A preliminary study of the displacement of sulphite by tetraphenylborate on trityl derivatives

Collens, Stephen John (1980) A preliminary study of the displacement of sulphite by tetraphenylborate on trityl derivatives. Masters thesis, Memorial University of Newfoundland.

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Abstract

Nucleophilic substitution is and has been an active area of investigation for physical organic chemists. Ingold's original mechanistic descriptions, SN1 and SN2, were initially controversial because of the proposal of ionic intermediates in the SN1 process. Now the controversy is over the lack of intermediates in the SN2 process. Winstein proposed that the SN1 ionization is better described in a stepwise fashion with the substrate forming first an intimate ion-pair with no solvent separation followed by ion-pairs succesively separated by layers of solvent until the kinetically free ions are obtained. Sneen extended this idea and claims that all substrates in nucleophilic substitution reactions ionize at least to the intimate ion-pair stage before nucleophilic attack. Thus, Sneen maintains that SN1 and SN2 are operationally equivalent to two extremes of a common mechanism through which all nucleophilic substitution reactions proceed. Sneen's proposal is criticized due to the difficulty in establishing the existence of the intimate ion-pair in reactions which display SN2 kinetics. -- The present study is concerned with the displacement of sulphite ion by tetraphenylborate ion on two trityl derivatives, namely malachite green and paramethoxymalachite green, using water as the solvent. A mechanistic description is derived, using spectrophotometric methods, which must include two different ionized forms of the substrate (differing by amount of solvent separation) which suggests that the Winstein ionization scheme is followed. Also, this reaction provides a method of measuring the rate of solvolysis of the sulphite ion from these trityl derivatives which has not been done before.

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