Organic Chemistry,5th Edition L.G.Wade,Jr. Chapter 8 Reactions of Alkenes Jo Blackburn Richland College,Dallas,TX Dallas County Community College District ©20o3,Prentice Hall
Chapter 8 Reactions of Alkenes Jo Blackburn Richland College, Dallas, TX Dallas County Community College District © 2003, Prentice Hall Organic Chemistry, 5th Edition L. G. Wade, Jr
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. 二> Chapter8 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. C-C Step 2:Nucleophile attacks the carbocation. E Nuc +Nuc: => Chapter 8 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 Type of Addition [Elements Added Product H OH hydration halogenation [HO] [X2],an oxidation OH hydrogenation halohydrin formtion [H],a reduction [HOX],an oxidation OH OH hydroxylation HX addition [HOOH],an oxidation HX灯 oxidative cleavage C=0 [O2l.an oxidation cyclopropanation epoxidation ICH] [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 CHs￥一CH-CH CH3 H CH3-C=CH-CH3 +Br H-Br 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) CH3 CH3 CH3-C-CH-CH3 Br H H-Br H3 H3 CH3-C-CH-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.” 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. => Chapter8 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 1111 Peroxide O-O bond breaks easily to form free radicals. R-OOR heat R-O·+·O-R Hydrogen is abstracted from HBr. R-0.Br R-O-H Br => Chapter 8 Electrophile 9
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 111 Bromine adds to the double bond. 衣· 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