Chapter 12 Alkene Reactions I.Catalytic Hydrogenation A.Thermodynamics of addition reactions 1)C=C  -bond is weak and thus reactive 2)Addition reactions: 3)  H = (DH o  + DH o AB ) – (DH o CA + DH o CB ) = -  H 4)CA and CB single  -bonds stronger than AB +  -bond 5)Additions usually occur spontaneously and -  H is released (Table 12-1) B.Hydrogenation of Alkenes 1)Addition of H 2 to C=C requires a catalyst to lower E a 2)Reaction occurs at the metal surface (Pd/C or PtO 2, or Ra Ni) 3)Solvent is usually MeOH, EtOH, or HOAc

3)Hydrogenation is stereospecific at one face of C=C (syn addition)

4)Steric Bulk may dictate which side can approach the metal surface  -bond as Nucleophile: HX Additions A.A  -bond is an e- rich cloud that electrophiles can attack 1)H + is a strong electrophile 2)Low temperature reduces chance of rearrangement

B.Markovnikov Rule 1.H + goes to the least substituted C, and X - goes to the most substituted C 2.Formation of the most stable carbocation directs the reaction. Initial protonation gives the most stable carbocation. TS-1 TS-2

III.Alcohol Synthesis bye Electrophilic Hydration A.Strong aqueous mineral acid gives H 2 O addition 1)This reaction obeys Markovnikov Rule 2)Mechanism is the reverse of the acid catalyzed alcohol dehydration B.Alkene Hydration vs. Alcohol Dehydration 1)All steps in the mechanism are reversible: Equilibrium 2)H + acts as catalyst and is not consumed 3)Favor alcohol with low temperature and excess water 4)Favor alkene with concentrated acid and heat

C.Thermodynamic Control 1.When reversible protonation can happen, an equilibrium mixture exists 2.The most stable product will be major, because minor products will be converted back to the cation, then to the most stable product 3.We can use acid to interconvert alkene isomers to most stable one IV.Halogen Addition A.Halogen gases (Cl 2 ) don’t seem very electrophilic, but will add to alkenes 1)Cl 2 and Br 2 in CCl 4 solvent at room temperature best conditions 2)F 2 reacts violently; I 2 doesn’t react at all (  H o = 0) 3)Disappearance of red-brown Br 2 upon addition to unknown signals alkene

B.Halogen Addition Mechanism 1.Anti addition is always observed 2.Bromonium ion a.Br—Br has a very large, polarizable  -bond b.C=C  -bond nucleophile attacks the  + end of Br—Br (like S N 2) c.The result is a Bromonium cation and Br - anion racemic meso

3)The last step is nucleophilic attack by Br - on the bromonium ion V.Other Additions A.Halonium cation can trap other nucleophiles 1)Cl 2 works just like Br 2 = chloronium ion 2)Mixed products can be useful synthetic intermediates

B.Regioselectivity of Halonium ion mixed products 1.Halogen ends up on the less-substituted C. Greater  + on more subst. C 2.Nucleophile ends up on more substituted C 3.Markovnikov-like addition because electrophile (H +, Br + ) behaves same 4.Other reagents behave as electrophile-nucleophile pair (in that order) a.Br—Cl b. Br—CN c.I—Cl d. RS—Cl d.XHg—X (X = acetate)

C.Oxymercuration—Demercuration 1)Oxymercuration proceeds in an anti addition, just like Br 2 addition 2)Demercuration replaces the Hg with H 3)The result is Markovnikov addition just like acidic hydration reaction 4)The advantage is that no rearrangement can take place