Unusual Solvent Effect on a S_N2 Reaction. A Quantum-Mechanical and Kinetic Study of the Menshutkin Reaction between 2-Amino-1-methylbenzimidazole and Iodomethane in the Gas Phase and in Acetonitrile

Abstract

The quaternization reaction between 2-amino-1-methylbenzimidazole and iodomethane was investigated in the gas phase and in liquid acetonitrile. Both experimental and theoretical techniques were used in this study. In the experimental part of this work, accurate second-order rate constants were obtained for this reaction in acetonitrile from conductivity data in the 293-323 K temperature range and at ambient pressure. From two different empirical equations describing the effect of temperature on reaction rates, thermodynamic functions of activation were calculated. In the theoretical part of this work, the mechanism of this reaction was investigated in the gas phase and in acetonitrile. Two different quantum levels (B3LYP/[6-311++G(3df,3pd)/LanL2DZ]//B3LYP/[6-31G(d)/LanL2DZ] and B3LYP/[6-311++G(3df,3pd)/LanL2DZ]//B3LYP/[6-31+G(d)/LanL2DZ]) were used in the calculations, and the acetonitrile environment was modeled using the polarized continuum model (PCM). In addition, an atoms in molecules (AIM) analysis was made aiming to characterize possible hydrogen bonding. The results obtained by both techniques are in excellent agreement and lead to new insight into the mechanism of the reaction under examination. These include the identification and thermodynamic characterization of the relevant stationary species, the rationalization of the mechanistic role played by the solvent and the amine group adjacent to the nucleophile nitrogen atom, the proposal of alternative paths on the modeled potential energy surfaces, and the origin of the marked non-Arrhenius behavior of the kinetic data in solvent acetonitrile. In particular, the AIM analysis confirmed the operation of intermolecular hydrogen bonds between reactants and between products, both in the gas phase and in solution. It is also concluded that the unusual solvent effect on this Menshutkin reaction stems from the conjunction of a nucleophile possessing a relatively complex chemical structure with a dipolar aprotic solvent that is protophobic.

The Menshutkin Reaction in the Gas Phase and in Aqueous Solution: A Valence Bond Study

“…Intrinsic reaction coordinate (IRC) calculations provide the relationship between the bond distances d' (of NÀC) and d (of CÀCl). For a given value of Q, the individual d' and d lengths can be determined by combining the d-d' relationship derived from the IRC with their relationship in Equation (10). For the reaction in aqueous phase, the values of a are the same with those in gas phase in order to compare the location of the TS.…”

Activation parameter changes as a mechanistic tool in SN2 reactions in solution

“…It should be noted that the variation of the activation parameters for SN2 reactions depend on the reacting species and solvents [120][121][122][123][124][125][126]. For instance, the activation parameters ΔН ≠ and ΔS ≠ for the reaction of sodium 4nitrophenoxide and iodomethane in acetone-water mixtures at 25-35 °C form the compensation dependence reflecting electrostatic and specific interactions between a nucleophile and a solvent mixture [120].…”

Nucleophilic Aliphatic Substitution

“…The effect of solvent on the S N 2 reaction between methyl iodide and 2-amino-1methylbenzimidazole has been investigated experimentally in acetonitrile and in the gas phase at two different levels of theory, B3LYP/ 79 The effect of the acetonitrile was determined using the PCM model. AIM calculations were also used to investigate hydrogen bonding in each intermediate.…”