Substitution reaction

A substitution reaction is a reaction in which one atom or group of atoms in a molecule is replaced by another atom or group. In Carbon Compounds it is the characteristic reaction of the saturated alkanes. Malay: tindak balas penggantian · Chinese: 取代反应.

Because an alkane has only single bonds, it cannot open a bond to accept new atoms; instead a hydrogen atom is swapped for another atom. Alkanes react with halogens such as chlorine or bromine by substitution, and this happens only in the presence of ultraviolet light. One hydrogen atom is replaced by a halogen atom, and a molecule of hydrogen halide is released as the second product. The ultraviolet light is essential because it supplies the energy needed to start the reaction; in the dark, an alkane and a halogen simply do not react at room temperature. Because there are several hydrogen atoms that could be replaced, substitution can continue step by step, replacing more than one hydrogen if excess halogen is present.

Example. Methane, CH₄, reacts with chlorine in ultraviolet light: a hydrogen atom is replaced by a chlorine atom to give chloromethane, CH₃Cl, and hydrogen chloride, HCl. Without the light, the reaction does not take place at room temperature.

Confusion to avoid. Do not confuse substitution with addition. Substitution needs ultraviolet light, works on a saturated molecule, replaces an atom and gives two products; addition works on an unsaturated molecule, opens a double bond, needs no light and gives a single product. Substitution is the mark of the alkanes, addition the mark of the alkenes.

Substitution shows why the saturated alkanes are relatively unreactive and sets up the contrast with the reactive alkenes, a comparison that is a favourite of the Carbon Compounds questions in SPM Chemistry.