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Reactivity of Substituted Benzotrichlorides toward Granular Iron, Cr(II), and an Iron(II) Porphyrin:  A Correlation Analysis

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journal contribution
posted on 01.07.2006, 00:00 authored by Tamar Kohn, William A. Arnold, A. Lynn Roberts
Cross-correlations of rate constants between a system of interest and a better-defined one have become popular as a tool in studying transformations of organic pollutants. A slope of unity (if the correlation is conducted on a log−log basis) in such plots has been invoked as evidence of a common mechanism. To explore this notion, benzotrichloride and several of its substituted analogues were reacted with Cr(H2O)62+, an iron(II) porphyrin (iron meso-tetra(4-carboxyphenyl)porphine chloride, Fe(II)TCP), and granular iron. The first two reductants react with organohalides by dissociative inner sphere single-electron transfer, while mechanism(s) for organohalide reduction by granular iron are still debated. Apart from sterically hindered compounds, good correlations were obtained in comparing any two systems, although slopes (on a log−log basis) deviated from unity. We argue that a slope of unity is neither necessary nor sufficient evidence of a common mechanism. Overall rate constants may be composite entities, consisting in part of rate or equilibrium constants for adsorption onto surfaces or for precursor formation in solution; these components may differ between systems in their susceptibility to substituent effects. Cross-correlations may prove useful in predicting reactivity in the absence of steric effects, but additional evidence is required in deducing reaction mechanisms.