Ozonolysis: cut the C=C.

Build an alkene from substituents on each sp² carbon; ozonolysis cuts the C=C into two carbonyl fragments, and reverse mode joins given carbonyls back into the parent alkene.

Reverse mode: given one ethanal and one propanone, assemble the parent alkene (2-methylbut-2-ene); goal checks the substituent sets.

Placing the cut at a C–C single bond; reversing to the wrong isomer.

• 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.

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