An alkyl group is a fragment derived from an alkane by removing one hydrogen, leaving one open (free) valency through which it bonds to the rest of a molecule. It matters because alkyl groups appear throughout organic chemistry as substituents and also donate electron density, stabilising nearby positive charge. They are named by replacing the alkane '-ane' ending with '-yl'; for example, methane gives the methyl group and ethane gives the ethyl group.

烷基是由烷烃去掉一个氢而得到的片段,留下一个开放(自由)价,通过它与分子其余部分相连。它之所以重要,是因为烷基作为取代基遍布于有机化学中,并能给出电子云、稳定邻近的正电荷。它们以烷烃词尾“-ane”改为“-yl”来命名;例如甲烷给出甲基,乙烷给出乙基。