Participation by neighbouring groups in addition reactions. Part I. Hydroxy-group participation in the bromination and iodination of olefins

Abstract

Second-order rate coefficients have been determined and product analyses carried out for the molecular bromination (in water and in methanol) and iodination (in water) of the series of compounds CH₂CH·[CH₂]_n·OH for n= 1, 2, 3, and 4. For both halogens a rate maximum is found for n= 3 and cyclised products (tetrahydrofuran derivatives) are formed. Rate coefficients are estimated for the addition reactions in the absence of participation by assuming a regular fall-off of the inductive effect of the –OH group. For n= 3 the ratios of rate coefficients for reaction with participation to those for the unassisted reaction kΔ : k_s are 1·5 for bromination in methanol, 0·7 for bromination in water, and ca. 60 for iodination; for n= 4 the corresponding values are 0·2, 0·4 and ca. 8. The results are interpreted in terms of the formation of oxonium ions containing five- and six-membered rings with a corresponding stabilisation of the transition states resulting in increased rates of reaction. The markedly bigger assistance for iodination suggests that the response by the neighbouring group is greatest for the most selective electrophilic reagent.