allyl

Allyl refers to the allyl group, a substituent derived from propene by removal of a hydrogen atom from the terminal carbon. Its basic structure is CH2=CH-CH2-. When attached to a molecule it is usually written as -CH2-CH=CH2. The allyl group also exists as reactive species such as the allyl radical and the allyl cation, and these forms participate in many reactions.

The allyl group is conjugated, with electron delocalization spanning the three carbon atoms. This resonance stabilizes allyl cations, radicals, and anions, making them reactive intermediates in numerous transformations. In organometallic chemistry, allyl ligands readily form η3-allyl metal complexes, which facilitate a variety of catalytic processes.

Chemically, allyl groups are central to allylic substitution reactions, where the allyl moiety is replaced by nucleophiles. These reactions can proceed via SN2' or SN2 pathways and are frequently mediated by transition metals, notably palladium in Tsuji-Trost-type allylation, enabling formation of C–C, C–N, and C–O bonds under mild conditions.

Common compounds containing the allyl group include allyl alcohol (CH2=CH-CH2OH), allyl chloride (CH2=CH-CH2Cl), and various allyl ethers and esters. The allyl group is widely used in protecting group strategies (allyl ethers), in polymer chemistry, and as a versatile handle in organic synthesis. Some allyl compounds, such as allyl chloride, are hazardous and lachrymatory alkylating agents; allyl alcohol is flammable and toxic.