Gas-phase SN2 and E2 reactions of alkyl halides

Article abstract of DOI:10.1021/ja00180a003

Rate coefficients have been measured for the gas-phase reactions of methyl, ethyl, n-propyl, isopropyl, tert-butyl, and neopentyl chlorides and bromides with the following set of nucleophiles, listed in order of decreasing basicity: HO^-, CH₃O^-, F^-, HO^- (H₂O), CF₃CH₂O^-, H₂NS^-, C₂F₅CH₂O^-, HS^-, and Cl^-. For methyl chloride the reaction efficiency first falls significantly below unity with HO^- (H₂O) as the nucleophile and for methyl bromide with HS^- as the nucleophile; in both cases the overall reaction exothermicity is about 30 kcal mol^-1. Earlier conclusions that these halides react slowly with stronger bases are shown to be in error. In the region where the rates are slow oxygen anions react with the alkyl chlorides and bromides by elimination while sulfur anions of the same basicity react by substitution. This difference is due to a slowing down of elimination with the sulfur bases; sulfur anions show no increased nucleophilicity as compared to oxy anions of the same basicity. Rate coefficients have also been measured for reaction of methyl fluoride with HO^- and CH₃O^- and ethylene oxide with HO^-, CH₃O^-, and F^-. All of these rates are slow but measurable; combining the results of these experiments with those of the alkyl chlorides and bromides suggests that the gas-phase barrier to the symmetrical S_N2 reaction of F^- with methyl fluoride is lower than previous estimates. We have also measured rates for reaction of allyl chloride with F^-, H₂NS^-, and HS^-, chloromethyl ether with H₂NS^- and HS^-, chloroacetonitrile with F^-, H₂NS^-, HS^-, and ³⁷Cl^-, bromoacetonitrile with Cl^- and ⁸¹Br^-, and α-chloroacetone with H₂NS^-, HS^-, and ³⁷Cl^-. Our results also imply that the gas-phase S_N2 barrier for Br^- reacting with methyl bromide is nearly equal to the ion-dipole attraction energy of the reactants, in agreement with previous estimates.

Full text of DOI:10.1021/ja00180a003

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Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.