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The base-catalyzed elimination reactions of 2-(2,4-dinitrophenyl)ethyl halides in aqueous solution have been investigated.The relative rate constants for the hydroxide ion catalyzed reactions of the fluoride, chloride, bromide, and iodide are 1:2:9:14, respectively, and there is no hydrogen-deuterium exchange into either the fluoride or chloride substrates when the reactions are run in deuterium oxide with deuteroxide ion as the base catalyst. The reactions are general-base catalyzed with Brensted ß values increasing from 0.42 for the iodide to 0.54 for the fluoride, but decreasing as the ß-phenyl substituent is made more electron withdrawing in the series 2-(p-nitrophenyl)ethyl bromide (0.61) and 2-(2,4-dinitrophenyl)ethyl bromide (0.46). These results are consistent with an E2 mechanism for these substrates and a reaction coordinate that has a major proton transfer component, as described on More O'Ferrall-Jencks energy surfaces. The decrease in ß values as the /3-phenyl substituent is made more electron withdrawing reverses previously reported trends of increasing ß values in the series 2-phenylethyl bromide (0.51) and 2-(p-nitrophenyl)ethyl bromide (0.61). This inversion in the trend of ß values means that substrate selectivity undergoes an inversion with increasing substrate reactivity; it is consistent with a clockwise rotation of the E2 reaction coordinate, from one with a major diagonal component for 2-phenylethyl halides to one with a major proton transfer component for the nitro-activated derivatives.

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