Solvolytic eliminations

Phenyl acetylene and acetophenone are also the reaction products observed when /3-bromostyrene is solvolysed in aqueous ethanol at 100°C . This reaction is also strongly accelerated by electron-releasing aromatic substituents but electron-withdrawing substituents render the substrates almost inert. The possibility that the ketone arises from the addition of water to the olefin followed by an Eco reaction (see Section 4.3) seems unlikely, as the rate of solvolysis is not influenced by varying the concentration of added triethylamine although this reagent induces bimolecular elimination from the strongly acidic p-nitro substrate. Moreover, as silver-ion catalysis is observed, it seems that the mechanism is of the El type, (214). 

It would be interesting to investigate the analogous reactions of the a-chloro-and a-iodo-styrenes. Halogen-exchange experiments might also prove useful in establishing a carbonium-ion intermediate -.  

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It could be claimed that such an El solvolytic elimination would be indistinguishable kinetically from a bimolecular (E2) elimination, in which EtOH was acting as base, because the [EtOH] term in the E2 rate law,... 

Full details of a study of leaving group-promoted solvolytic elimination reactions of 1-(1-methyl-l-arylethyl)pyridinium cations in 25 vol.% acetonitrile (aqueous) have been reported. Reactions of (34) and (35) are found to proceed via a common carbocation intermediate of ion-molecule pair type to give the suhstitution product (36) and elimination product (37) (Scheme 4). The total rate of reaction of (35) exceeds that for (34) by 1100-fold, corresponding to a Bronsted parameter of )S g = —0.93, and the fraction of (37) obtained is governed by = 0.12 for the dehydronation (kg) of the ion-molecule pair by the leaving group the product ratio is hardly affected hy the presence of substituted pyridines. For (34) and (35), = 1.85 0.10 (60 °C) and... 

A similar explanation has been suggested to account for the variation in the product distribution from the decomposition of the 2-phenyl-2-butyl cation generated from a variety of substrates-. In solvolytic elimination reactions of l,l,4.4-tetramethylcyclodecyl-6- or -7-tosylates jy/i-elimination appears to predominate in both acidic and basic solvents, an ion pair mechanism being suggested to account for the observed results . It is obviously unsound to generalise when considering stereochemistry of El reactions. 

To explain the predominance of anti stereospecificity in dehydration over alumina, participation by both acidic and basic sites of the catalyst is invoked. Furthermore, as the alumina must surround the substrate, it has been regarded as a solvating agent and an analogy to solvolytic elimination has been drawn ... 

While there is a paucity of information on neighboring sulfur involvement in elimination reactions, the results of McCabe and Livingston suggest participation in the solvolytic elimination of hydrogen chloride from the chloroalkene (91) and in the pyrolytic elimination of adipic acid from the adipate ester (92). 

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