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The rates of gas-phase pyrolysis of @-substituted ethyl acetates AcOCH2CH2X where X = Si Me,, Si Et,, GeEt31 and SiMe,Ph, and of 1 -aryl-2-trimethylsilylethyl acetates haveeach been measured over a minimum of 50 "C, between 282 and 397 "C; the rates of pyrolysis of 2-aryldimethylsilylethyl acetates have been measured at 396.9 and 378.2 "C. The P-organometallic substituents are not themselves eliminated (except at considerably higher temperatures) but strongly accelerate the normal elimination of acetic acid, the relative rates per p-hydrogen at 327 "C for X being : H, 1 .O ; Si Me,, 125 ; Si Et,, 179 ; GeEt,, 108 ; SiMe2Ph, 144. These groups appear to activate by a combination of increasing the acidity of the P-hydrogen via stabilisation of the forming P-carbanion through (p --.+ d ) x bonding, stabilisation of the incipient a-carbocation by C-X hyperconjugation, and steric acceleration. The effect of substituents in the aryl ring of 1 -aryl-2-trimethylsilylethyl acetates gave an excellent correlation with cr+ values with p -0.52 at 327 "C. The lower p-factor for this reaction compared to that for 1 -arylethyl acetates ( -0.66) is consistent with either conjugative stabilisation of the a-carbocation or increased P-hydrogen acidity. Substituents in the aryl ring of 2-aryldimethylsilylethyl acetates gave a very small positive p-factor indicating that overall their effect on P-hydrogen acidity is larger than that on the forming carbocation. The product of pyrolysis of AcO*CH2*CH2*GeEt, viz. CH2=CHGeEt, underwent increasingly rapid elimination of successive moieties, believed to be ethylene, in a reaction of stoicheiometry 4.0, which did not occur with the silicon analogue.

While we explained the effect of water in terms of electrophilic solvation, as in (a), we were concerned there only with the relative effects of water and methanol.

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