E2-E1CB BORDERLINE - ELIMINATION-REACTIONS OF 2-(2,4,6-TRINITROPHENYL)ETHYL HALIDES IN AQUEOUS-SOLUTION

The base-promoted elimination reactions of 2-(2,4,6-trinitrophenyl)ethyl halides in aqueous solution have been investigated. The relative rate constant ratios for elimination of fluoride, chloride, bromide, and iodide are 1:1.6:4.7:3.9,1:1,9:4.3:8.5, and 1:2.2:10.2:14.7 for the hydroxide ion, trifluoroethoxide ion, and hexafluoroisopropoxide ion promoted reactions, respectively. This small spread in relative halogen leaving group abilities extends a previously reported trend of decreasing sensitivity to the identity of the leaving halogens as the β-aryl substituent is made more electron withdrawing in the series 2-phenylethyl, 2-(p-nitrophenyl)ethyl, 2-(2,4-dinitrophenyl)ethyl, and 2-(2,4,6-trinitrophenyl)ethyl halides. The fluoride to chloride rate constant ratio, however, shows little or no change for the 2-(2,4-dinitrophenyl)ethyl (2.0) and 2-(2,4,6-trinitrophenyl)ethyl (1.6) derivatives. The reactions are general-base promoted, with Brönsted β values increasing in the order 0.37:0.41:0.49:0.54 for the iodide, bromide, chloride, and fluoride substrates. The Brönsted β values reported here are consistent with a previously reported trend of decreasing β values for strongly activated 2-arylethyl derivatives as the β-aryl substituent is made more electron withdrawing, e.g,, β = 0.61,0.46, and 0.41, for 2-(p-nitrophenyl)ethyl bromide, 2-(2,4-dinitrophenyl)ethyl bromide, and 2-(2,4,6-trinitrophenyl) ethyl bromide, respectively. A similar decrease is observed for the 2-(2,4-dinitrophenyl)ethyl (β = 0.43) and 2(2,4,6-trinitrophenyl)ethyl (β = 0.37) iodides, but little or no change is observed for the corresponding chlorides and fluorides (β.sim.0.50). Rate constants for the hydroxide ion promoted reaction are about 100-fold smaller than expected on the basis of reactions of other 2-arylethyl derivatives, including the p-nitrophenyl and the 2,4-dinitrophenyl compounds, and suggest steric hindrance to resonance by the two o-nitro groups. Taken together, these results are consistent with an E2 (AxhDHDN) mechanism for the iodide and bromide substrates and a reaction coordinate that has a large component of hydron transfer. The mechanisms of reaction of the fluoride and chloride substrates are, however, not established by these results, but a change to an ElcBirr (AxhDH + DN) mechanism appears possible. © 1990, American Chemical Society. All rights reserved.