Properties and reactions of alkenes

Alkenes C₂ to C₄ are gases, then fourteen members are liquids, and higher ones are solid. Ethene has a faintly sweet smell; other alkenes have none. None of them dissolves in water, but all dissolve fairly well in non-polar solvents such as benzene and petroleum ether. Each added CH₂ raises the boiling point by 20–30 K.

Bromine or chlorine adds to give a vicinal dihalide (through a cyclic halonium ion); iodine does not add under normal conditions. Loss of the reddish orange colour of bromine in CCl₄ is a test for unsaturation.

Hydrogen halides add to give alkyl halides, with reactivity HI > HBr > HCl. A symmetrical alkene gives one product: ethene + HBr gives bromoethane.

Markovnikov rule (1869): when an unsymmetrical reagent adds to an unsymmetrical alkene, its negative part joins the double-bond carbon that carries fewer hydrogens. Propene + HBr gives mainly 2-bromopropane.

Mechanism: H⁺ attacks first, forming the more stable carbocation (secondary rather than primary), which forms faster; Br⁻ then bonds to that positive carbon.

Peroxide (Kharash) effect: with benzoyl peroxide, HBr adds to propene against Markovnikov's rule, giving 1-bromopropane, by a free-radical chain through the more stable secondary radical. Hex-1-ene gives 2-bromohexane without peroxide and 1-bromohexane with it.

Only HBr shows the peroxide effect. H–Cl (430.5 kJ mol⁻¹) is too strong for the radical to break, and I• radicals join to form I₂ rather than add, even though H–I (296.8 kJ mol⁻¹) is weaker than H–Br (363.7 kJ mol⁻¹).

Cold concentrated H₂SO₄ adds by Markovnikov's rule to give alkyl hydrogen sulphates; water with a few drops of concentrated H₂SO₄ adds by the same rule to give alcohols.

Cold dilute aqueous KMnO₄ (Baeyer's reagent) turns alkenes into vicinal glycols, and its loss of colour is a test for unsaturation. Acidic KMnO₄ or K₂Cr₂O₇ cleaves them to ketones and/or acids; but-2-ene gives two molecules of ethanoic acid.

Ozonolysis adds O₃ to form an ozonide, which Zn–H₂O splits into smaller carbonyl compounds; the fragments reveal where the double bond was. Polymerisation at high temperature and pressure with a catalyst turns ethene into polythene and propene into polypropene.