Phase behaviour zwitterions

Perhaps the most spectacular success of explanations based on solvation of ground states, published to date, is the dissection of activation parameters for solvolysis of t-butyl chloride in mixtures of ethanol and water, first discussed by Winstein and Fainberg (1957). The complex variation of AH and AS (Fig. 21) has been shown to be due almost entirely to ground state solvation effects, at least for the solvents ethanol—40% ethanol/water studied by Arnett et al. (1965). For 90%, 80%, 70%, 60%, 50% and 40% ethanol/water the parameter AH1 for solvation of the transition state (by transfer from the gas phase) was calculated to be linearly proportional to the corresponding value of AS, as expected from the behaviour of simple salts. The point for pure ethanol did not fall on the calculated line, and this was attributed to nucleophilic solvent assistance. The variation in AG, AH and AS (Fig. 21) can be reproduced remarkably well using ethane and the zwitterionic a-amino acid, glycine, as model compounds (Abraham et al., 1975 see also Abraham, 1974 Abraham and Abraham, 1974). 

This reaction requires a specific alignment of the migrating substituent and the amino group of an adjacent molecule and does not proceed in isotropic or nematic solvents. It does however, proceed in the neat crystalline phase when /t=CH3 (where the crystal structure of the lattice holds adjacent molecules in the proper orientation for reaction), and in the smectic phases of a large variety of liquid crystals. Initial studies of the kinetics of quatemization reported complex concentration and temperature dependence, including an inverse dependence of the rate constant on solute concentration and non-linear Arrhenius behaviour [359, 363]. Later studies have shown that these initial observations were the result of biphasic solubilization phenomena, with the reactant partitioned between the smectic phase and a nematic or isotropic phase [357]. The coexisting lower order phase (isotropic or nematic) was shown to participate in the reaction by providing a pool of reactant molecules which enter the smectic phase, react, and diffuse out as the zwitterionic product (see above). 

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