chapter 8

chapter 8 - Addition Reactions General Example: R2 R1 R4 R3...

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Addition Reactions X and Y add to same side X and Y add to opposite sides Y R 1 R 2 R 3 R 4 R 1 R 2 R 3 R 4 + X Y X and/or Y R 1 R 2 R 3 R 4 X SYN ANTI General Example: Addition of HBr (HX) H H H H H H H H H Br H Br + addition elimination
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Stepwise Mechanism: Br H H H H H H H H H base, proton acceptor + acid, proton donor + (1) acid/base rxn H - Br H H H H H H H H H H Br Br H H H H H Br + (2) acid/base rxn same as S N 1 top bottom top bottom syn anti the same compound from top and bottom
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Free Energy Diagram for the Hydrohalogenation Reaction C=C + H-X C C H X C C H + + X - C C H X !" ! + ! + C C H X ! + !" ! G step 1 ! G step 2 The rate-determining (slow) step is formation of the high energy intermediate--the carbocation . Free Energy Reaction Coordinate
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Regiochemistry of the Addition Reaction Regiochemistry means the specific carbons of the alkene to which the H and X attach. In unsymmetrical alkenes, there are two possible regiochemistries: CH 3 CH 2 CH 2 Br 1-bromopropane very little formed CH 3 CHCH 2 Br CH 3 1-bromo-2-methyl- propane very little formed + H Br propene 2-bromopropane CH 2 =CHCH 3 CH 3 CHCH 3 Br 2-methylpropene + H Br 2-bromo-2-methylpropane C=CH 2 CH 3 CH 3 CH 3 CCH 3 CH 3 Br Regioisomer
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Addition of Hydrogen Halides to Alkenes Markovnikov's Rule The reactivity order of HX in these additions is HI > HBr > HCl Vladimir Markovnikov (University of Kazan) in 1869: "The hydrogen of the acid attaches to the carbon that already holds the greater number of hydrogens." Hydrogen halides (HCl, HBr, HI) add to alkenes: H H CH 3 CH 3 + HX H H CH 3 CH 3 X = Cl, Br, I C=C C C H X (less substituted carbon)
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H H H H H H H H Br Br Br H H H Br H H H H Br H top bottom racemic mixture top bottom different from top and bottom Stereochemistry Generating one stereocenter --> enantiomers (achiral) Generating two stereocenters --> diastereomers enantiomers (achiral)
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CH 3 + HI CH 3 CH 3 + HI Predict the products of the following addition reactions. Indicate important stereochemical details in the products. achiral CH 3 I four products: two diastereomers, each as a racemic form (2 pairs of enantiomers) + CH 3 CH 3 CH 3 CH 3 I I H H A B + CH 3 CH 3 CH 3 CH 3 I I H H + C D
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Acid-Catalyzed Direct Hydration of Alkenes CH 3 C=CH 2 CH 3 + H 2 O H + CH 3 CCH 3 CH 3 OH Mechanism of Direct Hydration of Alkenes Note: Hydronium ion is reformed, so the reaction is catalyzed by acid . Step 1: + + slow + + H 2 O electrophilic addition CH 3 C=CH 2 CH 3 H-O-H H CH 3 CCH 3 CH 3 Step 2: + + : : fast nucleophilic addition + tert-butyloxonium ion CH 3 CCH 3 CH 3 O-H H CH 3 CCH 3 CH 3 O-H H Step 3: + + : fast deprotonation + + CH 3 CCH 3 CH 3 O-H H O-H H CH 3 CCH 3 CH 3 O-H H-O-H H
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Rearrangements may occur during the acid-catalyzed hydration of alkenes at the carbocation stage : CH 3 CCH=CH 2 CH 3 CH 3 3,3-dimethyl-1-butene H + H 2 O CH 3 C-CHCH 3 CH 3 CH 3 HO 2,3-dimethyl-2-butanol
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chapter 8 - Addition Reactions General Example: R2 R1 R4 R3...

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