A vinyl group is the CH2=CH– group, in which the point of attachment is a carbon that is part of a carbon–carbon double bond (C=C). It matters because attaching a leaving group directly to such an sp²-hybridised carbon makes it very unreactive: vinyl halides resist nucleophilic substitution, because the C–X bond is strengthened and the resulting cation would be unstable. This behaviour parallels that of the aryl group.

乙烯基是 CH2=CH– 基团,其连接点是一个属于碳碳双键(C=C)的碳。它之所以重要,是因为把离去基团直接连在这样一个 sp² 杂化碳上会使其非常不活泼:乙烯基卤化物不易发生亲核取代,因为 C–X 键被增强,且生成的阳离子会不稳定。这种行为与芳基相似。