Addition reaction

An addition reaction is a reaction in which atoms add across a carbon–carbon double bond, C=C, opening it into a single bond so that no other product is formed. It is the characteristic reaction of the unsaturated alkenes. Malay: tindak balas penambahan · Chinese: 加成反应.

Because the double bond is a region of extra bonding, it readily accepts new atoms on each of the two carbons. Alkenes therefore undergo several addition reactions: they add hydrogen over a nickel catalyst (hydrogenation) to give an alkane, add bromine to decolourise bromine water, add hydrogen halides, and add steam over a catalyst to form an alcohol. In each case one molecule is added and one product results, with no small molecule given off. This is why an alkene has just one product from each addition while a saturated molecule reacting by substitution always produces a second, separate molecule as well. The same double bond that makes addition possible is also what lets many alkene molecules add to one another in polymerisation to build a polymer.

Example. Ethene, C₂H₄, reacts with bromine by addition: the brown colour of bromine water disappears at room temperature as the bromine adds across the double bond to give a colourless product. This decolourising is the standard test for a carbon–carbon double bond.

Confusion to avoid. Do not confuse addition with substitution. Addition opens a double bond and needs no light, giving a single product; substitution replaces an atom on a saturated molecule, needs ultraviolet light for alkanes, and releases a second product such as a hydrogen halide. Addition belongs to alkenes; substitution belongs to alkanes.

The addition reaction is the engine of alkene chemistry and the reason alkenes feed into alcohols, polymers and hardened fats, making it one of the most tested ideas in Carbon Compounds for SPM Chemistry.