Chapter 12 Alkene Reactions Part 2 I.Hydroboration—Oxidation A.Hydroboration 1)Borane, BH 3, is stable in ether or THF because they stabilize the Lewis acid by bonding as Lewis bases 2)BH 3 will add to alkenes: 3)Mechanism: BH 3 electrophile,  -bond nucleophile Filled p-orbitals Empty p-orbital Alkylborane

4)Boron binds to the less hindered Carbon (regioselective and syn addition) due primarily to steric factors B.Oxidation of Alkylborane gives an Alcohol 1.Oxidation Mechanism 2.Result of Hydroboration—Oxidation is Anti-Markovnikov H 2 O addition 3.Example

II.Oxidations of Alkenes A.Peroxycarboxylic Acids are useful oxidation reagents for alkenes 1)Generic: 2)MCPBA (metachloroperbezoic acid) is often used in small scale reactions B.Epoxidation is the common name for the oxidation of an alkene to an oxacyclopropane (epoxide) 1)The reaction is syn stereospecific 2)Mechanism 3)More substituted alkenes react faster MCPBA

C.Hydrolysis of Oxacyclopropanes leads to Vicinal Anti Dihydroxylation D.Osmium Tetroxide oxidation gives Vicinal syn Dihydroxylation 1)Overall Reaction:

2)Mechanism 3)KMnO 4 reacts identically and is used as a test for C=C E.Ozonolysis 1.Ozone = O 3 Reacts with Alkenes to give two carbonyl products

2)Mechanism 3)Reduction step is complex 4)Example

III.Radical Additions: Anti-Markovnikov Products A.Radical Hydrobromination 1)HBr addition to alkenes is cleanly Markovnikov 2)If peroxides are present (RO—OR), the addition is Anti-Markovnikov 3)Peroxides provide entry into a Radical Addition Mechnism Initiation Propagation

4)The bromine radical attacks least substituted carbon, yielding the carbon radical that is most stabilized—the most substituted carbon 5)Only HBr works, HCl and HI reactions are endothermic by radicals 6)Popular Initiating peroxides: B.Other Anti-Markovnikov Radical Additions 1)Thiols can add to alkenes by a radical mechanism initiated by peroxide 2)Halomethanes can add to alkenes by a radical mechanism initiated by peroxide

IV.Polymerization: producing plastics, and synthetic materials A.Acid catalyzed polymerization 1.Alkenes can react with themselves if catalytic H + is present 2.Cations (even carbocations) attack the  -system 3.An acid is needed to initiate the polymerization: H 2 SO 4, HF, BF 3, etc… 4.Repeated attack leads to polymerization

5)Controlled reactions can be stopped at dimers or trimers or oligomers B.Radical Polymerization is initiated by a radical instead of an acid Oligomer or Polymer Initiation Propagation

1)Polyethylene has a branched structure and a MW of 1,000,000 2)Polyvinylchloride (PVC) is made with regioselective polymerization a)Radicals only add to unsubstituted end b)Radicals next to Cl are very stable, so they are formed preferentially c)A head-to-tail structure results C.Anionic Polymerization 1)Strong bases can also initiate polymerization of alkenes