Concerted Bimolecular Substitution Reactions of 1-Phenylethyl Derivatives

Substituted 1-phenylethyl derivatives with σ⁺ > −0.08 exhibit bimolecular substitution reactions with azide ion in 20% acetonitrile in water. The reactions with 1-phenylethyl chlorides follow a Hammett correlation with p = −2.9, compared with p = −5.6 (r⁺ = 1.15) for solvolysis. Swain-Scott correlations give values of r = 0.46 and 0.22 for l-(4-nitrophenyl)ethyl chloride and tosylate, respectively; there are large positive deviations for azide ion and water and negative deviations for cyanide ion. The value of β_nuc is 0.09 for reactions of substituted acetates with the chloride. The reactions exhibit “synergism” between the nucleophile and leaving group that favors the bimolecular reaction with Me₂S, Br⁻ > Cl⁻ > OTs⁻ leaving groups. The bimolecular reaction with azide follows the Grunwald-Winstein Y correlation with m = 0.8 in methanol-water mixtures. Bimolecular reactions with less reactive nucleophiles in the series N₃⁻, CN⁻, AcO⁻, and ROH appear at progressively larger σ values, as the carbocation becomes less stable. It is concluded that these reactions are S_N2 displacements that proceed through an open, “exploded” transition state that closely resembles a carbocation. Specific salt effects are small in water but are significant in acetonitrile-water mixtures and could be mistaken for normal or induced common ion rate depressions. No evidence was obtained for nucleophilic assistance to the formation of a carbocation intermediate. Concurrent S_N1 and S_N2 pathways occur in the reactions with solvent and azide of [l-(4-methylphenyl)ethyl]dimethylsulfonium ion, l-(4-fluorophenyl)ethyl chloride, l-(3-methoxyphenyl)ethyl chloride, and, probably, l-(3-nitro-4-methoxyphenyl)ethyl chloride. Crude estimates of the lifetime of the carbocation intermediate in the presence of the nucleophile are consistent with the hypothesis that the concerted reactions are enforced by the absence of a significant lifetime of the carbocation in the presence of the nucleophile and that stepwise mechanisms are followed when the intermediate has a significant lifetime; the change from a stepwise to a concerted mechanism occurs when the intermediate ceases to have a lifetime in the presence of a nucleophile.