Canonical chemical theories exemplified by the methylolation of urea and melamine

Abstract

Old and recent data on equilibria and kinetics of methylolation of urea and melamine are analysed. The theoretical framework affords a sequence, claimed to be canonical, of successive phenomenological approximations. The theory, expounded earlier, begins with two successive schemes whose essentials go back to early work by Pauling and by Flory. The two stages here suffice to show that all the data from eight laboratories, covering the six-membered family of methylol ureas and the ten-membered family of methylol melamines, are in quantitative agreement, a circumstance unsuspected by other workers in the field. The statistical-thermodynamic and kinetic principles involved are therefore expounded in more detail than before. Using them, very small substituent effects (e.g. 0.35 kJ mol^–1) are deduced with high significance and accuracy. Owing largely to the recent measurements by Tomita, the methylol melamines are claimed currently to be the thermodynamically best characterised family in the chemical literature. A challenge arises for theoretical chemists to test quantum-theoretical techniques against the measured energetics of substituent effects in this family, and to exploit for other families the graph-theoretical analysis of molecular additivity which underlies the approximation schemes, which are already well tested.