Section 9.5 Radical Substitution of Benzylic and Allylic Hydrogens 347N-Bromosuccinimide (NBS) is frequently used to brominate allylic positions be-
cause it allows a radical substitution reaction to be carried out without subjecting the
reactant to a relatively high concentration of that could add to its double bond.
The reaction involves initial homolytic cleavage of the bond in NBS. This
generates the bromine radical needed to initiate the radical reaction. Light or heat
and a radical initiator such as a peroxide are used to promote the reaction. The bro-
mine radical abstracts an allylic hydrogen to form HBr and an allylic radical. The
allylic radical reacts with forming the allyl bromide and a chain-propagating
bromine radical.
The used in the second step of the preceding reaction sequence is produced in low
concentration from the reaction of NBS with HBr.
The advantage to using NBS to brominate allylic positions is that neither or HBr is
formed at a high enough concentration to add to the double bond.
When a radical abstracts a hydrogen atom from an allylic carbon, the unpaired elec-
tron of the allylic radical is shared by two carbons. In other words, the allylic radical
has two resonance contributors. In the following reaction, only one substitution prod-
uct is formed, because the groups attached to the carbons are the same in both res-
onance contributors:
If, however, the groups attached to the two carbons of the allylic radical are notthe
same in both resonance contributors, two substitution products are formed:
CH 3 CHCH CH 2 CH 3 CH + HBrBr 2Br + CH 3 CH 2 CH CH 2 CHCH 2CH 3 CH CHCH 2 Br
1-bromo-2-buteneCH 3 CHCH CH 2 + + Br
3-bromo-1-buteneBrsp^2CH 2 CH CH 2 CH 2 + HBrBr 2Br + CHCH 2BrCH 2 CH CH 2
3-bromopropene+ BrCH 3 CH CH 2sp^2Br 2+ N +OON HOOBr HBr Br 2Br 2Br + HBr +Br 2Br+ BrBr 2 ,N¬BrBr+ + NOO
succinimideN peroxideh HOO
N-bromosuccinimide
NBSBr^ or^ ∆Br 2+ HX∆
CH 2 + X 2 CH 2 CH CH 2allylic substituted
productXCH 3 CHTutorial:
Radicals: common terms3-D Molecule:
Allyl radical