Solvated solvent exchange reaction, rate

Valuable information on mechanisms has been obtained from data on solvent exchange (4.4).The rate law, one of the most used mechanistic tools, is not useful in this instance, unfortunately, since the concentration of one of the reactants, the solvent, is invariant. Sometimes the exchange can be examined in a neutral solvent, although this is difficult to find. The reactants and products are however identical in (4.4), there is no free energy of reaction to overcome, and the activation parameters have been used exclusively, with great effect, to assign mechanism. This applies particularly to volumes of activation, since solvation differences are approximately zero and the observed volume of activation can be equated with the intrinsic one (Sec. 2.3.3). 

With an increase in the acceptor strength of the solvent, the stability of the solvate of the halid Y" increases, which inhibits the formation of the transition product therefore, the rate of the exchange reaction decreases if the acceptor strength of the solvent increases. This effect is manifested in such a clear-cut manner that the solvent dependence of the rate constant can be described unambiguously by means of the Krygowski-Fawcett [Kr 75] model (cf. Chapter 4). 

Non-aqueous Solvents.—Mention has already been made of the evidence for an associative (/a) mechanism for the exchange of DMSO with the [Cr(DMSO) ] + ion. Both the entropy and volume of activation are large and negative. A recent study of the exchange process by n.m.r. in DMSO-MeNOj mixed solvents (nitro-methane is an inert, non-co-ordinating diluent which is known to have very little rate effect) shows that the exchange rate is approximately constant above 0.2 mole fraction of DMSO, but drops off sharply below this concentration. On the other hand the fraction of DMSO molecules in the solvation shell of the [Cr(DMSO) J + ion decreases immediately with the decrease in the DMSO mole fraction. This difference in concentration effects is postulated to arise from a unique outer-sphere solvation site which preferentially binds DMSO molecules and preferentially participates in the exchange reaction. 

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