Substituent and neighbouring group effects in the bromination of olefins

Abstract

Second-order rate constants have been determined for the bromination of the series CH(_2)C(X)CH(_2)C1 (for -X = -H, -F, -CI and -Br) in 50% by volume methanol-water and trifluoroacetic acid solvents. 2-Halo substitution caused deactivation with respect to the unsubstituted allyl chloride, but enhanced reactivity was observed for the 2-fluoro substituted olefin compared to the 2-chloro and 2-bromo substituted compounds. The above data were compared with the rate coefficients for the addition of trifluoroacetic acid to a series of 2-halopropenes. The difference in the scale of rate enhancement for the 2-fluoro substituted olefins in the two reactions is discussed in terms of the relative contributions of the cyclic bromonium ion structure and of the classical open carbonium ion structure to the transition state for electrophilic addition to olefins. Second-order rate constants have been measured for the addition of bromine to allyl chloride and bromide and benzyl chloride and bromide in acetic acid-water and trifluoroacetic acid solvents. The relative rate enhancement for addition to allyl bromide in trifluoroacetic acid compared to allyl chloride was discussed in terms of solvent enhancement of the inductive effects of the halo substituents by hydrogen bonding, and compared to the rates of electrophilic substitution of bromine into the analogous aromatic compounds. The second-order rate constants for bromination of the two series CH(_2)=CH(CH(_2))(_n-2)OH and CH(_2)=CH(CH(_2))(_n-2)Br (where n = 3 to 6) were measured in acetic acid-water and trifluoroacetic acid solvents, Neighbouring group participation was not observed in the bromo-substituted series of olefins.