Reactions in the solid state ionic crystals

In 1937, dost presented in his book on diffusion and chemical reactions in solids [W. lost (1937)] the first overview and quantitative discussion of solid state reaction kinetics based on the Frenkel-Wagner-Sehottky point defect thermodynamics and linear transport theory. Although metallic systems were included in the discussion, the main body of this monograph was concerned with ionic crystals. There was good reason for this preferential elaboration on kinetic concepts with ionic crystals. Firstly, one can exert, forces on the structure elements of ionic crystals by the application of an electrical field. Secondly, a current of 1 mA over a duration of 1 s (= 1 mC, easy to measure, at that time) corresponds to only 1(K8 moles of transported matter in the form of ions. Seen in retrospect, it is amazing how fast the understanding of diffusion and of chemical reactions in the solid state took place after the fundamental and appropriate concepts were established at about 1930, especially in metallurgy, ceramics, and related areas. 

In recent years, ionic assemblies have received significant attention for [2 -I- 2] cycloaddition reaction in the solid state for several reasons greater stability due to robust and directional charge assisted H-bonded ionic heterosynthons over their neutral assemblies combination of more than one cooperative non covalent interaction in order to stabilise crystal packing, better control of the stoichiometry better solubility and easy separation of the photoproduct from the ionic auxiliary template. 

Crystal structure, crystal defects and chemical reactions. Most chemical reactions of interest to materials scientists involve at least one reactant in the solid state examples inelude surfaee oxidation, internal oxidation, the photographie process, electrochemieal reaetions in the solid state. All of these are critieally dependent on crystal defects, point defects in particular, and the thermodynamics of these point defeets, especially in ionic compounds, are far more complex than they are in single-component metals. I have spaee only for a superficial overview. 

To this point, all the examples presented have been ones in which the origin of the asymmetric induction has been unimolecular in nature, that is, the molecules adopt homochiral conformations in the solid state that favor the formation of one enantiomer over the other, usually through the close intramolecular approach of reactive centers bimolecular crystal packing effects appear to play little or no role in governing the stereochemical outcome of such reactions. This raises the interesting question of whether the soUd-state ionic chiral auxiUary approach to asymmetric synthesis could be made to work for conformationally unbiased reactants, i.e., those possessing symmetrical, conformationally locked structures. Two such cases are presented and discussed below. 

We are cognizant that the structural data on hydrogen bonds derived from crystal structure analyses refer particularly to the hydrogen bond in the solid state. These data are subject to crystal field effects caused by other intermolecular forces. Just as with any discussion of covalent or ionic bonds observed in crystals, these crystal field effects have to be taken into consideration when extrapolating from the precise data available from the crystalline state to the imprecise data that applies to the liquid state in which most chemical and biochemical reactions take place. 

The recently reported correlation of reactivities, and one-electron oxidation potentials of nucleophiles is examined with new data for hydrazine in aqueous solution and several nucleophiles in (CH3)2SO solution. The correlation fails to apply to these reactions. A thermodynamic cycle is utilized to estimate the free energies of ionization of pyronin-nucleophile adducts both in solution and in the solid state. A satisfying rationalization of the dichotomy of ionic and covalent crystals of these and similar compounds is obtained. The equilibrium constants for reactions of nucleophiles with several types of cations are examined as indicators of specific bonding effects such as steric and gem interactions. 

There are other phenomena that do not present unconditional proof of the mechanism. The activation energies of polymerizations in the solid state are often unusually low (see below). Since no activation energy is needed with a radiation-induced start reaction, these low activation energies can also be due to the special conditions in the crystal. The same applies to solid state polymerizations of monomers that can only be polymerized ionically in the fluid phase. 

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