Organic Chemistry

Section 8.7 Thermodynamic Versus Kinetic Control of Reactions 311

chemists thought it was because the transition state for formation of the 1,2-addition
product resembles the contributing resonance structure in which the positive charge is
on a secondary allylic carbon. In contrast, the transition state for formation of the
1,4-addition product resembles the contributing resonance structure in which the pos-
itive charge is on a less stable primary allylic carbon.

However, when the reaction of is carried out under kinetic con-
trol, essentially the same relative amounts of 1,2- and 1,4-addition products are obtained
as are obtained from the kinetically controlled reaction of The
transition states for formation of the 1,2- and 1,4-addition products from 1,3-pentadiene
should both have the same stability because both resemble a contributing resonance
structure in which the positive charge is on a secondary allylic carbon. Why, then, is the
1,2-addition product still formed faster?

When the electrons of the diene abstract from a molecule of undissociated
DCl, the chloride ion can better stabilize the positive charge at C-2 than at C-4 simply
because when the chloride ion is first produced, it is closer to C-2 than to C-4. So it is
a proximity effectthat causes the 1,2-addition product to be formed faster. A proximity
effectis an effect caused by one species being close to another.

PROBLEM 9

a. Why does deuterium add to C-1 rather than to C-4 in the preceding reaction?

b. Why was DCl rather than HCl used in the reaction?

+

−

CH CHCH 3 CH 2 CH CH CHCH 3

D

CH

Cl

CH 2

D Cl−

+

secondary allylic cation

Cl– is closer to

C-2 than to C-4

p D+

1,3-butadiene+HBr.

1,3-pentadiene+DCl

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

HBr + +

+

CH 3 CHCH CH 2

+Br– Br–

Br

transition state for

formation of the

1,2-addition product

δ−

δ+

CH 3 CH CHCH 2

Br

transition state for

formation of the

1,4-addition product

δ−

‡‡δ+

secondary allylic carbon primary allylic carbon

CH 2 CHCH CHCH 3 + DCl CH 2 CHCH CHCH 3 CH 2 CH CHCHCH 3

D Cl D Cl

+

1,2-addition product

78%

1,3 pentadiene

− 78 °C

1,4-addition product

22%