Analysis of the Nucleophilic Solvation Effects in Isopropyl Chlorothioformate Solvolysis

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.

Correlation of the Rates of Solvolysis of t-Butyl Fluoroformate Using the Extended Grunwald-Winstein Equation

“…The l/m ratio has been suggested as a useful mechanistic criterion and the values of Table 4 divide nicely into two classes with values of 1.8 to 2.8 for those entries postulated to represent addition-elimination pathway (Scheme 1) and 0.54 to 0.71 for those believed to represent ionization pathway (Scheme 2). The l/m ratio of 0.63 obtained for 1 is similar to those previously reported for i-PrOCOCl, 1(c) EtSCOCl, 1(g) i-PrSCOCl, 13 and t-butyl chloride (l = 0.34 ± 0.04, m = 0.84 ± 0.02, and l/m = 0.40) 14 in all solvents except the less ionizing and more nucleophilic solvents, consistent with the unimolecular pathway. The higher m-values for the solvolyses of fluoroformates, relative to chloroformates except alkyl chlorothioformates, may reflect the need for increased solvation of the developing positive charge on the carbonyl carbon the presence of the more electronegative fluorine attached at the carbonyl carbon.…”

Influence of Sulfur for Oxygen Substitution in the Solvolytic Reactions of Chloroformate Esters and Related Compounds

“…Indeed, only for 100% ethanol was there an upward deviation from the correlation line, indicating for this solvent a superimposed A-E pathway [47]. It has been suggested [9,35] that a likely mechanism for solvolyses of 2° and 3° alkyl chloroformates involves a concerted fragmentation reaction (Scheme 3) leading, in the present context, to the isopropyl carbenium ion, with this then combining with the chloride ion, deprotonating to give propene plus HCl, or adding a solvent molecule.…”

Detailed analysis for the solvolysis of isopropenyl chloroformate

“…The two-term extended [1] Grunwald-Winstein equation was found to be very efficient [2] in correlation studies for elucidating solvolytic mechanisms of reaction for a variety of chloroformate esters (ROCOCl) [2–28], and their corresponding sulfur-for-oxygen substituted analogs (RSCOCl, ROCSCl and RSCSCl) [2,23,26,29–34]. In the extended (equation 1) Grunwald-Winstein equation [1], k and k o are the specific rates of solvolysis in a given solvent and in the standard solvent (80% ethanol), respectively, l governs the sensitivity to changes in solvent nucleophilicity ( N ), m represents the sensitivity to changes in the solvent ionizing power Y (initially set at unity for tert -butyl chloride solvolyses), and c is a constant (residual) term [2].…”

“…Replacement of the ether oxygen in 2 with sulfur yields isopropyl chlorothioformate ( i -PrSCOCl), that was recently shown [34] to solvolyze by a dominant stepwise S N 1 mechanism with moderate rear-side nucleophilic solvation of the developing acylium ion. This conclusion [34] for a dominant unimolecular pathway for i -PrSCOCl in all solvents except 100% EtOH, occurred with no alkyl-sulfur bond fission.…”