Organic Chemistry

340 CHAPTER 9 Reactions of Alkanes • Radicals

PROBLEM 1

Show the initiation, propagation, and termination steps for the monochlorination of

cyclohexane.

PROBLEM 2

Write the mechanism for the formation of carbon tetrachloride, from the reaction of

methane with

PROBLEM 3 SOLVED

If cyclopentane reacts with more than one equivalent of at a high temperature, how

many dichlorocyclopentanes would you expect to obtain as products?

SOLUTION Seven dichlorocyclopentanes would be obtained as products. Only one iso-

mer is possible for the 1,1-dichloro compound. The 1,2- and 1,3-dichloro compounds have

two asymmetric carbons. Each has three stereoisomers because the cis isomer is a meso

compound and the trans isomer is a pair of enantiomers.

9.3 Factors that Determine Product Distribution

Two different alkyl halides are obtained from the monochlorination of butane. Substi-

tution of a hydrogen bonded to one of the terminal carbons produces 1-chlorobutane,

whereas substitution of a hydrogen bonded to one of the internal carbons forms

2-chlorobutane.

The expected (statistical) distribution of products is 60% 1-chlorobutane and 40%

2-chlorobutane because six of butane’s 10 hydrogens can be substituted to form

1-chlorobutane, whereas only four can be substituted to form 2-chlorobutane. This as-

sumes, however, that all of the bonds in butane are equally easy to break. Then,

the relative amounts of the two products would depend only on the probability of a

chlorine radical colliding with a primary hydrogen, compared with its colliding with a

secondary hydrogen. When we carry out the reaction in the laboratory and analyze the

product, however, we find that it is 29% 1-chlorobutane and 71% 2-chlorobutane.

Therefore, probability alone does not explain the regioselectivity of the reaction. Be-

cause more 2-chlorobutane is obtained than expected and the rate-determining step of

C¬H

Cl

CH 3 CH 2 CH 2 CH 3 + Cl 2 CH 3 CH 2 CH 2 CH 2 Cl + CH 3 CH 2 CHCH 3

2-chlorobutane

expected = 40%

experimental = 71%

1-chlorobutane

expected = 60%

experimental = 29%

butane

+ HCl

h

Cl

Cl

Cl

H

Cl

H

Cl

H

H

Cl

Cl

H

H

Cl

Cl

H

Cl

H

Cl

H

H

Cl

Cl

H

H

Cl

meso compound enantiomers

meso compound enantiomers

Cl 2

Cl 2 +hv.

CCl 4 ,