Organic Chemistry

Cl 2

F 2

Br 2

I 2

Halogens

Section 9.4 The Reactivity–Selectivity Principle 345

The more reactive a species is, the less

selective it will be.

reactants all have approximately the same energy, so there will be only a small differ-
ence in the activation energies for removal of a hydrogen atom from a primary, sec-
ondary, or tertiary carbon. In contrast, the reaction of a bromine radical with an alkane
is endothermic, so the transition states resemble the products more than they resemble
the reactants. Because there is a significant difference in the energies of the product
radicals—depending on whether they are primary, secondary, or tertiary—there is a
significant difference in the activation energies. Therefore, a chlorine radical makes
primary, secondary, and tertiary radicals with almost equal ease, whereas a bromine
radical has a clear preference for formation of the easiest-to-form tertiary radical
(Figure 9.1). In other words, because a bromine radical is relatively unreactive, it is
highly selective about which hydrogen atom it abstracts. In contrast, the much more
reactive chlorine radical is considerably less selective. These observations illustrate
the reactivity–selectivity principle, which states that the greater the reactivity of a
species, the less selective it will be.
Because chlorination is relatively nonselective, it is a useful reaction only when
there is just one kind of hydrogen in the molecule.

PROBLEM 8

If 2-methylpropane is brominated at 125°C in the presence of light, what percent of the

product will be 2-bromo-2-methylpropane? Compare your answer with the percent given

in Problem 4 for chlorination.

PROBLEM 9

Take the same alkanes whose percentages of monochlorination products you calculated in

Problem 6, and calculate what the percentages of monobromination products would be if

bromination were carried out at 125°C.

By comparing the values for the sum of the two propagating steps for the
monohalogenation of methane, we can understand why alkanes undergo chlorination
and bromination but not iodination and why fluorination is too violent a reaction to
be useful.

Br

I

CH 4

Br 2 CH 3 Br

+ HBr

Br

105

46

18

−24

− 87 =

− 70 =

+

++

CH 3

CH 3

∆H° = −6 kcal/mol

CH 4

I 2 CH 3 I

+ HI

I

105

36

34

−21

− 71 =

− 57 =

+

++

CH 3

CH 3

∆H° = 13 kcal/mol

F CH 4

F 2 CH 3 F

+ HF

F

105

38

−31

−70

(or −423 kJ/mol)

(or −100 kJ/mol)

(or −25 kJ/mol)

(or 54 kJ/mol)

− 136 =

− 108 =

+

++

CH 3

CH 3

∆H° = −101 kcal/mol

Cl CH 4

Cl 2 CH 3 Cl

+ HCl

Cl

105

58

2

−26

− 103 =

− 84 =

+

++

CH 3

CH 3

∆H° = − 24 kcal/mol

¢H°

+ Cl 2 + HCl

h

Cl

Tutorial:

Reactivity–selectivity