Activation energy solvent exchange

In 1961 Jolly 152) reported the fact that BH4" exchanges its incipient hydride ions with deuterium in the solvent pool by two different pathways, one pH-independent, and measurable at very high pH s, while the other path is catalyzed by acid and occurs in solutions undergoing hydrolysis at ordinary temperatures. The activation energy for exchange is less than that for hydrolysis. It has been reported 159,160) that micromolar amounts of metal ion can catalyze this exchange process and the former observations may have to be re-examined. 

J mol ). This is additional evidence in favor of rate limitation by inner diffusion. However, the same reaction in the presence of Dowex-50, which has a more open three-dimensional network, gave an activation energy of 44800 J mol , and closely similar values were obtained for the hydrolysis of ethyl acetate [29] and dimethyl seb-acate [30]. The activation energy for the hydrolysis of ethyl acetate on a macroreticular sulphonated cationic exchanger [93] is 3566 J mol . For the hydrolysis of ethyl formate in a binary system, the isocomposition activation energy (Ec) [28,92] tends to decrease as the solvent content increases, while for solutions of the same dielectric constant, the iso-dielectric activation energy (Ed) increases as the dielectric constant of the solvent increases (Table 6). 

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). 

TABLE 25. Solvent effect on activation free energies for exchange of NMe groups in 189b211... 

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