Solvent effects mixed solvolysis

Extensive studies have been carried out on the concentrated salt effects on the solvolysis reaction rates of aliphatic halides and related compounds in acetone-water mixed solvents. The main outcome of the complicated results presented appears to be that Tt is proposed that one could simply distinguish 5n1 from 5n2 reactions merely by observing a substantial increase in the solvolysis rate constant at 1.0 mol dm LiC104 in aqueous mixed solvents. ... 

In contrast, the acid-catalyzed hydrolysis of alkyl selenates is A-2158. The actual species which undergoes decomposition to alcohol and sulfur trioxide is probably the zwitterion as in the case of phosphate monoester monoanions. Evidence for sulfur trioxide as the reactive initial product of the A-1 solvolysis is obtained from the product compositions arising with mixed alcohol-water solvents. The product distribution is identical to that found for sulfur trioxide solvolysis, with the latter exhibiting a three-fold selectivity for methanol. Although the above entropies of activation and solvent deuterium isotope effects do not distinguish between the conventional A-l mechanism and one involving rate-limiting proton transfer, a simple calculation, based on the pKa of the sulfate moiety and the fact that its deprotonation is diffusion controlled. 

Gilbert and Martin have described an excellent kinetics experiment which illustrates solvolysis of 2-chloro-2-methylbutane via an SnI mechanism (10). The reaction is carried out in a green solvent mixture of water and 2-propanol. In addition, we have scaled down to the use of only 118 microliters of 2-chloro-2-methylbutane per student, and carry out our titrations using reusable plastic syringes instead of burets (77). This lab exercise provides experimental support for the proposed SnI mechanism and illustrates the effect of solvent polarity (increasing percentages of water in the solvent mix) on reaction rate. 

The differing effects of hydrogen ion or of base concentration on thiocyanate substitution rates into rans-[CoCl2 enal and into trans-[CoCl(N02)en2]+ in methanol or ethanol support an earlier hypothesis that the mechanisms are not identical for these two complexes, but that there is considerable solvent assistance in the case of the chloronitro-compound. However, one should recall an earlier study in mixed aqueous alcoholic solvents which indicated no diflFerence in substitution mechanisms for these two complexes. Chloride and thiocyanate substitution at cis-[CoCl2diars]+ in methanol both involve formation of the same inter-mediate. Solvolysis of optical and geometrical isomers of [CoCl2en2]+ in an optically active solvent, propane-1,2-diol, results in stereospecific substitution. ... 

Solvent Variation.—References to the aquation of cobalt(iii) complexes in mixed aqueous-organic solvents will be found in earlier sections. The aquation of the [Co(NH3)sDMSO] + ion and the equilibrium between Cl and [Co(NH3)50H2] ions have been investigated in ethanol-water mixtures, and references to a few studies of the solvolysis of dioximato-complexes in mixed non-aqueous solvents are given in Table 10. The effect of solvent variation is the subject of Chapter 5. 

The required investigation in mixed solvents has yet to be carried out, but /3-effects in solvent water, at the extreme end of the scale, have been reported by Leffek et al. (150). Their results, shown in Table XI, are quite compatible with the considerations of See. VA, 2(c) in which water solvolyses were classified as essentially SnI, borderline, or Sn2 on the basis of the a-isotope effect. Solvolysis of terf-butyl chloride, which is certainly dissociative, shows an effect close to that in non-aqueous media. The isopropyl sulfonates, which we concluded also solvolyze by an essentially SnI mechanism, are subject to effects almost as large whereas those on solvolysis of the isopropyl halides. 

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