Halogenation of Alkanes Br Br CH3一CH2-CH3+Br2 h,125℃3 CH3-CH2-CH2 CH3-CH-CH3 primary bromide,3% secondary bromide,97% Bromination is highly selective: 3°carbons>2°carbons>1°carbons Chapter6 16
Chapter 6 16 Halogenation of Alkanes • Bromination is highly selective: 3ºcarbons > 2ºcarbons > 1ºcarbons
Allylic Halogenation allylic positions allylic hydrogens HH H Br Br2 HBr cyclohexene HH 3-bromocyclohexene (80%) Copyright 2010 Pearson Prentice Hall,Inc. Allylic radical is resonance stabilized. Bromination occurs with good yield at the allylic position (sp3 C next to C-C). Chapter6 17
Chapter 6 17 Allylic Halogenation • Allylic radical is resonance stabilized. • Bromination occurs with good yield at the allylic position (sp3 C next to C C)
N-bromosuccinimide N-Br HBr N-H Br2 N-bromosuccinimide (NBS) regenerates a low concentration of Br2 Copyright2010 Pearson Prentice Hall.Inc. N-bromosuccinimide (NBS)is an allylic brominating agent. Keeps the concentration of Br2 low. Chapter6 18
Chapter 6 18 N-bromosuccinimide • N-bromosuccinimide (NBS) is an allylic brominating agent. • Keeps the concentration of Br2 low
Reaction Mechanism 一的一 allylic shift Copyright 2010 Pearson Prentice Hall,Inc. The mechanism involves an allylic radical stabilized by resonance. Both allylic radicals can react with bromine. Chapter 6 19
Chapter 6 19 Reaction Mechanism • The mechanism involves an allylic radical stabilized by resonance. • Both allylic radicals can react with bromine
Substitution Reactions Nucleophilic substitution +Nuc: H :X: H Nuc Copyright 2010 Pearson Prentice Hall,Inc. The halogen atom on the alkyl halide is replaced with a nucleophile (Nuc ) 。 Since the halogen is more electronegative than carbon,the C-X bond breaks heterolytically and X leaves. Chapter6 20
Chapter 6 20 Substitution Reactions • The halogen atom on the alkyl halide is replaced with a nucleophile (Nuc- ). • Since the halogen is more electronegative than carbon, the C—X bond breaks heterolytically and Xleaves