A Guidebook to Mechanism in Organic Chemistry

Halogenation

The lower reactivity of bromine compared with chlorine is associated,

as often happens, with greater selectivity in the position of attack, so

that the difference in reactivity of tertiary, secondary and primary

hydrogen is considerably more marked in bromination than chlorin-

ation. Direct iodination is not normally practicable, for though I' is

readily formed it is not reactive enough to abstract a hydrogen

atom, the bond energy of the H—I that would be formed being low.

Radical halogenation by reagents other than the halogens them­

selves, e.g. N-bromosuccinimide (XXIII), is of considerable synthetic

importance. TW^eagent will brominate a number of positions but is

especially useful for attack on hydrogen attached to a carbon atom

a- to a double bond (an allylic carbon). Thus with cyclohexene

(XXIV), 3-bromocyclohexene (XXV) is formed:

CH,—CO

| ^>NBr

CH,—CO

(XXIII)

I hv or

1 initiators

CH,—CO

I + I >•

CH,—CO

(XXIV)

CH,—co

I _>

CH,—CO

(XXIII)

>NBr

CH 2 —CO

CH 2 —CO

(XXV)

More recently, however, it has oeen suggested that the function of
the N-bromosuccinjmide, in some cases at any rate, is to act as a
source of a low concentration of molecular bromine which itself
effects the actual bromination. The particularly ready attack on an
allylic, or on the similar, benzylic position, is due to stabilisation of