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|Title:||Cleavage of the aryl-germanium bonds.|
|Authors:||Pande, Kailash Chandra.|
|Presented at:||University of Leicester|
|Abstract:||Sixty two new organic derivatives of the IVB Group metals have been prepared. A linear relationship has been observed between the refractive indices of aryltriethylgermanes and those of analogous aryltrimethylsilanes. The rates of cleavage of a large number of aryltriethylgermanes have been measured either in methanol-perchloric acid-water or in acetic acid-sulphuric acid-water. The results of these cleavage studies provide a detailed picture of substituent effects in an electrophilic aromatic substitution, and these effects are discussed in relation to substituent effects in other such substitutions. The rates of cleavage of some mono-substituted phenyltricyclohexyl pluiribanes, XC6H4Pb (C6H11)3, have been measured in a ethanol-perchloric acid-water medium. Electron-releasing substituents were found to increase the rate of cleavage and electron-withdrawing substituents to retard it. The effects of substituents in this cleavage could not be correlated with those in acid cleavage of trimethylphenylsilanes or of triethylphenylgermanes but a linear free-energy relationship was found to exist between the substituent effects in cleavage of the tricyclohexylphenylplumbanes and those in cleavage of tricyclohexylphenylstannanes. It is suggested that in the cleavage of aryl-Pb and aryl-Sn bonds the +E effects of the substituents operate less effectively than in most electrophilic aromatic substitutions. The rates of cleavage of some ArMR3 compounds, where M = Si, Ge, Sn and Pb, by aqueous ethanolic perchloric acid have been measured. The relative reactivities of PhMEt3 compounds were found to be approximately (M=) Si 1, Ge 36, Sn 2.5 x 105, Pb 2 x 108, a reactivity sequence which appears to be unrelated to the electronegativities of the metals concerned. It is suggested that the predondnant effects in the series (M=) Si, Ge, Sn and Pb may be increasing availability of d-orbitals in the valency shell and decreasing C-M bond strength. Possible mechanisms for the cleavage are discussed. The apparent dissociation constants of the acids, P-XCH2C6H4C02H (where X = H, Me3Ge and Me3Si), have been determined spectrophotometrically in 39 wt. % aqueous methanol, and the rates of cleavage of the compounds, XCH2C6 H4SiMe3 (where X = H, Me3Ge and Me3 Si) have been measured in a mixture of methanol and aqueous perchloric acid. It is concluded from the results that electron release by the group X is in the order (X=) Me3Ge> Me3 Si> H.|
|Rights:||Copyright © the author. All rights reserved.|
|Appears in Collections:||Theses, Dept. of Chemistry|
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