Addition of HX(2) 1114 CH3 CH3 CH3-C-CH-CH3 CH3-C-CH-CH3 Br H-Br H3 H3 HI-CH3 →CH -CH-CH3 Br: Chapter 8 6
Chapter 8 6 Addition of HX (2) CH3 C CH3 CH CH3 H Br CH3 C CH3 CH CH3 H + + Br _ CH3 C CH3 CH CH3 H + Br _ CH3 C CH3 CH CH3 Br H =>
Regiospecificity Markovnikov's Rule:The proton of an acid adds to the carbon in the double bond that already has the most H's."Rich get richer.” o More general Markovnikov's Rule:In an electrophilic addition to an alkene,the electrophile adds in such a way as to form the most stable intermediate. HCI,HBr,and HI add to alkenes to form Markovnikov products. > Chapter 8 7
Chapter 8 7 Regiospecificity • Markovnikov’s Rule: The proton of an acid adds to the carbon in the double bond that already has the most H’s. “Rich get richer.” • More general Markovnikov’s Rule: In an electrophilic addition to an alkene, the electrophile adds in such a way as to form the most stable intermediate. • HCl, HBr, and HI add to alkenes to form Markovnikov products. =>
Free-Radical Addition of HBr In the presence of peroxides,HBr adds to an alkene to form the "anti- Markovnikov"product. Only HBr has the right bond energy. HCI bond is too strong. HI bond tends to break heterolytically to form ions. > Chapter8 8
Chapter 8 8 Free-Radical Addition of HBr • In the presence of peroxides, HBr adds to an alkene to form the “antiMarkovnikov” product. • Only HBr has the right bond energy. • HCl bond is too strong. • HI bond tends to break heterolytically to form ions. =>
Free Radical Initiation Peroxide O-O bond breaks easily to form free radicals. ROO-R heat >R-O·+O-R Hydrogen is abstracted from HBr. R-0.Br R-O-H Br Chapter8 Electrophile
Chapter 8 9 Free Radical Initiation • Peroxide O-O bond breaks easily to form free radicals. R O + H Br R O H + Br R O O R R O + O R heat • Hydrogen is abstracted from HBr. Electrophile =>
Propagation Steps Bromine adds to the double bond. 文· Hydrogen is abstracted from HBr. Br. Br H Chapter 8 Electrophile = 10
Chapter 8 10 Propagation Steps • Bromine adds to the double bond. + C Br C + H Br C Br C H Br Electrophile => C Br Br C C C + • Hydrogen is abstracted from HBr