离去基团（或称离去基）在化学反应中从一较大分子中脱离的原子或官能基。如下式中，Cl就是离去基团：

当离去基团共轭酸的pK_a越小，离去基团越容易从其他分子中脱离。原因是因为当其共轭酸的pK_a越小，相应离去基团不需和其他原子结合，以阴离子（或电中性离去基团）的形式存在的趋势也就增强。因而强碱往往不是很好的离去基团。

溴化物作为离去基团和氢氧化物（取其OH的功能）作为亲核试剂。

对于S_N1反应而言，以卤离子、拟卤离子和非配位阴离子作为离去基团较好，尤其是卤离子。可以加入银离子以生成难溶的卤化银，进一步向右拉动反应平衡。

In chemistry, a leaving group is a molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. Leaving groups can be anions or neutral molecules, but in either case it is crucial that the leaving group be able to stabilize the additional electron density that results from bond heterolysis. Common anionic leaving groups are halides such as Cl, Br, and I, and sulfonate esters such as tosylate (TsO). Fluoride (F) functions as a leaving group in the nerve-agent sarin gas. Common neutral molecule leaving groups are water and ammonia.