A Study on the Relative Stabilities of Selected Isomers of 1,3-Dimethyl-2,4-Pyrimidinedione and 2,4-Pyrimidinedione. The Effect of The Molecular Environment on Selected Lactim-Lactam Equilibria
White, James Matthew
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https://hdl.handle.net/2142/70186
Description
Title
A Study on the Relative Stabilities of Selected Isomers of 1,3-Dimethyl-2,4-Pyrimidinedione and 2,4-Pyrimidinedione. The Effect of The Molecular Environment on Selected Lactim-Lactam Equilibria
Author(s)
White, James Matthew
Issue Date
1982
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Organic
Abstract
The first experimentally based estimates for the relative stabilities in the vapor phase of 2,4-pyrimidinedione (1), 2,4-dihydroxypyrimidine (6), 4-hydroxy-(1H)-2-pyrimidone (3) are for 1-6 -22 (+OR-) 8.8 kcal/mole and for 1-3 -19 (+OR-) 6 kcal/mole. These values were derived from the free energies of isomerization of the methyl analogues 1,3-dimethyl-2,4-pyrimidinedione (7)-2,4-dimethoxypyrimidine (12) of -38 (+OR-) 4.2 kcal/mole and 4-methoxy-1-methyl-2-pyrimidone (9)- 7 of -27 (+OR-) 4.1 kcal/mole. The free energies of isomerization in the vapor for the methyl systems 12-7 and 9-7 were obtained by calorimetric measurement of the heats of isomerization in the neat liquids of -44 (+OR-) 2.6 and -24 (+OR-) 1.3 kcal/mole respectively and subsequent conversion to the gas phase with estimated heats of vaporization. It is noted that in these systems the energy differences between the dilactim or lactim and the dilactam structure was much greater than the energy difference between the dilactim and lactim structures. This behavior is qualitatively rationalized in terms of the delocalization energy present in the dilactam structures due to the presence of a second carbonyl. The comparison of the experimentally calculated relative stabilities of 6-1 and 3-1 with those calculated by molecular orbital methods showed none of the theoretical calculations to be satisfactory.
A quantitative model to describe the effect of the molecular environment upon lactim-lactam equilibria in terms of the specific solute-solvent interactions of reaction field stabilization and hydrogen-bonding was found to correlate the solvent effect on the equilibria for four systems. The attempts to use the model to correlate the solvent effect in the pyrimidone case, for other equilibria and for kinetic phenomena was successful in relatively a few systems and is reviewed.
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