Topotactic Reactions in Organic Crystals.

Crystalline organic compounds have been observed to react in a number of cases in a manner which suggests that the process is dependent on the geometry of the disposition of reactive groups in the crystal lattice. Such a process, first observed with inorganic reactions, has been designated by Kohlschuetter' as "topochemical," and several criteria may be used to show that a reaction falls into this category. The first depends on the demonstration that the reactivity of chemically similar crystalline substances is unrelated to the relative reactivity of these substances in the liquid state. A particularly powerful argument is provided if different crystalline modifications of the same reagent differ widely in their reactivity. A striking example of such effects was provided in detailed studies of the radiation-induced decomposition of choline chloride to acetaldehyde and trimethylammonium chloride. It was found that other choline salts do not react at comparable rates, that such minor alterations in the molecule of the reagent as the substitution of an ethyl for a N\-methyl group eliminates the reactivity, that dissolved choline chloride is quite unreactive,2-' and that an allotropic transformation in the temperature range of 73-780C eliminates the reactivity of crystalline choline chloride.5 6 The radiation-induced polymerization of alkali acrylates7 or of different modifications of calcium acrylate8 was also observed to proceed at vastly different rates, although the reactive species is the acrylate ion in all these cases. Another argument for topochemical control of a solid state reaction is the demonstration that the reaction product obtained from solid reagents is chemically distinct from the reaction product obtained in the liquid state, or else depends in a series of related solid reagents on properties of the crystal structure. The first report of such an effect in an organic reaction was, as far as we know, made by Sluyterman and his collaborators,9' 10 who found that tetraglycine methyl ester yielded polyglycine and methanol when heated in solution, but led to sarcosylglycine when heated in the crystalline state. MNore recently, a study was carried out of the photodimerization of cinnamic acid derivatives. Two isomeric dimers are possible, one with a center of symmetry and one with a plane of symmetry; it was strikingly demonstrated that the symmetry properties of the crystal structure of the reagent invariably correlated with the symmetry of the reaction product." A detailed review of topochemical effects in reactions of organic crystals will be given elsewhere.12 A different category of topochemical processes includes "all chemical solid state reactions that lead to a material with crystal orientations correlated with the crystal orientation of the reagent" which have been named by Lotgeringt3 topotactic. The phenomenon of topotaxy has been observed fairly frequently with inorganic processes, and this field has been recently reviewed by Glasser et al.14 A typical process of this class is the oxidation of magnetic Fe3O4 to Y-FeO03, which has been shown to l)roceed by the addition of oxygen layers to the surface of the