Chapter 12 Alkene Reactions

1. Chapter 12 Alkene Reactions • Catalytic Hydrogenation • Thermodynamics of addition reactions • C=C p-bond is weak and thus reactive • Addition reactions: • DH = (DHop + DHoAB) – (DHoCA + DHoCB) = -DH • CA and CB single s-bonds stronger than AB + p-bond • Additions usually occur spontaneously and -DH is released (Table 12-1) • Hydrogenation of Alkenes • Addition of H2 to C=C requires a catalyst to lower Ea • Reaction occurs at the metal surface (Pd/C or PtO2, or Ra Ni) • Solvent is usually MeOH, EtOH, or HOAc

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

3. Steric Bulk may dictate which side can approach the metal surface • p-bond as Nucleophile: HX Additions • A p-bond is an e- rich cloud that electrophiles can attack • H+ is a strong electrophile • Low temperature reduces chance of rearrangement

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

5. Alcohol Synthesis bye Electrophilic Hydration • Strong aqueous mineral acid gives H2O addition • This reaction obeys Markovnikov Rule • Mechanism is the reverse of the acid catalyzed alcohol dehydration • Alkene Hydration vs. Alcohol Dehydration • All steps in the mechanism are reversible: Equilibrium • H+ acts as catalyst and is not consumed • Favor alcohol with low temperature and excess water • Favor alkene with concentrated acid and heat

6. Thermodynamic Control • When reversible protonation can happen, an equilibrium mixture exists • The most stable product will be major, because minor products will be converted back to the cation, then to the most stable product • We can use acid to interconvert alkene isomers to most stable one • Halogen Addition • Halogen gases (Cl2) don’t seem very electrophilic, but will add to alkenes • Cl2 and Br2 in CCl4 solvent at room temperature best conditions • F2 reacts violently; I2 doesn’t react at all (DHo = 0) • Disappearance of red-brown Br2 upon addition to unknown signals alkene

7. Halogen Addition Mechanism • Anti addition is always observed • Bromonium ion • Br—Br has a very large, polarizable s-bond • C=C p-bond nucleophile attacks the d+ end of Br—Br (like SN2) • The result is a Bromonium cation and Br- anion racemic meso

8. The last step is nucleophilic attack by Br- on the bromonium ion • Other Additions • Halonium cation can trap other nucleophiles • Cl2 works just like Br2 = chloronium ion • Mixed products can be useful synthetic intermediates

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

10. C. Oxymercuration—Demercuration • Oxymercuration proceeds in an anti addition, just like Br2 addition • Demercuration replaces the Hg with H • The result is Markovnikov addition just like acidic hydration reaction • The advantage is that no rearrangement can take place