A functional group is a particular bonding feature, either a carbon–carbon multiple bond (C=C) or a carbon–heteroatom bond (C–X, where X is a heteroatom), that forms the reactive site of an organic molecule and largely determines its chemistry. It matters because molecules sharing the same functional group tend to react in the same way, which lets reactions be organised by group rather than learned one by one. By contrast, the carbon–carbon and carbon–hydrogen σ-framework (the sigma single-bond skeleton) is essentially unreactive.

官能团是一种特定的成键特征,即碳碳多重键(C=C)或碳–杂原子键(C–X,其中 X 为杂原子),它构成有机分子的反应活性部位,并在很大程度上决定其化学性质。它之所以重要,是因为含有相同官能团的分子往往以相同方式反应,从而可以按官能团而不必逐一记忆来归纳反应。相比之下,碳碳键与碳氢键组成的 σ 骨架(由西格玛单键构成的骨架)基本不发生反应。