Organic Chemistry

PROBLEM 10

a. When HBr adds to a conjugated diene, what is the rate-determining step?

b. When HBr adds to a conjugated diene, what is the product-determining step?

Because the greater proximity of the nucleophile to C-2 contributes to the faster

rate of formation of the 1,2-addition product, the 1,2-addition product is the kinetic

product for essentially all conjugated dienes. Do notassume, however, that the

1,4-addition product is alwaysthe thermodynamic product. The structure of the con-

jugated diene is what ultimately determines the thermodynamic product. For exam-

ple, the 1,2-addition product is both the kinetic product and the thermodynamic

product in the reaction of 4-methyl-1,3-pentadiene with HBr, because not only is the

1,2-product formed faster, it is more stable than the 1,4-product.

The 1,2- and 1,4-addition products obtained from the reaction of 2,4-hexadiene with

HCl have the same stability—both have the same number of alkyl groups bonded to

their carbons. Thus, neither product is themodynamically controlled.

PROBLEM 11 SOLVED

For each of the following reactions, (1) give the major 1,2- and 1,4-addition products and

(2) indicate which is the kinetic product and which is the thermodynamic product:

a. c.

b. d.

SOLUTION TO 11a First we need to determine which of the terminal carbons of

the conjugated system is going to be the C-1 carbon. The proton will be more apt to add

to the indicated carbon because the carbocation that is formed shares its positive

charge with a tertiary allylic and a secondary allylic carbon. If the proton were to add to the

carbon at the other end of the conjugated system, the carbocation that would be formed

would be less stable because its positive charge would be shared by a primary allylic and a

secondary allylic carbon. Therefore, 3-chloro-3-methylcyclohexene is the 1,2-addition

product and 3-chloro-1-methylcyclohexene is the 1,4-addition product. 3-Chloro-3-

methylcyclohexene is the kinetic product because of the chloride ion’s proximity to C-2,

and 3-chloro-1-methylcyclohexene is the thermodynamic product because its more highly

substituted double bond makes it more stable.

sp^2

sp^2

sp^2

+ HCl

CH 3 CH CHC CH 2 CH CHCH 3

CH 3

+ HCl

+ HCl

CH 3

+ HCl

CH 2

sp^2

312 CHAPTER 8 Reactions of Dienes • Ultraviolet and Visible Spectroscopy

CH 2 CHCH CCH 3

4-methyl-1,3-pentadiene

CH 3

CH 3 CHCH CCH 3

4-bromo-2-methyl-2-pentene

1,2-addition product

kinetic product

thermodynamic product

4-bromo-4-methyl-2-pentene

1,4-addition product

CH 3 CH 3

Br

CH 3 CH CHCCH 3

Br

++HBr

CH 3 CH CHCH CHCH 3 HCl CH 3 CH 2 CHCH CHCH 3 CH 3 CH 2 CH CHCHCH 3

2,4-hexadiene

Cl Cl

2-chloro-3-hexene

1,4-addition product

4-chloro-2-hexene

1,2-addition product

+

the products have the same stability