Reactions of organic solids

In the absence of defects, the reactivity of organic solids is mainly determined by molecular packing. Reactions in which the crystal structure holds sway over intrinsic molecular reactivity are said to be topochemically controlled (Thomas, 1974). A classic example of a topochemically controlled organic reaction in the solid state is the photodimerization of rrans-cinnamic acids studied by Schmidt et al. (see Ginsburg, 

An interesting example of crystal engineering and topochemical reactivity is provided by 2-benzyl-5-benzylidene-cyclopentanone(BBCP) derivatives which undergo single crystal-single crystal photodimerization (Jones et al, 1980). In the monomer 

Polymerization of diacetylene (Fig. 8.13) is one of the most elegant examples of the topochemical principle. Wegner (1971, 1979) showed that diacetylene monomers, R—C = C—C = C— R, polymerize in the solid state by a 1,4-addition reaction at the diacetylene group to produce a polymer that can be represented by the mesomeric structures  

Colourless diacetylene monomer crystals can be polymerized under heat, ultraviolet. X-ray or y-ray irradiation to form single-crystal, highly coloured polyacetylenes. The solid state reaction transforms the entire monomer crystal to polymer crystal without phase separation the polymer forms a solid solution with the monomer over the entire 

Systematic studies of topochemical reactions of organic solids have led to the possibility of asymmetric synthesis via reactions in chiral crystals. (A chiral crystal is one whose symmetry elements do not interrelate enantiomers.) (Green et al, 1979 Addadi et al, 1980). This essentially involves two steps (i) synthesis of achiral molecules that crystallize in chiral structures with suitable packing and orientation of reactive groups and (ii) performing a topochemical reaction such that chirality of crystals is transferred to products. The first step is essentially a part of the more general problem of crystal engineering. An example of such a system where almost quantitative asymmetric induction is achieved is the family of unsymmetrically substituted dienes  

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The scope of reactions of organic solids has been extended over the past decade to heterogeneous multi-phase systems. Here a crystalline solid is exposed to a gas or liquid with the expectation that the crystalline environment will dictate the stereochemistry of the products formed. For such reactions, model systems were chosen in our laboratories which are known to yield different products from different conformations in the dispersed phase. One may anticipate high selectivity for such reactions in the solid-state, for on crystallization the molecule will adopt a single conformation that will eventually determine the stereo-course of the reaction. 

Since single-crystal epr spectra contain information about electron spin distribution it is necessary to know how the electron spin is distributed about the nuclear framework of a radical in order to infer how the radical is oriented. When the spin distributions of isolated radicals are known from index>endent sources, epr gives detailed information about the arramgement of the radicals in pairs. Naturally knowing this arrangement cam be uniquely helpful in detemining the factors which influence reactions of organic solids. 

In spite of these problems, the study of electronic and steric effects in reactions of organic compounds over solid catalysts can be successful, especially when quantitative correlations are attempted (see Section III). The observation of unusual behavior sometimes can be more informative than the standard (expected) influence as it indicates some peculiarities of the mechanism. 

Subsurface environments under anoxic conditions may contain high levels of Fe(II) on the solid phase or dissolved within immobile pore water or groundwater. The role of Fe(II) species in reductive transformation reactions of organic and inorganic contaminants in the subsurface was reviewed by Haderlein and Pecher (1988). A major finding of current studies is that Fe(II) associated with solid phases is much more reactive than Fe(II) present in dissolved forms (e.g., Erbs et al. 1999 Hwang and Batchelor 2000). 

Curtin and Paul [56] contributed substantially also to the investigation of intra-solid reactions between crystals. In this context. Ref [57] constitutes, together with the other references quoted throughout, a useful entry in the recent literature of organic solid-state reactions, including those activated mechanochemically [57]. An early application to the preparation of charge-transfer systems was reported by Toda and Miyamoto [58]. 

In this volume, thermochemical and photochemical reactions in the solid state which have been studied mainly during the last five years by eight research groups are included. The editor hopes that this volume will contribute to the further development of organic solid-state reactions and solid-state chemistry. 

Dislocations are linear defects and were first invoked to account for the mechanical properties of solids, particularly the shear strengths. Dislocations play an important role in a variety of solid state phenomena from phase transitions to chemical reactions and the subject has been investigated and reviewed widely (Fine, 1973 Nembach, 1974). The effect of dislocations on the transformations and properties of organic solids has been recognized in recent years (Thomas Williams, 1971 Jones Thomas, 1979). 

Photochemical reactions in organic solids are important in practical fields as diverse as photography, biology, photoresist technology, polymerization, and the decomposition and stabilization of dyes, energetic materials, pharmaceuticals, and polymers [1], They have been equally important in basic research, particularly for preparing matrix-isolated reactive intermediates for spectroscopic investigation [2]. 

The same type of procedure can be used to estimate AH0 values for many other kinds of reactions of organic compounds in the vapor phase at 25°. Moreover, if appropriate heats of vaporization are available, it is straightforward to compute AH° for vapor-phase reactions of substances which are normally liquids or solids at 25°. The special problems that arise when solutions and ionic substances are involved are considered in Chapters 8 and 11. 

One of the great advantages in studying the catalytic activity of organic solids is that sometimes the heterogeneous reaction may be compared with reactions of the same molecule in solution, by which special solid state effects can be eliminated. An example of this is the study of hydrogen adsorption by anion radicals and di-anions of anthracene in tetrahydrofuran-solution. The following mechanisms have been proposed 43 ... 

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