Preparation of alkenes

Partial reduction of alkynes: H₂ over Lindlar's catalyst (palladised charcoal partly poisoned with sulphur compounds or quinoline) gives cis alkenes; sodium in liquid ammonia gives trans alkenes.

Ethyne and propyne add one H₂ over Pd/C to give ethene and propene. Propene shows no geometrical isomerism because one of its doubly bonded carbons carries two H atoms.

Dehydrohalogenation: an alkyl halide heated with alcoholic KOH loses HX to give an alkene. H leaves from the β carbon (next to the carbon bearing X), so this is a β-elimination.

The rate of dehydrohalogenation depends on the halogen (iodine > bromine > chlorine) and the alkyl group (tertiary > secondary > primary).

Dehalogenation: a vicinal dihalide (halogens on adjacent carbons) with zinc loses ZnX₂: CH₂Br–CH₂Br + Zn → CH₂=CH₂ + ZnBr₂.

Acidic dehydration: an alcohol heated with concentrated H₂SO₄ loses water to give an alkene; the –OH removes an H from the β carbon, so this too is a β-elimination.