Organic Chemistry

342 CHAPTER 9 Reactions of Alkanes • Radicals

The percent yield of each alkyl halide is calculated by dividing the relative amount

of the particular product by the sum of the relative amounts of all the alkyl halide

products

Radical monochlorination of 2,2,5-trimethylhexane results in the formation of five

monochlorination products. Because the relative amounts of the five alkyl halides total

the percent yield of each product can be

calculated as follows:

Because radical chlorination of an alkane can yield several different monosubstitu-

tion products as well as products that contain more than one chlorine atom, it is not the

best method for synthesizing an alkyl halide. Addition of a hydrogen halide to an alkene

(Section 4.1) or conversion of an alcohol to an alkyl halide (a reaction we will study in

Chapter 12) is a much better way to make an alkyl halide. Radical halogenation of an

alkane is nevertheless still a useful reaction because it is the only way to convert an inert

alkane into a reactive compound. In Chapter 10, we will see that once the halogen is in-

troduced into the alkane, it can be replaced by a variety of other substituents.

PROBLEM 4

When 2-methylpropane is monochlorinated in the presence of light at room temperature,

36% of the product is 2-chloro-2-methylpropane and 64% is 1-chloro-2-methylpropane.

From these data, calculate how much easier it is to abstract a hydrogen atom from a tertiary

carbon than from a primary carbon under these conditions.

PROBLEM 5

How many alkyl halides can be obtained from monochlorination of the following alkanes?

Neglect stereoisomers.

a. d. g.

b. e. h.

c. f. i.

CH 3 CH 3

CH 3

CH 3 CCH 2 CHCH 3

CH 3

CH 3

CH 3

CH 3 CHCH 2 CH 2 CH 3

CH 3 CH 3

CH 3 CH 3

CH 3 C CCH 3

CH 3

CH 3 CH 3

CH 3 CHCH 2 CH 2 CHCH 3

CH 3 CH 3

CH 3 CH 3

CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CCH 2 CCH 3

35 1 9.0+7.6+7.6+5.0+6.0= 352 ,

16 + 15 = 212.

9 × 1.0 = 9.0

9.0

35

= 26%

2 × 3.8 = 7.6

7.6

35

= 22%

CH 3 CH 3

CH 3 CH 3 CH 2 Cl

CH 3 Cl

CH 3 CH 3 CH 3

CH 3 CCH 2 CH 2 CHCH 3 + Cl 2 CH 3 CCH 2 CH 2 CHCH 3 ++CH 3 C CHCH 2 CHCH 3

2 × 3.8 = 7.6

7.6

35

= 22%

CH 3 Cl

CH 3 CH 3

1 × 5.0 = 5.0

5.0

35

= 14%

6 × 1.0 = 6.0

6.0

35

= 17%

CH 3 Cl

CH 3 CH 3

CH 3

CH 3 CH 3

CH 3 CCH 2 CHCHCH 3 ++CH 3 CCH 2 CH 2 CCH 3 CH 3 CCH 2 CH 2 CHCH 2 Cl

2,2,5-trimethylhexane

∆

+ HCl