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Also known as: Radical Chain Halogenation
Substitution of a C-H bond by C-X through a radical chain mechanism. Bromination is highly selective for tertiary C-H bonds (1600:80:1 for 3°:2°:1°), while chlorination is less selective (5:4:1).
Initiation: X₂ homolyzes under UV light or heat to give two halogen radicals (X•).
Homolytic cleavage — fishhook arrows
Propagation 1: X• abstracts a hydrogen from the alkane (preferentially from 3° > 2° > 1° C-H), forming HX and a carbon radical.
Selectivity determined here
Propagation 2: R• reacts with X₂ to form R-X product and regenerate X• for the chain to continue.
Chain continues until termination
Isobutane
Br₂, hν
tert-Butyl bromide (major)
Toluene
NBS, hν
Benzyl bromide
Simple method to functionalize unactivated C-H bonds. Bromination with Br₂ or NBS gives useful selectivity.