Rochow synthesis

Keywords compensation effect, isokinetic effect, Rochow synthesis, transition metal silicide phases, selective energy transfer model 

Summary Both in the Rochow synthesis of methylchlorosilanes and in the reaction of transition metal silicides with HCl, catalytic reactions of silicon, bound as metal silicide, with gaseous reactants are involved. With both reactions, the kinetic parameters ko and Ea. exhibit consequent compensation effects, with the isokinetic temperature positioned within the range of reaction temperatures investigated. In this paper, we apply the model of selective energy transfer from the catalyst to adsorbed species to the kinetic data. With Rochow synthesis Si-CHs rocking frequencies, and with hydrochlorination of silicides Si-H vibration frequencies could correspond to the isokinetic temperatures observed. An interpretation in terms of accessibility of the reactive silicon atom to reactant molecules is given. 

Analyzing kinetic data of closely related reactions, the so-called compensation effect or, better, the isokinetic effect has often been found in chemistry, especially also in gas-solid reactions and heterogeneous catalysis, e.g., [1,2]. Provided validity of the Arrhenius equation 

That means that the phenomenon isokinetic effect is indicated by the observation that the Arrhenius lines of the closely related reactions intersect in only one point, characterized by the isokinetic pre-exponential factor kso and by the isokinetic temperature fso, or, in other words, there is a linear relationship between In ko and Ef, with the slope l/RTiso- 

In this paper, we would like to present two examples of the compensation effect in direct reactions of silicon and to discuss both of them in terms of the concept of Larsson [3 - 5], which is one of the established theoretical concepts to explain the occurrence of the compensation effect. 

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The Miiller-Rochow-Synthesis [16,17] (direct synthesis of methylchlorosilanes) provides as byproduct a high boiling fraction consisting essentially of 1,1,2-trimethyltrichlorodisilane and 1,2-dimethyltetrachlorodisilane [18]. Starting with these disilanes Wacker-Chemie has developed different ways to produce silicon carbide [19, 21] and silicon carbonitride [22] fibers. 

Compensation Effect with Rochow Synthesis of Methylchlorosilanes... 

Table 1. Parameters of the compensation effect in Rochow synthesis and in hydrochlorination...

Rochow synthesis Hydrochlorination of transition metal silicides... 

During the last years several synthetic pathways to various polysilane backbones have been extensively studied. One interesting polysilane synthesis has been developed based on the disproportionation of chloromethyldisilanes, which are byproducts of the industrial chloromethylsilane production (Miiller-Rochow Synthesis) [1]. 

With this paper, we want to return to two basic questions of Rochow synthesis, with which we have dealt in the last years and about which we have reported preliminary results ... 

In literature on Rochow synthesis, frequently it is usual to compare stationary state values of reaction rate and selectivities, or mean values over a certain reaction time. Fig. 4 shows that such practice can lead to completely misleading conclusions. In this figure, the catalytic activities of the contact masses CuCySitech, CuCb/Siteoh/Zn, and CuCySipure, p after 20, 60, 120, and 180 min are compared. 

Isolated silicon atoms show a similar behavior compared to the bulk contact mass of the Rochow synthesis. This result may support Okamoto s observation [7], who was able to show by scavenger reactions that the contact mass must contain highly reactive silicon areas which produce chloromethylsilylene. 

Silicones are now most commonly manufactured using the Miiller-Rochow synthesis. Finely ground elementary silicon is reacted with... 

The preparation of silicones is started from dimethyldichlorosilane, which is produced from silicon powder and methyl chloride by the Rochow-synthesis... 

The chlorine is obtained back from the hydrolysis process as hydrogen chloride, which is in turn reacted with methanol to give methyl chloride. In a principially closed loop, the methyl chloride is fed back into the Rochow synthesis. 

Uses The main use of CH3CI is in the production of silicones by the Rochow synthesis (see p. 179). In organic chemistry, it is u.sed as a methylating agent giving ethers of phenols, alcohols, and cellulose (methyl cellulose). Other uses are in Friedel-Crafts - reactions to give alkylbenzenes, in the production of quarternary ammonium salts and methylmercaptan. 

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