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Studies in gas chromatography with reference to volatile inorganic compounds



<p>The major part of the work described in this thesis is concerned with the development of the techniques of closed-circuit gas chromatography, as originally derived from conventional chromatography by Timms, for the separation and identification of certain volatile silicon and germanium compounds. These techniques were devised to extend the work, classically carried out in high-vacuum distillation systems, to more complex mixtures and smaller samples.</p> <p>In addition to the development work on the basic method, development work was carried out on components and systems, in particular, detector circuitry, a cheap and simple method of temperature control for chromatographic columns, the deoxidisation of nitrogen supplies, and traps for high efficiency removal of volatiles from gas streams.</p> <p>In its final form, the closed-circuit chromatographic apparatus consisted essentially of a preparative gas chromatograph, fitted with two columns and fraction collection traps, and equipped with facility for the recycling of samples, combined with an analytical system. The analytical system consisted of a two-column gas chromatograph equipped with a gas density detector for quantitative analysis of mixtures, provided the components could be identified. Analyses were either effected on products of reactions carried out in external apparatus, or on the products of the reaction of compounds, purified in the preparative system, passed in a nitrogen stream through a reactor containing anhydrous AuCl<sub>3</sub>. This reactor with its ancilliary apparatus was fitted as part of the analytical system.</p> <p>The system of a reactor containing gold chloride heated to 170&deg;C effectively in series with a quantitative gas chromatograph was devised by Timms as a technique for the elucidation of the empirical formulae of the silicon-germanium hydrides, Si<sub>n</sub>Ge<sub>m</sub>H<sub>2(n-hn)+2</sub>, which were quantitatively chlorinated to yield SiCl<sub>4</sub>, GeCl<sub>4</sub> and HCl. In this work this technique was developed for the analysis of compounds of silicon and germanium with alkyl groups. Initially it was attempted to convert the tetramethyls and tetraethyls to the tetrachlorides and chlorinated hydrocarbons by reactions under severe conditions. Although this proved feasible, it was realised that partial chlorination under milder conditions should give structural information as well as empirical formulae, provided that all the products could be identified.</p> <p>When the gold chloride reaction, under a variety of conditions, was carried out on GeMe<sub>4</sub>, and the methyl germanium chlorides, it was discovered that chlorination of the methyl groups could occur without Ge-C bond fission. In particular, loss of methyl groups from GeMe<sub>2</sub>Gl<sub>2</sub> and GeMeCl<sub>3</sub> mainly occurred after a stepwise chlorination to CCl<sub>3</sub>.GeMeCl<sub>2</sub> and CCl<sub>3</sub>.GeCl<sub>3</sub>. All of the members of the series CCl<sub>n</sub>H<sub>3-n</sub>.GeMe<sub>m</sub>Cl<sub>3-m</sub> (n = 1 to 3, m = 0 to 3) were tentatively identified by their reactions and by gas chromatographic data.</p> <p>In view of the large range of products obtained under these intermediate conditions, it was decided to attempt the reaction of gold chloride at temperatures only between 30° and 90°C, under which conditions no side chain chlorination would be expected to occur. As such, the technique was employed to determine the empirical formulae and, in particular, the structures of the permethyl silanes and permethyl germanes, as the anticipated products of Si-Si and Ge-Ge fission only, the methyl silicon and germanium chlorides were not expected to react further to any great extent. Reactions under these conditions proved to be successful and the following compounds were identified:</p> <p align=center>Si<sub>2</sub>Me<sub>6</sub>, Ge<sub>2</sub>Me<sub>6</sub>, Si<sub>3</sub>Me<sub>8</sub>, Ge<sub>3</sub>Me<sub>8</sub> and n-Si<sub>4</sub>Me<sub>10</sub>.