Structure and order in thermal dehydrations of crystalline solids

Abstract

Criteria capable of classifying solid-state dehydrations are explored with a view to determining whether systematic patterns of order can be identified within this extensive group of reactions. A classification scheme is proposed. Recognition of behavioral similarities, and differences, between different 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 water is evolved from crystalline reactants remain unrelated to other, generally 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 of 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 crystallographic structures in controlling the course of the bond rearrangements that occur during the conversion of a solid reactant into solid products. It is suggested that this feature of solid-state processes has been accorded less attention than it deserves in mechanistic proposals that usually focus attention on the movement of individual bonds. This use of extended features of the reactant (lattice) can be regarded as a chemical analogue of the development of band theory used to complement the valence bond model in theoretical explanations of the physical properties of solids. Such consideration 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 dehydrations are considered in the context of the problems that arise when interpreting experimental data. Kinetic measurements for these reactions are often sensitive to experimental conditions due to the influences of reversibility and of endothermicity. Problems in the characterization of structures at active reactant/product contacts and in identification of the factors that determine reactivity and rates of the interfaces steps that control water elimination are mentioned in the context of current theory. Aspects of the development of a theoretical framework that increases our understanding of dehydrations are discussed critically. The provision of an acceptable classification scheme is regarded as an essential step to advance theory and understanding of dehydrations and other crystolysis reactions.