Organic Chemistry,5th Edition L.G.Wade,Jr. Alkenes:Chapter 8 Reactions of Alkenes
Alkenes: Chapter 8 Reactions of Alkenes Organic Chemistry, 5th Edition L. G. Wade, Jr
Reactivity of C=C 111 Electrons in pi bond are loosely held. Electrophiles are attracted to the pi electrons. Carbocation intermediate forms. Nucleophile adds to the carbocation. Net result is addition to the double bond. => Chapter 8 2
Chapter 8 2 Reactivity of C=C • Electrons in pi bond are loosely held. • Electrophiles are attracted to the pi electrons. • Carbocation intermediate forms. • Nucleophile adds to the carbocation. • Net result is addition to the double bond. =>
Electrophilic Addition Step 1:Pi electrons attack the electrophile. Step 2:Nucleophile attacks the carbocation. E Nuc C-C- => Chapter8 3
Chapter 8 3 Electrophilic Addition • Step 1: Pi electrons attack the electrophile. C C + E + C E C + C E C + + Nuc: _ C E C Nuc => • Step 2: Nucleophile attacks the carbocation
Types of Additions 111 Type of Addition [Elements Added Product H OH hydration halogenation [H2O] [X2l.an oxidation OH hydrogenation halohydrin formtion [H],a reduction [HOX],an oxidation OH OH hydroxylation HX addition [HOOH],an oxidation HX灯 oxidative cavage C=0 [O2l.an oxidation cyclopropanation epoxidation [CH] [O],an oxidation "These are not the reagents used but simply the groups that appear in the product
Chapter 8 4 Types of Additions =>
Addition of HX(1) Protonation of double bond yields the most stable carbocation.Positive charge goes to the carbon that was not protonated. CH3 CH3 CHs￥CH-C4 H CH3-C=CH-CH3 Br H-Br X CH3 CH3-C-CH-CH3 H => Chapter 8 5
Chapter 8 5 Addition of HX (1) Protonation of double bond yields the most stable carbocation. Positive charge goes to the carbon that was not protonated. X => + Br _ + + CH3 C CH3 CH CH3 H CH3 C CH3 CH CH3 H H Br CH3 C CH3 CH CH3
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