eor vidnal syn ofakne 12-12 Oxidative Clea The otassium Perm <·一←· in dry 2ec8t他ee futhaofdency 8 12-RadAn-Markovnikov Product ation A CH.CCHCH 6ae6reatcMgrSt8vrpae3lensaquencemeath haheaghee8nmyastgtoahnegaemhomHer Both prcoagation stens are e mnainrdcalrocom ion or by some other remo ur-emtdm 中等·华r 99 An older reagent for vicinal syn dihydroxylation of alkenes is KMnO4. This reagent is less useful than OsO4 because of its tendency towards overoxidation. The deep purple KMnO4 is converted into a brown precipitate, (MnO2) during the reaction, which can serve as a useful test for the presence of alkenes. 12-12 Oxidative Cleavage: Ozonolysis The mildest reagent capable of breaking both the σ and π bonds in a double bond is ozone, O3. This process is known as “ozonolysis.” Ozone is produced by an electrical discharge in dry oxygen in a instrument called an ozonator. The initial product of the reaction of ozone with an alkene is an ozonide which is then directly reduced to two carbonyl products. The mechanism of ozonolysis proceeds through a molozonide, which breaks apart into two fragments, which then recombine to form the ozonide: Radical Additions: Anti-Markovnikov Product Formation 12-13 Hydrogen bromide can add to alkenes in anti￾Markovnikov fashion: a change in mechanism. The reaction products from the treatment of 1-butene with HBr depend upon the presence or absence of molecular oxygen in the reaction mixture: In the presence of oxygen, a radical chain sequence mechanism leads to the anti-Markovnikov product. Small amounts of peroxides (RO-OR) are formed in alkene samples stored in the presence of air (O2). The peroxides initiate the radical chain sequence mechanism, which is much faster than the ionic mechanism operating in the absence of peroxides. The halogen’s attack is regioselective, generating the more stable secondary radical rather than the primary one. The alkyl radical subsequently abstracts a hydrogen from HBr which regenerates the chain-carrying bromine atom. Both propagation steps are exothermic. Termination is by radical recombination or by some other removal of the chain carriers. Commonly used peroxides for initiating radical additions include: