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-Br HBr -H Br2 => Chapter6 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. Bnhv→2Br. Br HBr Br. Br => H Br 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 一C一C一 +Nuc:" X: 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.= Chapter6 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 X:+HB H X 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). => Chapter6 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