Intermolecular Methyl Migration in the Solid State

Abstract

The conversion of ammonium cyanate to urea, discovered by Friedrich Wöhler in 1848, is widely considered to mark the beginning of organic chemistry as a separate field. However, it is not a well-known fact that this particular transformation occurs in the solid state. Chemical reactions in organic crystals are, therefore, certainly not new to the scientific community. However, for many years there was no systematic development of the subject and many solid-state reactions remained untouched and considered to be ‘nature curiosities’. Organic chemical reactions were carried out in solutions and the development of this field was due to the progress in theories and experiments centered on molecular properties and reactivity. The difficulties associated with the understanding of solid-state reactions arise mostly from the fact that the reactivity of the system is a characteristic of the entire assembly of molecules and there were, at that time, no experimental and theoretical methods to explore the structure of such an assembly. The development of the technique of X-ray crystallography provided the means with which the link between the structure of the assembly of molecules and the solid-state reactivity could be established. The basic rules for solid-state photochemistry in the crystal were formulated [1] by using the term ‘topochemical’ which was coined by Hertel [2]. This rule states that a reaction in the solid state occurs with a minimum amount of atomic or molecular movement. Such a statement implies that there should exist an upper limit for such distances beyond which reaction can no longer occur. Nevertheless, many exceptions to this rule have been found and they were classified as non-topochemically controlled reactions. Major obstacles to the progress of the field are the lack of techniques that enable the study of the structures of short-lived intermediates and the shortage of examples of single-crystal-to-single-crystal transformation. Therefore most of the conclusions are based on the crystal structure of the pre- and post-reacted compound. There is no doubt that with a more comprehensive understand of packing and of topochemical effects, solid-state organic reactions could be planned and exploited in organic chemistry.