Organic Chemistry

344 CHAPTER 9 Reactions of Alkanes • Radicals

2 kcal

2 kcal

2 kcal

2 kcal

alkane

1 °

2 °

3 °

1 °

2 °

3 °

+Cl

alkane

+Br

Progress of the reaction

Potential energy

a.

Progress of the reaction

Potential energy

b.

less than 1-kcal

difference

in Ea

chlorination is

exothermic;

transition states

resemble reactants

about 1.5-kcal

difference

in Ea

bromination is endothermic;

transition states resemble products

Why are the relative rates of radical formation so different when a bromine radical

rather than a chlorine radical is used as the hydrogen-abstracting reagent? To answer

this question, we must compare the values for the formation of primary, sec-

ondary, and tertiary radicals when a chlorine radical is used, as opposed to when a

bromine radical is used. These values can be calculated using the bond dissocia-

tion energies in Table 3.1 on p. 129. (Remember that is equal to the energy of the

bond being broken minus the energy of the bond being formed.)

We must also be aware that bromination is a much slower reaction than chlorina-

tion. The activation energy for abstraction of a hydrogen atom by a bromine radical

has been found experimentally to be about 4.5 times greater than that for abstraction of

a hydrogen atom by a chlorine radical. Using the calculated values and the ex-

perimental activation energies, we can draw reaction coordinate diagrams for the for-

mation of primary, secondary, and tertiary radicals by chlorine radical abstraction

(Figure 9.1a) and by bromine radical abstraction (Figure 9.1b).

Because the reaction of a chlorine radical with an alkane to form a primary, sec-

ondary, or tertiary radical is exothermic, the transition states resemble the reactants

more than they resemble the products (see the Hammond postulate, Section 4.3). The

¢H°

¢H°

¢H°

¢H°

Cl CH 3 CH 2 CH 3

CH 3 CH 2 CH 3

CH 3 CHCH 3

Cl

+

CH 3

HCl

HCl

HCl

101

99

97

−2

−4

−6

− 103 =

− 103 =

− 103 =

+

++

Cl + CH 3 CCH 3 +

CH 3 CHCH 3

CH 3 CH 2 CH 2

∆H° (kcal/mol)

CH 3

Br CH 3 CH 2 CH 3

CH 3 CH 2 CH 3

CH 3 CHCH 3

Br

+

CH 3

HBr

HBr

HBr

101

99

97

14

12

10

− 87 =

− 87 =

− 87 =

+

++

Br + CH 3 CCH 3 +

CH 3 CHCH 3

CH 3 CH 2 CH 2

∆H° (kcal/mol)

−8

−17

−25

∆H° (kJ/mol)

59

50

42

∆H° (kJ/mol)

CH 3

Figure 9.1

(a) Reaction coordinate diagrams for the formation of primary, secondary, and tertiary alkyl radicals as a result of

abstraction of a hydrogen atom by a chlorine radical. The transition states have relatively little radical character because

they resemble the reactants.

(b) Reaction coordinate diagrams for the formation of primary, secondary, and tertiary alkyl radicals as a result of

abstraction of a hydrogen atom by a bromine radical. The transition states have a relatively high degree of radical

character because they resemble the products.