Control of Chemical Selectivity by Effect of Shape Similarity between Solute and Solvent Molecules

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In order to develop a more "precise" and novel type of method for control of chemical selectivity, we examined correlation of the selectivity and the pattern of temperature dependence of the selectivity in oxidation of a pair of associating thiols (I and II) , each having R^1 or R^2 as a substituent, with shape similarity between substituents of solute and solvent molecules.1. The selectivity (R^1=Ph, R^2=n-C_5H_<11>) was higher in methanol-n-propyl alcohol than in methanol-isopropyl alchol (at 20 and 35ﾟC).2. The selectivity (R^1=Ph, R^2=i-C_5H_<11>) in hydrocarbon-water-n-propyl alcohol (mole fraction of hydrocarbon=0.50) increased in going from cyclohexane, n-hexane, and to isohexane.3. The selectivity (R^1=Ph, R^2=i-C_5H_<11>) exhibited a maximum at ca.35ﾟC in benzene-ethanol (mole fraction of ethanol=0.50) , showed a minimum in isohexane-ethanol, and decreased with temperature in n-hexane-ethanol.4. The selectivity (R^1=cyclo-C_5H_9, R^2=n-C_4H_9) increased with temperature both in methanol-n-butyl alcohol (mole fraction of methanol=0.50) and in methanol-isobutyl alchol.It is evident from these results that (i) the selectivity increases when a substituent of a thiol is similar in shape to that of a solvent molecule and (ii) the pattern of temperature dependance of the selectivity alters markedly with the degree of shape similarity between substituents of solute and solvent molecules. This indicates that shape similarity effect becomes a novel type of method for control of chemical selectivity.