Allyl group
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. Another example of a simple allyl compound is allyl chloride. Compounds containing the allyl group are often referred to as being allylic. Substituted versions of the parent allyl group, such as the trans-but-2-en-1-yl or crotyl group (CH3CH=CH-CH2-), may also be referred to as allylic groups.
The site of the saturated carbon atom, where the vinyl 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.
Allylic carbons are sp³ hybridized; vinylic carbons are sp² hybridized.
Allylic methylene groups show special reactivity such as demonstrated in allylic oxidations, ene reactions and the Trost asymmetric allylic alkylation.
The term homoallylic refers to the position on a carbon skeleton next to an allylic position.