A substituent is an atom or group, such as an alkyl group or a halogen, that is attached to the parent chain in place of a hydrogen atom. It matters because each substituent is cited as a prefix in the name, together with a locant showing the carbon it is attached to, so that the full structure can be read off the name. Thus the parent chain supplies the root of the name and the substituents supply its prefixes.

取代基是连接在主链上、取代一个氢原子的原子或基团,如烷基或卤素。它之所以重要,是因为每个取代基都在名称中以前缀形式给出,并附有标明其所连碳的位次,使整个结构可以从名称中读出。因此,主链提供名称的词根,取代基提供名称的前缀。