Alkene

An alkene is an unsaturated hydrocarbon with the general formula CₙH₂ₙ that contains a carbon–carbon double bond, C=C. Malay: alkena · Chinese: 烯烃.

The double bond makes alkenes far more reactive than alkanes, and their characteristic reaction is addition, in which atoms add across the C=C and open it into a single bond. Alkenes add hydrogen with a nickel catalyst to give alkanes, add bromine to decolourise bromine water, add steam to form alcohols, and add to one another in polymerisation to build polymers such as poly(ethene). Alkenes are named like the alkanes but with the ending -ene, and a locant shows where the double bond sits.

Example. Ethene (C₂H₄) is the first alkene. Bubbled through bromine water it removes the brown colour at room temperature, and heated with hydrogen over nickel it becomes ethane. Passed over a catalyst with steam it forms ethanol, which is one industrial route to alcohol.

Confusion to avoid. Do not confuse an alkene with an alkane. The alkene is unsaturated, fits CₙH₂ₙ and decolourises bromine water; the alkane is saturated, fits CₙH₂ₙ₊₂ and does not. Also, be careful to say addition, not substitution, substitution belongs to the alkanes, addition to the alkenes.

Every one of the alkene reactions traces back to the same double bond, which is why this series feels like a single idea rather than a list of unrelated facts. Once you can picture the C=C opening up, hydrogenation, the bromine test, hydration to an alcohol and polymerisation all become the same move applied to different added atoms. Ethene itself is produced on a large scale by cracking long-chain fractions obtained from petroleum, and it is one of the most important raw materials in the whole chemical industry, which is why the alkenes reward careful study.

Alkenes are where the reactive, transformable side of carbon chemistry appears, feeding directly into alcohols, polymers and fats, so they are among the most heavily tested members of the Carbon Compounds chapter in SPM Chemistry.