An allyl group is an alkene hydrocarbon group with the formula H₂C=CH-CH₂-. It is made up of a vinyl group, CH₂=CH-, attached to a methylene -CH₂. For example allyl alcohol has the structure H₂C=CH-CH₂OH . Compounds containing the allyl group are often referred to as being allylic.

The site of the saturated carbon atom, where the allyl group attaches (e.g. next to the OH in allyl alcohol) is called the allylic position or allylic site. A group, such as -OH, attached at an allylic site is sometimes described as allylic. Two examples of simple allyl compounds are allyl chloride and allyl alcohol.

Substituted versions of the above, such as the trans-but-2-en-1-yl or crotyl group (CH₃CH=CH-CH₂-) may be also referred to as allylic groups.

Allylic methylene groups show special reactivity such as demonstrated in allylic oxidations, ene reactions and the Trost asymmetric allylic alkylation.

Alkenes | functional groups

أليل | Allyl | Grupo alilo | アリル化合物 | Allyl