SULPHONIUM SALT SOLVOLYSIS: PART II. COMMON-ION EFFECT AND THE ION-PAIR MECHANISM IN THE SOLVOLYSIS OF DIMETHYL-t-BUTYL SULPHONIUM SALTS

Abstract

The kinetic effect of added common anion in the solvolysis of dimethyl-t-butyl s~~lphonium halides in ethanol-water mixtures is analyzed in terms of a mechanism involving solvolysis through a n ion-pair intermediate. The simple ion-pair role, however, does not account for the kinetic behavior a t higher added common-anion concentrations. A normal salt effect and further involvement of the ion pair in Sx2 attacl; by common anion are compared as possible explanations for the observed kinetic behavior. INTRODUCTIOXEvidence presented in the previous paper of tliis series ( I ) was interpreted as indicative of the increasing role of an ion-pair niechanism in the solvolysis of dimethyl-t-butyl sulphonium salts as the dielectric constant of the mediuni is reduced. Further experimental evidence in support of this proposed nlechanism is presented in this paper. I t is clear from equation [ I ] tliat such a mechanism should be affected by the concentration of coiiinion anion X-. Higher concentrations should favor ion-pair forniation and a higher proportion of the over-all solvolysis should proceed through the ion-pair intermediate. The over-all rate of solvolysis (kobs) of dimethyl-t-butyl sulphoiiiuin iodide and chloride has therefore been measured as a functioi~ of both added coninion anion and solvent coniposition.I11 the previous paper of tliis series it was silo\\-ii tliat if the ion-pair niecl~anism is correct, k , , > k+. Consequently, added common anion should increase the over-all rate of solvolysis (kobs) since a greater proportion of tlie solvolytic process will proceed through the faster ion-pair pathway. As the dielectric constant of the solvolytic mecli~~iii is reduced, tlie ion-pair association constant ( K , ) will increase and together witli increasing concentration of common anion these factors \\-ill result in virtually complete conversion of the sulphonium ion to the ion-pair state. The over-all rate of solvolysis slioulcl therefore increase witli added common anion and lower dielectric constant but should approach an upper limiting value as the degree of ion pairing approaches 100%. EXPERIMEXTAL METHOD .AND RESULTSAll rates were measured by tlie radiochemical flow counting technique of Hyne and Wolfgang (2). C14-labelled dimethyl-t-butyl sulphonium iodide was prepared as described previously ( 2 ) and converted to the chloride by metathesis witli freshly prepared silver chloride. Solvent mixtures were prepared by weight from doubly distilled water (lo-' mhos) and redistilled absolute ethanol. Reagent grades of lithium chloride and iodide 'Manuscript

Solvent effects on the intramolecular conversion of trimethylsulfonium chloride to dimethyl sulfide and methyl chloride

“…43 The linear ion pair is not a stationary point (Section 3.3.2) and is greyed to distinguish it from the calculated stationary points; however, the reaction path from the ions (CH 3 ) 3 S + and Cl À at infinite separation dissolved in polar solvents to the transition state for a backside attack passes through this geometry. 18) and dimethyl-t-butyl sulfonium chloride (B100 L mol À1 ref. The Cl atom is shown in green, S in purple, C in orange and H in grey/white.…”

“…The dominating reaction path in polar solvents is marked in red and that in non-polar solvents in blue. 18) indicate significant ion pair concentrations even in pure water. The formation of the tripod ion pair is exothermic, but becomes endergonic at a cmf value of 0.90 AE 0.03 (e = 28.5 AE 1.1).…”

“…13,14,[17][18][19] The effective barrier is the result of weighing the relative transition state energies by the molar fraction of the corresponding reactants. 13,14,[17][18][19] The effective barrier is the result of weighing the relative transition state energies by the molar fraction of the corresponding reactants.…”

“…[17][18][19][20][21] The solvolysis proceeds via a solvent stabilised t-butyl cation 17 and the ion pair concentration increases as the permittivity of the solvent decreases. [17][18][19][20][21] The solvolysis proceeds via a solvent stabilised t-butyl cation 17 and the ion pair concentration increases as the permittivity of the solvent decreases.…”

“…The solvolysis of various sulfonium salts R 3 0 SX in a series of X À + R 3 0 S + + ROH -R 2 0 S + R 0 OR + HX (2) has been subjected to many kinetic investigations. 11,[13][14][15][17][18][19][20] The solvolysis of dimethyl-t-butylsulfonium halides R 2 R 0 SX and the role of ion pairs R 2 R 0 S + ÁX À in the solvolysis process…”