Topochemical reaction

A tantalum substrate has been dipped in molten silver so that it is partially covered by a thin adherent layer of solid silver. Just as in Tammann s [48] original experiment, the silver is iodized in an atmosphere of iodine gas. The Agl surface layer grows according to a parabolic rate law. Since Agl is virtually a purely ionic conductor (i.e. 1), the product layer will be 

It is important to note here that the average values (designated by bars) are not arithmetic means of the values at the Ag/AgI and Agl/l2 boundaries, but rather, they are averages with respect to chemical potential across the Agl layer as in eq. (8-9). The activity dependence of the transport coefficients is determined by the disorder type and by defect thermodynamics, as has been discussed many times in this monograph. 

There are many solid state reactions with interesting morphologies in which other modes of transport besides ionic transport in the reaction product can be very important. As a first example, let us consider a reaction which normally occurs by the counter-diffusion of cations, as illustrated in Fig. 2-1 for the formation of spinels in the reaction couple AO/AB2 O4/B2 O3. It was assumed in this case that the mobility of the oxygen ions is negligible. However, if the reactant oxides AO and B2 O3 are very porous, then the reaction product AB2 O4 will also be porous. Furthermore, if the reaction product is an electronic conductor 1), as is generally 

The reaction rate will then be determined by the diffusion of the faster of the cations, since the transport number of the electronic charge carriers is essentially unity, and, for a sufficiently high porosity, the oxygen gas will always be available everywhere at the reaction front. This reaction mechanism is illustrated in Fig. 6-3 where the differences between this and other reaction mechanisms can clearly be seen. It appears that such a reaction mechanism involving the gas phase probably occurs during the production of ferrites or chromites [37]. 

The numerical factor 8 in eq. (8-37) reflects the fact that the reaction mechanisms at the surface and in the interior are different, so that the reaction volumes for the passage of one ion equivalent are not the same in each case. 

Essentials of Inorganic Materials Synthesis, First Edition. C.N.R. Rao and Kanishka Biswas. 2015 John Wiley Sons, Inc. Published 2015 by John Wiley Sons, Inc. 

Topochemical dehydration has been used for sometime to prepare new metastable solids (e.g. the synthesis of Ti Nb O, from HTiNbOj [12]). This strategy has been extended to perovskites [13]. Thus, upon heating of a solid acid with 

FIGURE 10.1.4 Different WO3 phases obtained by the dehydration of WOj l/SH O at different temperatures. 

FIGURE 10.1.5 Schematic arrangement of MnO octahedra and MnOj square-pyramids in CaMnO, 

A typical example is given by the following reaction of methyl transfer in Eq. (7.6), similar to intramolecular rearrangements of Eqs. (7.1), (7.3) considered earlier  

In crystal, this reaction proceeds at a considerable rate even at room temperature, whereas in concentrated solutions under heating and in melt it develops rather slowly [20]. The reason for such behavior becomes clear from 

X-ray structural data [19,20]. The molecules XVIa are packed in parallel stacks in which methyl sulfonyl and amino groups lie pairwise one over another in conformations which exactly correspond to steric demands of the transition structure of type III for cooperative methyl 0, iV-transfer. 

Understanding of the nature of many other known—see the reviews of Refs. [18,19, 21]—topochemical reactions and prediction of the new ones are based on the principle of correspondence of the crystal packing peculiarities to steric demands of the intrinsic mechanism of reactions. 

Tenood L, Farooq S, Seibl J, Eshenmoser A (1970) Helv Chim Acta 53 2059 

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If the flow is accompanied with CBA decomposition, the G value in Eq. (5) should be substituted with its time function, G(t). In the general case, thermal decomposition of a solid substance with gas emission is a heterogeneous topochemical reaction [22]. Kinetic curves of such reactions are S -shaped the curves representing reaction rate changes in time pass a maximum. At unchanging temperature, the G(t) function for any CBA is easily described with the Kolrauch exponential function [20, 23, 24] ... 

The intermediate formed in the two-stage decomposition of ZnS04 to ZnO has been variously reported as Zn502(S04)3 [781,798] and Zn30(S04)2 [784,799] reported values of E (Table 15) show little consistency. Decomposition has been described [799] as a topochemical reaction and the reverse process is slow if the ZnO (product) is not finely divided. 

Topochemical reactions of mixed crystals, inclusion complexes and... 

