Abstract
Correlation of the solvent effects through application of the extended Grunwald-Winstein equation to the solvolysis of isopropyl chlorothioformate results in a sensitivity value of 0.38 towards changes in solvent nucleophilicity (l) and a sensitivity value of 0.72 towards changes in solvent ionizing power (m). This tangible l value coupled with the negative entropies of activation observed indicates a favorable predisposition towards a modest rear-side nucleophilic solvation of a developing carbocation. Only in 100% ethanol was the bimolecular pathway dominant. These observations are very different from those obtained for the solvolysis of isopropyl chloroformate, where dual reaction channels were proposed, with the addition-elimination reaction favored in the more nucleophilic solvents and a unimolecular fragmentation-ionization mechanism favored in the highly ionizing solvents.
Highlights
In the simple [2] (Equation 1) and extended [3] (Equation 2) Grunwald-Winstein Equations, k and ko are the specific rates of solvolysis in a given solvent and in the standard solvent (80% ethanol), respectively, m represents the sensitivity to changes in the solvent ionizing power Y, and c is a constant term
For bimolecular (SN2 and/or E2) reactions, NT scales based on the solvolyses of the S-methyldibenzothiophenium ion [37,38]
On the other hand, superimposed mechanisms [1,10,12,13,16,18,19,20,21,24,30,57,58,59,60,61,62] were observed for the alkyl chloroformate (ROCOCl) and chlorothioformate (RSCOCl) esters and the ranges of dominance for each were found to be heavily dependent on the nature of the substituent (R), solvent nucleophilicity, and on the ionizing ability of the solvent
Summary
The application [1] of the simple [2] and extended [3] (two-term) Grunwald-Winstein Equations for the analyses of the specific rates of solvolyses of chloroformates [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24], and their corresponding sulfur-for-oxygen substituted analogs (thiono-, thio-, and dithiochloroformate esters) [1,10,21,25,26,27,28,29,30]. On the other hand, superimposed mechanisms [1,10,12,13,16,18,19,20,21,24,30,57,58,59,60,61,62] were observed for the alkyl chloroformate (ROCOCl) and chlorothioformate (RSCOCl) esters and the ranges of dominance for each were found to be heavily dependent on the nature of the substituent (R), solvent nucleophilicity, and on the ionizing ability of the solvent These solvolyses could be treated, by application of Equation 2, in terms of two correlations with very different sets of l and m values, believed to correspond to the side-by-side operation of the bimolecular addition-elimination and unimolecular ionization (SN1) pathways. Rate constants at additional temperatures and the calculated activation parameters are reported
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