LFER Studies Evaluating Solvent Effects on an α-Chloro- and two β,β,β-Trichloro-Ethyl Chloroformate Esters

Abstract

To provide insight and to identify the occurrence of mechanistic changes in relation to variance in solvent-type, the solvent effects on the rates of solvolysis of three substrates, 2,2,2-trichloro-1,1-dimethylethyl chloroformate, 2,2,2-trichloroethyl chloroformate, and 1-chloroethyl chloroformate, are analyzed using linear free energy relationships (LFERs) such as the extended GrunwaldWinstein equation, and a similarity-based LFER model approach that is based on the solvolysis of phenyl chloroformate. At 25.0 °C, in four common solvents, the α-chloroethyl chloroformate was found to react considerably faster than the two β,β,β-trichloro-substituted analogs. This immense rate enhancement can be directly related to the proximity of the electron-withdrawing α-chlorine atom to the carbonyl carbon reaction center. In the thirteen solvents studied, 1-chloroethyl chloroformate was found to strictly follow a carbonyl addition process, with the addition-step being rate-determining. For the two β,β,β-trichloro-substrates, in aqueous mixtures that are very rich in a fluoroalcohol component, there is compelling evidence for the occurrence of side-by-side addition-elimination and ionization mechanisms, with the ionization pathway being predominant. The presence of the two methyl groups on the α-carbon of 2,2,2-trichloro-1,1-dimethylethyl chloroformate has additive steric and stereoelectronic implications, causing its rate of reaction to be significantly slower than that of 2,2,2-trichloroethyl chloroformate.

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

“…In the trichloro-derivative of tert -butyl chloride, 2,2,2-trichloro-1,1-dimethylethyl chloroformate, the electron-withdrawing chlorides lead to an A-E mechanism [55,56] and an increased value of 2.14 ± 0.03 is obtained [56]. Similarly, values of 2.17 ± 0.03 have been obtained for n -propyl chloroformate [57] and of 2.03 ± 0.01 for isobutyl chloroformate [58] methanolyses.…”

The Influence of a Terminal Chlorine Substituent on the Kinetics and the Mechanism of the Solvolyses of n-Alkyl Chloroformates in Hydroxylic Solvents

“…In addition to the solvolyses of 2-chloroethyl chloroformate, the solvolyses of the isomeric 1-chloroethyl chloroformate were also studied [ 25 ]. Here, the movement of the substituent closer to the reaction center increases its influence.…”

“…The l and the m values included in the Table give a measure of the importance of solvent nucleophilicity and solvent ionizing power in the substitution process. The N T and the Y Cl values are from earlier tabulation [ 16 , 19 , 21 , 22 , 25 , 26 , 27 ]. The statistical analyses were carried out using the Excel 2016 package from the Microsoft Corporation.…”

“…e From [ 1 ] with omission of values in 97% TFE and 70% and 50% HFIP. f From Table 3 of [ 25 ] with the value in 90% HFIP omitted. g From [ 25 ].…”

Correlation analysis of the rates of solvolysis of 4-bromopiperidine: A reaction following a Grob fragmentation pathway

“…Unfortunately, changes in mechanisms from one involving a dominant nucleophilic attack by solvent towards one involving ionization of the substrate as the solvent nucleophilicity is decreased and the solvent ionization power is increased, by a fluoroalcohol being incorporated within the solvent, can complicate this goal [22,23]. For the 29 solvents incorporated, the specific rates can be adequately correlated by the one-term equation and there is only a negligible increase in the correlation coefficient on going to the two-term equation, accompanied by an appreciable reduction in the F -test value.…”