Kinetic Studies of Bimolecular Nucleophilic Substitution. IV. Rates of the SN2 and E2 Reactions of β-Substituted Ethyl Chlorides with Sodium Acetate in Aqueous Solutions

Abstract

Rates in the SN2 and E2 reactions of a series of β-substituted ethyl chloride (RCH2CH2Cl, R=Et, MeO, PhO, Gl, and H) in aqueous solutions have been measured at temperatures ranging from 90 to 150°C, using sodium acetate as a nucleophile. The mild rate-retarding effect of β-subtituents in the SN2 reactions, observed in the rate order, H>Et>MeO>Cl>PhO, may be attributed to the steric effects. In a similar fashion, the retardation of E2 reaction rates by the β-substituents, which is in the rate sequence H>MeO>Et>PhO>Cl, may be ascribed to the steric effects, which outweigh the electronic effects. The β-substituent effects of several other substituents, which are known in the literature, have also been discussed.

Perspective on Diabatic Models of Chemical Reactivity as Illustrated by the Gas-Phase SN2 Reaction of Acetate Ion with 1,2-Dichloroethane

“…This is probably due to the delocalized nature of the charge in the acetate anion. The overall free energy of activation is 21.3 kcal/mol, smaller than the experimental value of 28.2 kcal/mol 24,27,230. at 373 K. Note that solute-solvent charge transfer is not taken into account by the present treatment.…”

Comparison of Reaction Energetics and Leaving Group Interactions during the Enzyme-Catalyzed and Uncatalyzed Displacement of Chloride from Haloalkanes

“…14 The experimental ∆G ‡ for this reaction in water at 120°C is 29.6 kcal/ mol. 17 The dependence of rate constant on temperature over a narrow (20 degrees) temperature range gives an activation enthalpy ∆H ‡ ) 19.6 kcal/mol and an activation entropy ∆S ‡ ) -26.5 cal M -1 K -1 , from which ∆G ‡ of 27.5 kcal/mol can be obtained at 25°C. As in the case of the formate plus methyl chloride reaction, the calculated ∆G ‡ value is slightly below the extrapolated experimental value.…”

“…8 Experimental rate constants for the uncatalyzed S N 2 displacement of chloride by carboxylates have been determined at elevated temperatures. [16][17][18] The rate constants at room temperature can be obtained by extrapolation of known rate constants, 27 estimated from similar reactions, 14 or calculated quantum mechanically.…”

“…15 Rate constants for reactions of several haloalkanes with the acetate anion in water have been determined above the normal boiling point of water in sealed vials. [16][17][18] The reaction between hydroxide anion and methyl chloride in aqueous solution has been also studied experimentally, and an activation barrier of 24.6 kcal/mol has been determined. 19 The origin of barrier lowering by haloalkane dehalogenases has been a subject of several previous studies.…”

Functional polyisobutylenes via a click chemistry approach

“…Complete reaction of both isomers was achieved after about 10 h. As suspected, the C 3 isomer reacted considerably faster than the C 2 isomer. This is probably a steric effect, since for S N 2 reactions, steric effects are generally more important than electronic effects for substituents located at the β position or more remotely 40. A similar analysis performed on the isomeric 2‐ and 3‐PIB‐1‐(3‐bromopropyl)pyrroles showed that for the case of a 3‐carbon tether, the two isomers displayed more nearly the same reactivity.…”