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Estimation of a p K a for Protonated Dimethoxycarbene
Author(s) -
Guthrie J. P.,
More O'Ferrall R. A.,
O'Donoghue A. C.,
Waghorne W. E.,
Zrinski I.
Publication year - 2003
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.663
Subject(s) - chemistry , carbene , protonation , methanol , carbocation , reaction rate constant , acetonitrile , solvent , medicinal chemistry , thermodynamics , organic chemistry , kinetics , catalysis , ion , physics , quantum mechanics
Abstract A p K a is estimated for the protonation of dimethoxycarbene based on the relationship p K a = p K R −p K 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 p K a = 15.5 in aqueous solution is derived based on p K R = −5.7 for the carbocation and an estimate of p K H 2 O = −21.2 based on a heat of formation of the carbene Δ H ° f = −38.8 ± 2 kcal mol −1 , calculated at the G3 level in the gas phase. This value for the p K a compares satisfactorily with p K a = 11 inferred from the dependence of experimental rate constants for reaction of the carbene with alcohols and acetic acid upon the p K 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. Copyright © 2003 John Wiley & Sons, Ltd.