

resonance (i.e., by electron delocalization). If the electrophile were to add to one of the

internal sp^2 carbons, the resulting carbocation would not be stabilized by resonance.

sp^2

p

p

sp^2

Br 2

CH 3 CH CH CH CHCH 3 CH 3 CH CH CH CHCH 3 + CH 3 CH CH CH CHCH 3

2,4-hexadiene

RCHCHCHCH

4,5-dibromo-2-hexene

1,2-addition product

2,5-dibromo-3-hexene

1,4-addition product

Br

R

1234

the conjugated system

Br Br Br

an allylic cation

a primary

carbocation

CH 2 CH CH CH 2 + H CH CH 2

+

+

+

+

Br

CH 2 CH 2 CH CH 2 CH 3 CH CH CH 2

Br

1,3-butadiene

3-bromo-1-butene

1,2-addition product

1-bromo-2-butene

1,4-addition product

+

mechanism for the reaction of 1,3-butadiene with HBr

CH 3 CH CH CH 2

Br

Br + Br

CH 3 CH CH CH 2 CH 3 CH

––

CH

δ+δ+

CH 3 CH CH 2