CH2CHCH2X

CH2CHCH2X, typically written in shorthand as CH2=CH-CH2-X, denotes an allyl halide, a class of organohalides in which a halogen atom X is attached to the terminal carbon of an allyl group. The allyl group is a 3-carbon moiety with a double bond between C1 and C2 and a CH2 substituent at C3, giving the general formula C3H5X with X = Cl, Br, or I. Allyl halides are widely used as reactive intermediates in organic synthesis because the allylic position is activated toward nucleophilic attack and rearrangement.

Structure and nomenclature: The halogen is bonded to the primary allylic carbon, while the adjacent C=C bond provides resonance stabilization to allyl cations or radicals formed during reactions. This conjugation influences reactivity, often enabling rapid SN2 reactions at the allylic site and facile participation in catalytic allylation processes.

Preparation: Allyl halides are typically prepared by converting allyl alcohols or related derivatives with reagents such as thionyl chloride, phosphorus halides (e.g., PCl3, PBr3), or hydrogen halides. Alternative routes include halogenation of propene derivatives or dehydration routes followed by halogenation, depending on the target halide and scale.

Reactions and applications: Allyl halides participate in a variety of transformations. They undergo SN2 substitutions at the allylic carbon and can form carbon–carbon bonds in allylation reactions, including the Tsuji–Trost type catalytic processes. They are key intermediates in the synthesis of fragrances, pharmaceuticals, and polymerizable materials, where the allyl group serves as a versatile handle for further functionalization or crosslinking.

Safety: Allyl halides are typically lachrymatory and corrosive; they should be handled with appropriate PPE and in well-ventilated or fume-controlled environments.

See also: allyl group, allylation, vinyl halide, propenyl derivatives.