CH2CHCH2R

CH2CHCH2R denotes an allyl substituent, typically written as CH2=CH-CH2-R, where R represents the rest of the molecule. Systematically, this group is called the prop-2-en-1-yl group and is commonly referred to as the allyl group. The three-carbon fragment features a terminal double bond and a methylene connecting to R, placing the CH2 adjacent to the double bond in an allylic position.

Properties and reactivity

The allylic position in CH2=CH-CH2-R is stabilized by resonance, which makes allylic substrates particularly reactive in a variety of transformations. Allyl groups participate in nucleophilic substitutions, radical additions, and coordinated cross-coupling processes. Because the allyl carbon (the CH2 attached to R) is allylic to the double bond, reactions can often proceed with regioselectivity at the γ-position (SN2′) or, less commonly, at the α-position, depending on the mechanism.

Nomenclature and related groups

When the allyl fragment is bound to a larger molecule, the substituent is called allyl or prop-2-en-1-yl. The parent allyl group is CH2=CH-CH2-. For substructures where R is a leaving group, the fragment is often encountered in allylic chlorides, bromides, or esters (for example, allyl chloride, CH2=CH-CH2-Cl).

Applications

Allyl groups are versatile in organic synthesis. They enable efficient allylation reactions, including palladium-catalyzed allylic substitutions, such as the Trost or Tsuji–Trost reactions. They also serve as protecting groups for alcohols and as handles in polymer chemistry and material science.

Safety

Allyl compounds vary in hazard; many are irritants or reactive with air. Proper handling and disposal per laboratory guidelines are advised.

See also

Allyl group; allylation; prop-2-en-1-yl; Trost reaction; allylic substitution.