Journal of the Chemical Society. Perkin transactions II p. 979 - 982 (1983)
Update date:2022-08-28
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Chevrier, Marianne
Guillerez, Jean
Dubois, Jacques-Emile
In neutral water, the interconversion of the two tautomers of the lactim-lactam equilibrium of 6-methoxy-2-pyridone occurs essentially via a pH-independent process.The influence of temperature on the position and on the dynamics of this tautomeric equilibrium has been studied by temperature-jump relaxation kinetics.The equilibrium and activation thermodynamic parameters obtained (ΔH0 26.8 kJ mol-1, ΔS0 58 J K-1 mol-1; lactam -> lactim, ΔHC(excit.) 46 kJ mol-1, ΔSC(excit.) -40 J K-1 mol-1) indicate a high value for the kinetic entropy term.These results indicate that the interconversion mechanism is ionic (rather than concerted) and involves the anionic form of the substrate in a cyclic transition state.In this transition state, at least two solvent molecules would temporarily ensure a hydrogen bond connection between the sites which undergo tautomerism.
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