</p> <p>The permethyl silanes were prepared by the Wurtz-Fittig reaction of potassium on mixtures of SiMe<sub>3</sub>Cl and SiMe<sub>2</sub>Cl<sub>2</sub> in stainless steel bombs at 125&deg;C, as originally carried out (but using sodium) by Kumada. Fairly high yields, particularly of n-Si<sub>4</sub>Me<sub>10</sub>, were obtained. Although this type of coupling had already been carried out on GeMe<sub>3</sub>Cl, it had not hitherto been carried out on mixtures of GeMe<sub>3</sub>Cl and GeMe<sub>2</sub>Cl<sub>2</sub>. This proved successful with potassium at 125°C for 24 hours, and reasonable yields of the permethyl germanes were obtained. The presence of GeMeCL<sub>3</sub> in the reaction mixtures did not apparently lead to the production of branched-chain volatile products. There was, however, strong evidence for the production of straight-chain compounds of both series with longer chains than those identified by the gold chloride technique. In common with most other true homologous series, plots of logarithmic retention volumes (or proportional values) against 'the number of atoms in the chain' of the known members of the two series were approximately linear. Extrapolation of these plots enabled values to be predicted for the next members of the series. These values were very close to the values actually obtained for compounds prepared in these reactions. In this way the following compounds were tentatively identified in the reaction products:</p> <p align=center>n-Ge<sub>4</sub>Me<sub>10</sub>, n-Si<sub>5</sub>Me<sub>12</sub>, n-Ge<sub>5</sub>Me<sub>12</sub> and n-Si<sub>6</sub>Me<sub>12</sub>.</p> <p>This work was extended to the preparation and identification of a new series of compounds, the permethyl silico-germanes, Si<sub>n</sub>Ge<sub>m</sub>Me<sub>2(n+m)+2</sub>, only the first member of which had previously been prepared. Wurtz -Fittig coupling of mixtures of SiMe<sub>3</sub>Cl, SiMe<sub>2</sub>Cl<sub>2</sub>, GeMe<sub>3</sub>Cl and GeMe<sub>2</sub>Cl<sub>2</sub> by potassium at 130°C for 24 hours, yielded, in addition to the permethyl silanes and permethyl germanes, at least 12 new compounds in quantities (in terms of volatiles recovered) between 0.2% and 6.5%. Similar preparations, with SiMe<sub>3</sub>Cl or SiMe<sub>2</sub>Cl<sub>2</sub> omitted, gave higher yields of specific members of this series of compounds and gave some indication of their structure. This information was conditioned, to an extent, by evidence that some rearrangement of groups at silicon and germanium was taking place under the reaction conditions.</p> <p>The principal method employed for the identification of the permethyl silico-germanes was the gold chloride reaction carried out at temperatures between 30° and 50&deg;C. It was found that samples of the order of 0.8 mg. were sufficient for three separate determinations of the empirical and structural formulae of these compounds. These determinations, under reasonably ideal conditions, gave results within 5% of the predicted values, and on larger samples, within 3%. In addition to this evidence, chromatographic retention data gave an indication as to their probable molecular formulae. Ultimately, a self-consistent hypothesis, confirmed to a large extent by a chromatographic determination of heats of solution, was devised for chromatographic prediction of their structures.</p> <p>The following compounds were identified by the gold chloride technique and the other information as outlined above:</p> <p> <table align=center cellspacing="0" cellborder="0"> <tr> <td></td> <td></td> <td></td> <td>SiMe<sub>3</sub>.GeMe<sub>3</sub></td> <td></td> <td></td> </tr> <tr> <td></td> <td></td> <td>SiMe<sub>3</sub>.GeMe<sub>2</sub>.SiMe<sub>3</sub></td> <td></td> <td>GeMe<sub>3</sub>.SiMe<sub>2</sub>.GeMe<sub>3</sub></td> <td></td> </tr> <tr> <td></td> <td></td> <td>SiMe<sub>3</sub>.GeMe<sub>2</sub>.SiMe<sub>3</sub></td> <td></td> <td>GeMe<sub>3</sub>.GeMe<sub>2</sub>.SiMe<sub>3</sub></td> <td></td> </tr> <tr> <td></td> <td>SiMe<sub>3</sub>.</td> <td>SiMe<sub>3</sub>.GeMe<sub>2</sub>.SiMe<sub>3</sub></td> <td></td> <td>GeMe<sub>3</sub>.GeMe<sub>2</sub>.GeMe<sub>2</sub>.SiMe<sub>3</sub></td> <td></td> </tr> <tr> <td></td> <td>SiMe<sub>3</sub>.</td> <td>SiMe<sub>3</sub>.GeMe<sub>2</sub>.SiMe<sub>3</sub></td> <td></td> <td>GeMe<sub>3</sub>.GeMe<sub>2</sub>.SiMe<sub>2</sub>.GeMe<sub>3</sub></td> <td></td> </tr> <tr> <td></td> <td>SiMe<sub>3</sub>.</td> <td>SiMe<sub>3</sub>.GeMe<sub>2</sub>.SiMe<sub>3</sub></td> <td></td> <td>GeMe<sub>3</sub>.SiMe<sub>2</sub>.SiMe<sub>2</sub>.GeMe<sub>3</sub></td> <td></td> </tr> <tr> <td>and &nbsp;</td> <td>SiMe<sub>3</sub>.