Time-resolved infrared absorption measurements of product formation in Cl atom reactions with alkenes and alkynes

The reactions of Cl with hydrocarbons have been the subjects of study since the early days of experimental chemical kinetics. The reactions of Cl atom s with alkanes are prototypical atom abstractions;the reactions with alkene s and alkynes display a richer chemistry because of the possibility of addi tion to form a chlorinated hydrocarbon radical. This review discusses recen t infrared absorption investigations of the reactions of Cl with unsaturate d hydrocarbons, with an emphasis on deconvolution of the addition and metat hesis (i.e. abstraction or addition-elimination) pathways. These experiment s are described in relation to other Cl + hydrocarbon reactions, and severa l examples illustrate the sensitivity of the kinetic and mechanistic behavi our of the Cl + alkene reactions to the thermochemistry of the individual s ystems. In the Cl + C2H4 reaction, where metathesis is endothermic, additio n dominates except at elevated temperatures or low pressures, but the exoth ermic metatheses of Cl with C-3 unsaturated species effectively compete wit h addition even at room temperature. An example of thermochemical differenc es creating qualitatively different kinetic behaviours is given by the reac tions of Cl with the C3H4 isomers allene (CH2=C=CH2) and propyne (CH=C-CH3) . Whereas Cl+propyne displays kinetics similar to other Cl + alkene systems , the Cl + aliene reaction shows unusual behaviour driven by the formation of the resonance-stabilized chloroallyl radical. Characterization of the na ture of abstraction in Cl + alkene systems and examination of the role of a ddition-elimination in exothermic reactions are suggested as important area s for further investigation.