Allylic Halogenation Allylic radical is resonance stabilized Bromination occurs with good yield at the allylic position(sp3c neXt to C=C) Avoid a large excess of Br2 by using N-bromosuccinimide(nbs)to generate Br2 as product HBr is formed -Br+ HBr H+ B > Chapter 6 16
Chapter 6 16 Allylic Halogenation • Allylic radical is resonance stabilized. • Bromination occurs with good yield at the allylic position (sp3 C next to C=C). • Avoid a large excess of Br2 by using N-bromosuccinimide (NBS) to generate Br2 as product HBr is formed. N O O Br + HBr N O O H + Br2 =>
Reaction mechanism Free radical chain reaction >initiation, propagation, termination Br2 hv 2 Br Br HB y-8 > Br t Br Chapter 6 17
Chapter 6 17 Reaction Mechanism Free radical chain reaction ➢initiation, propagation, termination. H H Br H + HBr Br Br H Br + Br => Br2 2Br h
Substitution reactions C-C t nuc H X H Nuc The halogen atom on the alkyl halide is replaced with another group Since the halogen is more electronegative than carbon the c-x bond breaks heterolytically and X leaves The group replacing x is a nucleophile. = Chapter 6 18
Chapter 6 18 Substitution Reactions • The halogen atom on the alkyl halide is replaced with another group. • Since the halogen is more electronegative than carbon, the C-X bond breaks heterolytically and Xleaves. • The group replacing Xis a nucleophile. => C C H X + Nuc:- C C H Nuc + X:-
Elimination reactions B: C=C X: HB The alkyl halide loses halogen as a halide ion and also loses ht on the adjacent carbon to a base A pi bond is formed. Product is alkene Also called dehydrohalogenation (-HX > Chapter 6 19
Chapter 6 19 Elimination Reactions • The alkyl halide loses halogen as a halide ion, and also loses H+ on the adjacent carbon to a base. • A pi bond is formed. Product is alkene. • Also called dehydrohalogenation (-HX). => C C H X + B:- + X:- C C + HB