12-2 Preparing Alkyl Halides from Alkenes: Allylic Bromination Methods of preparing alkyl halides Reactions of HX and X2 with alkenes in electrophilic addition reactions The hydrogen halides HCI,HBr,and HI react with alkenes by a polar mechanism to give the product of Markovnikov addition Bromine and chlorine undergo anti addition through a halonium ion intermediate to give 1,2-dihalogenated products HX H CH3 CH3 H CH3 X=CIor Br X=Cl,Br,or I O Cengege Loaming Al Rights Reserved
Methods of preparing alkyl halides ▪ Reactions of HX and X2 with alkenes in electrophilic addition reactions ▪ The hydrogen halides HCl, HBr, and HI react with alkenes by a polar mechanism to give the product of Markovnikov addition ▪ Bromine and chlorine undergo anti addition through a halonium ion intermediate to give 1,2-dihalogenated products 12-2 Preparing Alkyl Halides from Alkenes: Allylic Bromination
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 HH N-Br (NBS) Br Allylic positions hy,CCla H H Cyclohexene 3-Bromocyclohexene (85%)
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 Preparing Alkyl Halides from Alkenes: Allylic Bromination
Preparing Alkyl Halides from Alkenes: Allylic Bromination Mechanism of allylic bromination a 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 B 3 (HBr Br. Br2* HBr -Br 2
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 Preparing Alkyl Halides from Alkenes: Allylic Bromination
Preparing Alkyl Halides from Alkenes: Allylic Bromination Allylic radicals are more stable than related alkyl radicals by about 40 kJ/mol(9 kcal/mol)and are more stable than vinylic radicals by about 85 kJ/mol (19 kcal/mol) According to the Hammond postulate,allylic radicals should form faster Allylic 370 kJ/mol (88 kcal/mol) H H Alkyl 410 kJ/mol (98 kcal/mol) Vinylic 465 kJ/mol (111 kcal/mol)
Allylic radicals are more stable than related alkyl radicals by about 40 kJ/mol (9 kcal/mol) and are more stable than vinylic radicals by about 85 kJ/mol (19 kcal/mol) ▪ According to the Hammond postulate, allylic radicals should form faster Preparing Alkyl Halides from Alkenes: Allylic Bromination
Preparing Alkyl Halides from Alkenes: Allylic Bromination -Allylic radicals are stable due to resonance
▪ Allylic radicals are stable due to resonance Preparing Alkyl Halides from Alkenes: Allylic Bromination