Reactivity of substituted benzotrichlorides toward granular iron, Cr(II), and an iron(II) porphyrin: A correlation analysis.
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)6(2+), 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. Apartfrom 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.
Kohn
Tamar
T
Department of Geography and Environmental Engineering, Johns Hopkins University, 313 Ames Hall, 3400 North Charles Street, Baltimore, Maryland 21218-2686, USA.
Arnold
William A
WA
Roberts
A Lynn
AL
D002857:Chromium / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D007700:Kinetics; D008665:Metalloporphyrins / Q000737:chemistry*; D008956:Models, Chemical; D010084:Oxidation-Reduction; D012044:Regression Analysis; D014050:Toluene / Q000031:analogs & derivatives* / Q000737:chemistry
D008665:Metalloporphyrins; C515260:iron meso-tetra(4-carboxyphenyl)porphine chloride; D002857:Chromium; D014050:Toluene; D007501:Iron; C018847:benzotrichloride