Nitration transition-state solvation

Although the intermolecular selectivity of the nitration of alkylbenzenes by nitric acid in trifluoroacetic acid is controlled by both electronic and steric factors, it is argued that intramolecular selectivity is controlled by steric effects on transition state solvation. 

In aquation of the cations [M(NH3)5Br] + (M = Cr or Co) in alcohol-water mixtures, solvent composition variation has much more effect on A.S than on iiH. This is attributed to solvation-shell ordering effects in the transition state. Enhancement of reactivity in the reaction of cobalt(ii) with chlorophyllic acid in methanol on addition of lithium nitrate is attributed to inhibition of transition-state solvation. The effective radius of a transition state can be guessed from the variation of rate constant with dielectric constant. This approach has been used for the bromide-bromate reaction. In contrast to this concentration of attention on the transition state, it may be noted that it is the stabilization of the reactant in relation to water structure that is thought to control the variation of the racemization rate of the ( + )-[Co(phen)3] + cation in t-butyl alcohol-water mixtures. ... 

Effects of added salts on the Belousov-Zhabotinskii oscillating reaction have also been attributed to solvent structural factors, through the influence of the cations (Li+, Na+, K+, Me4N+) on the water structure. Specific effects of added salts on transition states have been proposed for the reactions of cobalt(ii) with chlorophyllic acid (in methanol) and the conversion of / - into -dodeca-molybdosilicate. In the former case added lithium nitrate is thought to inhibit solvation of the transition state, but in the latter added ions are believed actually to enter the transition state. Salt effects on the kinetics and mechanism of formation of monofluoro-complexes of a range of M + and Al + cations in aqueous media are described in a conference report comprehensible only to readers of Russian. 

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