Use of Empirical Correlations to Determine Solvent Effects in the Solvolysis of S-Methyl Chlorothioformate

Abstract

The specific rates of solvolysis of S-methyl chlorothioformate (MeSCOCl) are analyzed in 20 solvents of widely varying nucleophilicity and ionizing power at 25.0 °C using the extended Grunwald-Winstein Equation. A stepwise SN1 (DN + AN) mechanism is proposed in the more ionizing solvents including six aqueous fluoroalcohols. In these solvents, a large sensitivity value of 0.79 towards changes in solvent nucleophilicity (l) is indicative of profound rearside nucleophilic solvation of the developing carbocation. In twelve of the more nucleophilic pure alchohols and aqueous solutions, the sensitivities obtained for solvent nucleophilicity (l) and solvent ionizing power (m) are similar to those found in acyl chlorides where an association-dissociation (AN + DN) mechanism is believed to be operative.

Correlation of the Rates of Solvolysis of Phenyl Fluorothionoformate

“…The l/m ratio has been suggested as a useful mechanistic criterion and the values of Table 3 divide nicely into two classes with values of 1.7 to 3.38 for those entries postulated to represent addition-elimination pathway (Scheme 1) and 0.37 to 0.93 for those believed to represent ionization pathway (Scheme 2). The l/m ratio of 3.38 obtained for 1 is similar to those previously reported for 2 in all the solvents, and for methyl chlorothioformate (MeSCOCl), 18 i-PrOCOCl…”

Analysis of the Nucleophilic Solvation Effects in Isopropyl Chlorothioformate Solvolysis

“…As in the case of MeSCOCl [30], the entropies of activation (ΔS ≠ values) for 1 reported in the footnotes of Table 1, are negative. The negative ΔS ≠ values suggest a greater ordering of the solvent molecules in the transition state, encasing the developing carbocation and thus dispersing the electrical charge.…”

“…The observed sensitivity values of l = 1.66 and m = 0.56 are now taken as typical values [1,7,10,12–16,18,20–28,30,43] for attack at an acyl carbon proceeding by the addition-elimination mechanism, with the addition step being rate-determining. A sulfur-for-oxygen substitution for both oxygen atoms in 3 yields the dithio analog, phenyl dithiochloroformate (PhSCSCl).…”

Detailed analysis for the solvolysis of isopropenyl chloroformate

“…Furthermore, with equation 2 an h value of 0.42 ± 0.15 signified that there was minimal charge dispersion into the aromatic ring during the formation of the thioacylium transition state that was stabilized by intense rear-side nucleophilic solvation (as indicated by the large l value). These sensitivity values are now considered typical [2,7,23,27,29,30,33,34,49,56] for substrates believed to solvolyze with the formation of an acylium or a thioacylium ion in the transition state.…”