A Study of Solvent Effects in the Solvolysis of Propargyl Chloroformate

Abstract

The specific rates of solvolysis of propargyl chloroformate (1) are analyzed in 22 solvents of widely varying nucleophilicity and ionizing power values at 25.0 °C using the extended Grunwald-Winstein equation. Sensitivities to solvent nucleophilicity (l) of 1.37 and to solvent ionizing power (m) of 0.47 suggest a bimolecular process with the formation of a tetrahedral intermediate. A plot of the rates of solvolysis of 1 against those previously reported for phenyl chloroformate (2) results in a correlation coefficient (R) of 0.996, a slope of 0.86, and an F-test value of 2161. The unequivocal correlation between these two substrates attest that the stepwise association-dissociation (AN + DN) mechanism previously proposed for 2 is also operative in 1.

Kinetic Studies that Evaluate the Solvolytic Mechanisms of Allyl and Vinyl Chloroformate Esters

“…For propargyl chloroformate, the tetrahedral stepwise addition-elimination pathway was confirmed for all 22 solvents studied [58]. 2-butyn-1-yl chloroformate differs from the propargyl substrate by the presence of an adjoining methyl group on the β-triple bond.…”

“…So far the two alkynyl chloroformates to be evaluated using the extended Grunwald-Winstein Equation [Equation (2)] are; propargyl [58] and 2-butyn-1-yl chloroformate [31]. For propargyl chloroformate, the tetrahedral stepwise addition-elimination pathway was confirmed for all 22 solvents studied [58].…”

Use of Linear Free Energy Relationships (LFERs) to Elucidate the Mechanisms of Reaction of a γ-Methyl-β-alkynyl and an ortho-Substituted Aryl Chloroformate Ester

“…Also listed are the previously published rate data for the sovolysis of propargyl chloroformate ( 4 ) [35] at 25.0 °C and the literature N T [18–20] and Y Cl [13–17] values.…”

“…Additionally, we demonstrated [35] that 4 and 3 solvolyze by a similar bimolecular addition-elimination pathway with a rate-determining addition step in a variety of solvents including those with appreciable fluoroalcohol content. On the other hand, the former war gas isopropenyl chloroformate, was shown [37] to exhibit a superimposed unimolecular ionization (S N 1) mechanism in the strongly hydrogen-bonding 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and 97% 2,2,2-trifluoroethanol (TFE) mixtures with water.…”

Nucleophilic Aliphatic Substitution

“…Dispersions observed in Grunwald-Winstein correlations of the S N 1 solvolyses of substrates featuring adjacent π -electrons (e.g., aryl groups) can be corrected by an additional term, "hI ", where h represents the sensitivity of the solvolysis to changes in the aromatic ring parameter "I ". 96 For example, in the S N 1 solvolyses of cinnamyl chloride and bromide, the sensitivity factor "h" ≈ 1.0 indicates that one aromatic ring is conjugated with the developing positive charge in the resonance-stabilized transition state.…”