Comparison of gas-phase acidities of some carbon acids with their rates of hydron exchange in methanolic methoxide

The Competition of the Loss of a Beta Fluoride and Hydron Transfer Between Carbon and Oxygen During Exchange and Elimination Reactions

Reactions of methanolic sodium methoxide with the series p-CF3C6H4C i HClRX [RX is -CH2Cl, -CH2F, -CHClF, -CHF2, -CF2Cl or -CF3] are reported to probe reaction mechanisms for dehydrohalogenation reactions compared to hydron exchange. All of the dehydrochlorinations and dehydrofluorinations from CH2F occur by a two step mechanism via a hydrogen-bonded carbanion intermediate. Dehydrofluorinations from CHF2 occurs via a carbanion no longer stabilized by a hydrogen bond and the fluoride ion is eliminated faster than proton transfer from methanol. Reactions from the CF3 occur from a similar intermediate but exchange of the benzylic proton is faster than the dehydrofluorination. Reactions of ethanolic sodium ethoxide are also reported with the series p-CF3C6H4C i HFRX [RX is -CH2F, -CHF2, -CF2Cl or -CF3]. Density functional calculations using B3LYP/6-31+G(d,p) were used to probe the compounds and intermediates.