Published work on the cleavage of carbon-silicon bonds has boon reviewed, with particular reference to cleavage by alkaline and other nucleophilic reagents. Tho apparent dissociation constants of several acids of the type -R3SiCH2C6H4CO2H (where R is alkyl or aryl) p have been determined opectrophotometrically in aqueous methanol, and the results are discussed. Tho alkaline cleavage of trobenzyltrimethylsilane has been examined in aqueous dioxane. At high water concentrations increases in solvent polarity retard the cleavago, as required by a mechanism involving charge dispersion in the transition state. At high dioxano concentrations, increases in solvent polarity are accompanied by increases in the rate of reaction, and this is suggested to result from association between the solvent components. In cleavage of o-nitrobenzy1trimothylsilane by aqueous methanol it has been shown that there is no appreciable salt effect. Addition of various nucleophilic reagents has shown that nucleophilic power towards carbon is not an indication of nucleophilic power towards silicon. A kinetic study of the effect of 3ubstituent3, X, on the ease of alkaline cleavage of the bonzyl-silicon bond in (C113)3SiCH2C6H4X, has, conßirnted that electron-withdrawal in X accelerates the reaction, In cleavage of anions ofthe type n-R3SiCH2C6H4CO (where R iss alkyl or aryl) , the reaction is retarded by electron-release in R, and accelerated by electron-withdrawal in R. Bulky groups attached to silicon slow the reaction by storically hindering; hydroxide ion attack on i. licon. Similar conclusions were reached with cilanou of the typo p-R3SiCIi2C6H4C1 (where R is alkyl or aryl), and furthermore, in- compounds of the type p-XC6H4(Cii3) 2S'CH2C6H4Cl, electron-supply in X rotardD, and electron-withdrawal in X accelerates the reaction. All the experimental results are consistent with a mechanism involving nucleophilic attack on cilicon, either by a synchronous bimolecular (Sn2) process, or equally well, by one involving initial formation of a pontacovalent silicon inteztcdiat©.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:547621 |
| Date | January 1957 |
| Creators | Allcock, H. R. |
| Publisher | University of London |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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