Indian Journal of Vol. 23A. June 1984, pp. 522-523

Solvent Effects on Solvolysis of (0.8) in these mixtures is typical for an SNI r-Butyl Hydrogenphthalate mechanism. (For SNI mechanism, m~0.5 for chlorosulphates, 0.5 to 0.7 for arenesulphonates and K SARAMMA* & JOHN KURIAN 0.9 to l.l for alkyl halides). The m value for r-butyl hydrogenphthalate is less than that for alkyl halide Department of Chemistry. University of Kerala, Trivandrum undergoing solvolysis by the same mechanism. The 695034 obvious deduction is that the ester is less dependent on Received 20 JUlie 1983; revised and accepted JO January 1984 the ionising power of the medium than the alkyl halide. number n is derived from equation log k , The rates of solvolysis of t-butyl hydrogenphthalate have been determined in -. ethanol-water. acetone-water and = log k' + n log [H 20] where k , is the experimental dioxane-water systems. The solvent effect has been examined from pseudo-first order rate constant and k' is a constant. the view point of solvation number (II). solvent ionising power ( y) Though n cannot be directly equated with and sensitivity of the substrate to solvent ionising power (m). The of water, nevertheless the values of n provide a useful observed results (m ~ 0.8 and II ~ 5.3) indicate an S"l mechanism for empirical criterion of mechanism". Unimolecular the reaction. reaction is associated with larger solvation number It has already been shown that r-butyl hydrogen- than bimolecular reaction. For common alkyl phthalate solvolyses by BALI mechanism in water and derivative the value of n is approximately 5.7 for in aqueous organic ", The present note unimolecular mechanisms and 2.3 for bimolecular provides additional evidence from solvation number mechanisms:'. The plots of log k , versus log [H 20] for (n), ionising power of the solvent (Y) and sensitivity of r-butyl hydrogen phthalate in acetone-water and in the substrate to solvent ionising power (m). dioxane-water are linear (corr. coeff =0998) and the Aqueous methanol, aqueous ethanol, aqueous derived n values are given in Table 1. For r-butyl acetone and aqueous dioxane were used as solvents. hydrogenphthalate, n ~ 5.3 which is typical for an SNI Kinetic data for the solvolysis in different solvent mechanism. These results may be attributed to a mixturers are given in Table I. specific solvation of the transition state by water Plots of log k versus Yare linear for all the solvent molecules. bearing in mind that in the unimolecular mixtures (corr. coeff. = 0.996). The m value obtained mechanism the carbonium is formed within a

Table I-Rates of Solvolysis of r-Butyl Hydrogenphthalate. nand m Values in Aqueous Organic Solvents

[Ester] = 0.007 mol dm -3; temp = 70C % Organic component in aqueous organic mixture

10 20 30 40 50 60 80 Ethanolt 10' k , (s I) 2.80 1.48 0.148 0.064 0.015 y 3.312 3.051 1.655 l.l24 0 m= 0.71 Methanol

10'kl(s I) 9.90 5.79 3.27 1.65 y 3.279 3.025 2.753 2.391 m= 0.87 Acetone 10' k , (s I) 8.66 4.33 2.17 1.04 0.038 y 3.23 2.913 2.482 1.981 -0.673 m= 0.74 n=5.36 Dioxane 10' k , (S'I) 7.47 4.20 1.81 0.875 Y 1.945 2.455 2.877 3.217 m= 0.073 n=5.19 ttemp=60"C

522 NOTES

solvation shell of water. The critical number of water UGC, New Delhi for the award of research fellowship molecules required in the unimolecular mechanism (FIP) to one of them (JK). will be greater than those in the bimolecular mechanism since in the latter case the role of water References I Sivaramakrishna Pillai R, Ph D Thesis. Salt effect in the solvolysis molecule is limited to solvation of the incipient anion of some acid phthalate in water, Kerala University, 1980. to nucleophilic participation. 2 Bunce! E, Millington J P & Wiltshire J F, Can J Chern, 55 (1977) The authors wish to express their gratitude to Prof. 1401. R Anantaraman for his interest in this study and to the 3 Abraham M H, Prog phys org Chern, 11 (1974) I.

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