Structure‐reactivity relationships in dehydrohalogenation reactions of polychlorinated and polybrominated alkanes

Abstract

Current information is inadequate to predict the rates at which polyhalogenated alkanes undergo dehydrohalogenation reactions under environmental conditions, forming olefins that are frequently more toxic and more recalcitrant than the products of substitution reactions. To permit evaluation of the relative importance of different potential transformation products, qualitative relationships between structure and reactivity via substitution and dehydrohalogenation pathways are reviewed. In order to develop quantitative predictive capability, data were analyzed for dehydrohalogenation reactions of 28 polychlorinated and polybrominated compounds in aqueous solution. Rate constants (extrapolated to 25°C) that were known or could be reasonably inferred to represent E₂ reactions were fit to linear free‐energy relationships (LFERs) of the form log₁₀ k/k_o or log₁₀ ,k/k_o = p₁ • S#GS₁ + constant. For base‐promoted dehydrohalogenation reactions, the inductive parameter p, was approximately 9, indicating that reactivity increases sharply with increasing Sσ₁, whereas for pH‐indepen‐dent dehydrohalogenation reactions, rates were insensitive to inductive effects. Product information was rarely available for the pH‐independent reactions, making it difficult to rule out the possibility that the apparent absence of polar influences may result from attempting a correlation on substrates that react via different mechanisms. We believe, however, that these pH‐independent reactions do in fact reflect E₂ reactions, with H₂O participating as the effective base: a regression on the Br⊘nsted coefficient β (calculated from the base‐promoted and pH‐independent reaction rates) vs. Sσ₁ yields a fit with an intercept not significantly different from 0. Furthermore, β values predicted from our LFER results compare reasonably well with β values measured in aqueous solution by a variety of techniques. Our studies provide parameters needed for preliminary estimates of dehydrohalogenation reactivity for new compounds. In addition, they furnish information useful in designing future experiments in buffered aqueous solution or in reassessing existing data, such as an estimate of the pH at which the observed dehydrohalogenation rate changes from domination by a pH‐inde‐pendent to a base‐promoted mechanism.

Key words:

• elimination reactions
• dehydrohalogenation reactions
• structure‐activity relationships
• chloroalkanes
• bromoalkanes