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Estimation of a pKa for Protonated Dimethoxycarbene

A pK a is estimated for the protonation of dimethoxycarbene based on the relationship pK a =pK R -pK H 2 O , where K H 2 O and K R are equilibrium constants for hydration of the carbene and reaction of the dimethoxymethyl carbocation with water to form dimethoxymethanol, respectively. A value of pK a = 15.5 in aqueous solution is derived based on pK R = -5.7 for the carbocation and an estimate of pK H 2 O = -21.2 based on a heat of formation of the carbene ΔH° f = -38.8′ 2 kcal mol - 1 , calculated at the G3 level in the gas phase. This value for the pK a compares satisfactorily with pK a = 11 inferred from the dependence of experimental rate constants for reaction of the carbene with alcohols and acetic acid upon the pK a of the oxygen acid in acetonitrile, taking account of the difference in solvents for the calculated and measured values. The calculated heat of formation of dimethoxycarbene in the gas phase is compared with an experimental value of -52.6 kcal mol - 1 based on photoacoustic measurements of the heat of reaction of the carbene with methanol solvent to form trimethyl orthoformate. Reanalysis of the experimental data using a heat of solution of trimethyl orthoformate in methanol measured in this work suggests that the discrepancy arises in part from a difference in the heat of reaction of the carbene with methanol in solution and in the gas phase and partly (although less certainly) from a relatively high heat of solution of the carbene in methanol.