Structure and order in thermal dehydrations of crystalline solids

Criteria capable of classifying solid-state dehydrations are explored with a view to determining whether systematic patterns of order can be identifie d within this extensive group of reactions. A classification scheme is prop osed. Recognition of behavioral similarities, and differences, between diff erent reactants can contribute towards theory development in a field where reactivity controls and factors that determine mechanisms have not yet been established. At present, many fundamental studies of reactions in which wa ter is evolved from crystalline reactants remain unrelated to other, genera lly similar, rate processes and do not contribute to the development of an ordered subject. It is argued that the establishment of interrelationships between hitherto individual and unrelated chemical changes may be capable o f introducing coherence into an important and active research area that is currently composed of mainly separate contributions. The classification scheme proposed here discusses the role of extended crys tallographic structures in controlling the course of the bond rearrangement s that occur during the conversion of a solid reactant into solid products. It is suggested that this feature of solid-state processes has been accord ed less attention than it deserves in mechanistic proposals that usually fo cus attention on the movement of individual bonds. This use of extended fea tures of the reactant (lattice) can be regarded as a chemical analogue of t he development of band theory used to complement the valence bond model in theoretical explanations of the physical properties of solids. Such conside ration of the less localized structural properties of crystalline reactants enables progress to be made towards classifying the many and diverse water elimination reactions identified as 'dehydrations'. Shortcomings in the theory currently available for application to dehydrati ons are considered in the context of the problems that arise when interpret ing experimental data. Kinetic measurements for these reactions are often s ensitive td experimental conditions due to the influences of reversibility and of endothermicity. Problems in the characterization of structures at ac tive reactant/product contacts and in identification of the factors that de termine reactivity and rates of the interfaces steps that control water eli mination are mentioned in the context of current theory. Aspects of the dev elopment of a theoretical framework that increases our understanding of deh ydrations are discussed critically. The provision of an acceptable classifi cation scheme is regarded as an essential step to advance theory and unders tanding of dehydrations and other crystolysis reactions, (C) 2000 Elsevier Science B.V. All rights reserved.