</td> <td>SiMe<sub>3</sub>.GeMe<sub>2</sub>.SiMe<sub>3</sub></td> <td align=center>and/or</td> <td>SiMe<sub>3</sub>.GeMe<sub>2</sub>.SiMe<sub>2</sub>.GeMe<sub>3</sub></td> <td></td> </tr> </table> </p> <p>There was also chromatographic evidence for the presence of at least 10 isomeric forms of compounds with the following formulae:</p> <p align=center>n-Si<sub>4</sub>GeMe<sub>12</sub>, n-Si<sub>3</sub>Ge<sub>2</sub>Me<sub>12</sub>, n-Si<sub>2</sub>Ge<sub>3</sub>Me<sub>12</sub> and n-SiGe<sub>4</sub>Me<sub>12</sub></p> <p>For the preparation of the permethyl germanes and the permethyl silico—germanes it was necessary to prepare GeMe<sub>3</sub>Cl and GeMe<sub>2</sub>Cl<sub>2</sub> as reactants. Two methods for the preparation of these compounds were developed from existing techniques. The first of these was the 'rearrangement' reaction of GeMe<sub>4</sub>, with GeCl<sub>4</sub>, originally carried out with limited success in the presence of large amounts of AlCl<sub>3</sub>. In this work the reaction was attempted with small amounts of catalyst, in some cases prepared in situ, e.g. by the slow reaction of aluminium with GeCl<sub>4</sub>, in the presence of mercury. The reaction was found on occasion to work well with traces of AlCl<sub>3</sub> at 250°C, but was sometimes drastically, and rather unpredictable, inhibited, possibly by moisture. The reaction was shown to be either autocatalytic or catalysed by elemental germanium, to a small extent, at 250&deg;C. Mercurous chloride was found to be a good catalyst for the primary reaction to GeMe<sub>3</sub>Cl and GeMeCl<sub>3</sub>, but indifferent for further reaction to GeMe<sub>2</sub>Cl<sub>2</sub>. A mixture of aluminium and calomel was successfully used for the preparative work, presumably as it reacted to form a mixture of both successful catalysts. With this catalyst it was shown that an equilibrium prevented the formation of high yields of</p> <p> <table cellborder="0" cellspacing="0" align=center> <tr> <td>GeMe<sub>2</sub>Cl<sub>2</sub>:</td> <td></td> <td></td> <td></td> </tr> <tr> <td></td> <td>K = </td> <td align=center>[GeMe<sub>2</sub>Cl<sub>2</sub>] <sup>2</sup><Hr noshade size=1>[GeMe<sub>3</sub>Cl], [GeMeCl<sub>3</sub>]</td> <td>= 22 at 270&deg;C</td> </tr> </table> </p> <p>The equivalent constant for the formation of GeMe<sub>3</sub>Cl was apparently much higher. The preparative work was carried out in stainless steel bombs but some of the experimental work was carried out in small sealed glass tubes which established that the stainless steel was also having some effect on the reaction.</p> <p>The second preparative technique employed for the methyl germanium chlorides was the 'direct reaction' of methyl chloride with germanium in the presence of copper at 360&deg;C. This reaction, normally carried out as a streaming method, was carried out in a stainless steel bomb. It appeared to take place almost quantitatively and in view of the presence of germanium, and the high temperature employed, it was presumed that equilibrium had been reached. The product ratio indicated that K (see above) for GeMe<sub>2</sub>Cl<sub>2</sub> was approximately 53 at 360°C.</p> <p>The last part of this work was concerned with the design, construction and operation of a chromatograph for operation at cryogenic temperatures. The principal feature of this apparatus was the temperature controlled air bath in which the chromatographic columns were mounted. This consisted of a long vertical glass tube, with an internally mounted wall heater, which was immersed in liquid nitrogen. The air bath was stirred by a series of fans powered by a small electric motor and the temperature was measured by a thermocouple. Temperature control was effected by merely varying the current fed to the heater from a stabilised voltage A.C. supply. With this simple system control of the average air bath temperature within &pm;0.5°C was easily attained in the range -188° to -90°C with a maximum, but predictable temperature variation along a 2 X 25 cm. column of 2°C. This performance was better than any quoted in the literature. A conventional katharometer detection system was provided for the chromatograph.</p> <p>This chromatograph was used for studies of the interaction of permanent gases and other low-boiling compounds with metallic ions. This was achieved by measuring the heats of adsorption of these gases on various solid phases with surfaces modified with salts containing a common cation. The initial success of this method has lead to its present use by other workers.</p

Year: 1968
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