Organic Chemistry

PROBLEM 6

Calculate the percent yield of each product obtained in Problems 5a, b, and c if chlorina-

tion is carried out in the presence of light at room temperature.

9.4 The Reactivity–Selectivity Principle

The relative rates of radical formation when a bromine radical abstracts a hydrogen
atom are different from the relative rates of radical formation when a chlorine radi-
cal abstracts a hydrogen atom. At 125°C, a bromine radical abstracts a hydrogen
atom from a tertiary carbon 1600 times faster than from a primary carbon and ab-
stracts a hydrogen atom from a secondary carbon 82 times faster than from a prima-
ry carbon.

When a bromine radical is the hydrogen-abstracting agent, the differences in reac-
tivity are so great that the reactivity factor is vastly more important than the probabili-
ty factor. For example, radical bromination of butane gives a 98% yield of
2-bromobutane, compared with the 71% yield of 2-chlorobutane obtained when bu-
tane is chlorinated (Section 9.3). In other words, bromination is more highly regiose-
lective than chlorination.

Similarly, bromination of 2,2,5-trimethylhexane gives an 82% yield of the product in
which bromine replaces the tertiary hydrogen. Chlorination of the same alkane results
in a 14% yield of the tertiary alkyl chloride (Section 9.3).

PROBLEM 7

Carry out the calculations that predict that

a. 2-bromobutane will be obtained in 98% yield.

b. 2-bromo-2,5,5,-trimethylhexane will be obtained in 82% yield.

Br

CH 3 CCH 2 CH 2 CHCH 3 + Br 2 CH 3 CCH 2 CH 2 CCH 3 + HBr

2-bromo-2,5,5-trimethylhexane

82%

2,2,5-trimethylhexane

CH 3

CH 3

CH 3 CH 3

CH 3

h

CH 3

Br

CH 3 CH 2 CH 2 CH 3 + Br 2 CH 3 CH 2 CH 2 CH 2 Br + CH 3 CH 2 CHCH 3

2-bromobutane

98%

1-bromobutane

2%

+ HBr

h

tertiary >

1600

secondary >

82

primary

1

relative rates of radical formation by a bromine radical at 125 °C

increasing rate of formation

Section 9.4 The Reactivity–Selectivity Principle 343

A bromine radical is less reactive and

more selective than a chlorine radical.

Tutorial:

Radical bromination