Direct transformation from 2,5-DSP to poly-2,5-DSP through 2,5-DSP oligomer in the crystal is shown in Scheme 1. This new reaction was named a four-centre-type photopolymerization. As well as being the first example of a topochemical reaction in a pure sense, the four-centre-type photopolymerization of 2,5-DSP crystals was the first example of photopolymerization via a step-growth mechanism. 

Along with the guidepost (Wegner, 1972, 1973) based on the crystal-to-crystal transition from 2,5-DSP to poly-2,5-DSP, absolute asymmetric synthesis has been achieved by the topochemical reaction of a chiral crystal of an achiral diolefin compound in the absence of any external chiral reagents. 

This is the first example of a topochemical reaction of a molecular complex of a perfectly ordered polymer composite. Complex 2,5-DSP-l OEt is also obtained by simple grinding of homocrystals 2,5-DSP and l OEt, as is observed for the pair of diolefinic compounds described on p. 166. 

In topochemical reactions all steps, including that of nucleation of the new phase, occur exclusively at the interface between two solid phases, one being the reactant and the other the product. As the reaction proceeds, this interface gradually advances in the direction of the reactant. In electrochemical systems, topochemical reactions are possible only when the reactant or product is porous enough to enable access of reacting species from the solution to each reaction site. The number of examples electrochemical reactions known to follow a truly topochemical mechanism is very limited. One of these examples are the reactions occurring at the silver (positive) electrode of silver-zinc storage batteries (with alkaline electrolyte) ... 

When the course taken by a given solid-state reaction is determined by geometrical details of the crystal lattice, the reaction type falls under the general category of topochemistry. In a topochemical reaction, the reaction takes place in the solid state with a minimum amount of molecular motion. For example, bimolecular reactions are expected to take place between nearest neighbors, which then suggests that the product of the reaction would be a function of the geometric relation in the crystal structure of the reactant molecules. 

Similar to macroscopic polymer networks, microgels have a more or less fixed surface. Due to the large value of their surface/mass ratio, microgels may be used as models for studying topochemical reactions at polymer surfaces. 

When the molar volume of product, i.e. solid (AB) is less than that of solid (A), the product layer will be porous and the rate determining step is the chemical process occurring at the interface of solid (A). Such reactions are also known as topochemical reactions. The rate of reaction may be determined by the available surface area of A. For example, if reaction involves spherical particle, the rate of reaction (i.e. - dV/dt, where V is the volume of particle at time t) is given as... 

Monolayer Polymerization. Polymerization of the highly ori-ented monomer films can simply be achieved by UV irradiation under nitrogen (Figure 8.). The polymerization of the diacetylene monomers (2, 5-9) is a topochemical reaction (32,38) that only takes place, if the monomers are perfectly orderecTT i.e. in the crystalline state or in oriented mono- (32) or multilayers (39) and leads to the formation of conjugated Tlue and red colorecT polymer backbones (Eqn. I.). 

Some examples ambipolar diffusion, total rate of topochemical reaction, change in the light velocity when transiting from vacuum into the given medium, resultant constant of chemical reaction rate (initial product - intermediary activated complex - final product). 

C. Topochemical Reactions in Enantiopolar Crystals Cinnamide-Cinnamic... 

This is not a trivial problem, and has important implications for the mechanism of the reaction. However, the bulk of the evidence is for centrosymmetric rings, which would be in keeping with our experience in small-molecule systems. For the present purposes we assume this to be the case. On this basis DSP is one of a class of monomers of crystal structural type 100 that polymerize to polymers 101. Note that, as is typical of topochemical reactions, there are cases of polymorphism of the monomers, in which only those of structure 100 are reactive. Also small changes in the substitution of this molecule frequently result in changes in crystal structure and reactivity. 

Figure 6. Formation of allo-Ge from LiTGei2 by a topochemical reaction and the main structural units of LfGei2, allo-Ge, 4H-Ge and a-Ge.

The prototypes of intra-solid reactions are the [2-1-2] photochemically activated cycloaddition reactions that constitute the basis of Schmidt s work on topochemical reactions [36]. It is now clear, however, that the topochemical postulate, though correct and of wide applicability, cannot account for many solid-state processes, in particular those implying high molecular or ionic mo-... 

Most of the topochemical reactions, including the first finding of the topochemical polymerization of 1, were found accidentally or beyond any expectation [ 14]. During subsequent and extensive investigation, numerous studies have been carried out by trial-and-error approaches to establish any empirical rule accounting for a correlation between the geometry of functional moieties and reactions in the solid... 

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