converted into alcohols via the oxymercuration–demercuration reaction , the hydroboration–oxidation reaction or by Mukaiyama hydration. Halogenation In electrophilic halogenation the addition of elemental bromine or chlorine to alkenes yields vicinal dibromo- and dichloroalkanes (1,2-dihalides or ethylene dihalides), respectively. The decoloration of a solution of bromine in water is an analytical test for the presence of alkenes: CH2=CH2 + Br2 -> BrCH2-CH2Br Related reactions are also used as quantitative measures of unsaturation, expressed as the bromine number and iodine number of a compound or mixture. Hydrohalogenation Hydrohalogenation is the addition of hydrogen halides, such as HCl or HI, to alkenes to yield the corresponding haloalkanes: If the two carbon atoms at the double bond are linked to a different number of hydrogen atoms, the halogen is found preferentially at the carbon with fewer hydrogen substituents. This patterns is known as Markovnikov's rule. The use of radical initiators or other compounds can lead to the opposite product result. Hydrobromic acid in particular is prone to forming radicals in the presence of various impurities or even atmospheric oxygen, leading to the reversal of the Markovnikov result: Halohydrin formation Alkenes react with water and halogens to form halohydrins by an addition reaction. Markovnikov regiochemistry and anti-stereochemistry occur. CH2=CH2 + X2 + H2O -> XCH2-CH2OH + HX Oxidation Alkenes react with percarboxylic acids and even hydrogen peroxide to yield epoxides: RCH=CH2 + RCO3H -> RCHOCH2 + RCO2H For ethylene, the epoxidation is conducted on a very large scale industrially using oxygen in the presence of catalysts: C2H4 + 1/2 O2 -> C2H4O Alkenes react with ozone, leading to the scission of the double bond. The process is called ozonolysis. Often the reaction procedure includes a mild reductant, such as dimethylsulfide (): RCH=CHR' + O3 + SMe2 -> RCHO + R'CHO + O=SMe2 R2C=CHR' + O3 -> R2CHO + R'CHO + O=SMe2 When treated with a hot concentrated, acidified solution of , alkenes are cleaved to form ketones and/or carboxylic acids. The stoichiometry of the reaction is sensitive to conditions. This reaction and the ozonolysis can be used to determine the position of a double bond in an unknown alkene. The oxidation can be stopped at the vicinal diol rather than full cleavage of the alkene by using osmium tetroxide or other oxidants: R'CH=CR2 + 1/2 O2 + H2O -> R'CH(OH)-C(OH)R2 This reaction is called dihydroxylation. In the presence of an appropriate photosensitiser, such as methylene