Reaction Mechanism Free radical chain reaction oInitiation,propagation,termination. Br2hv 2Br. Br. HBr Br、 Br H Br + Br
Reaction Mechanism Free radical chain reaction ⚫Initiation, propagation, termination. H H Br H + HBr Br Br H Br + Br Br2 2Br h
Sec 1 Preparing Alkyl Halides from Alkenes: Allylic Bromination Alkyl halides can also be prepared from alkenes by reaction with N- bromosuccinimide(NBS)in the presence of light Bromine is substituted for hydrogen at the position next to the double bond-the allylic position N-Br (NBS) B Allylic positions hy,CCla H H Cyclohexene 3-Bromocyclohexene (85%) LerringlPhs Perd
Alkyl halides can also be prepared from alkenes by reaction with Nbromosuccinimide (NBS) in the presence of light ▪ Bromine is substituted for hydrogen at the position next to the double bond – the allylic position Sec 1 Preparing Alkyl Halides from Alkenes: Allylic Bromination
Sec 1 Preparing Alkyl Halides from Alkenes: Allylic Bromination Mechanism of allylic bromination Br radical abstracts an allylic hydrogen atom of the alkene forming an allylic radical plus HBr The allylic radical reacts with Br2 to yield the product and a Br. radical which cycles back to carry on the chain Br2 results from reaction of NBS with the HBr formed in the first step 3 (HBr Br2 HBr N-H
Mechanism of allylic bromination ▪ Br. radical abstracts an allylic hydrogen atom of the alkene forming an allylic radical plus HBr ▪ The allylic radical reacts with Br2 to yield the product and a Br. radical which cycles back to carry on the chain ▪ Br2 results from reaction of NBS with the HBr formed in the first step Sec 1 Preparing Alkyl Halides from Alkenes: Allylic Bromination
Sec 1 Preparing Alkyl Halides from Alkenes: Allylic Bromination -Allylic radicals are stable due to resonance
▪ Allylic radicals are stable due to resonance Sec 1 Preparing Alkyl Halides from Alkenes: Allylic Bromination
Sec 1 Preparing Alkyl Halides from Alkenes: Allylic Bromination Allylic bromination of an unsymmetrical alkene often leads to a mixture of products Products are not usually formed in equal amounts because the intermediate allylic radical is not symmetrical and reaction at the two ends is not equally likely CH3CH2CH2CH2CH2CH2CH-CH2 Oct-1-ene hr NBS,CCl4 CHCH.CH.CH.CH.CHCHH2 ←→ CH3CH2CH2CH2CH2CH=CHCH2 Br CH3CH2CH2CH2CH2CHCH=CH2 CH3CH2CH2CH2CH2CH=CHCH2Br 3-Bromooct-1-ene(17%) 1-Bromooct-2-ene(83%) (53 47 trans cis)
▪ Allylic bromination of an unsymmetrical alkene often leads to a mixture of products ▪ Products are not usually formed in equal amounts because the intermediate allylic radical is not symmetrical and reaction at the two ends is not equally likely Sec 1 Preparing Alkyl Halides from Alkenes: Allylic Bromination