Phase Example Menshutkin Reaction

In this section, VTST is applied to the bimolecular Menshutkin reaction in aqueous solution  

The SES calculations were carried out along the gas-phase MEP. In an SES calculation, the solvent is not considered when constructing the MEP, and solvent effects are added separately to create the potential of mean force using Eq. [311]. The solvation free energy was evaluated with the SMS.43 model, and therefore, the SES calculations are denoted as SM5.43/HE/ 6-31+G(d)//HE/6-31G(d) or simply as SM5.43/HE/6-31G(d)//g. 

In the gas phase, a transition state exists for reaction only because there is a slightly stable ion-pair structure, which disappears when the geometry is optimized in solution. The maximum of U s T) in the SES approximation is located at s = —1.60 Uq. The maximum of U(s T) along the reaction path at the SM5.43//HF/6-31G(d) level is much closer to reactants than in the gas phase, which was expected, because in solution, products are much more stabilized. 

Although the above reaction is quite simple, the similarity between the SES and the ESP profiles is stunning if we consider the great difference between the gas-phase and liquid-phase potentials. From this example, we can conclude that, although the ESP allows a more reliable description of the reaction in solution, the SES approach is an inexpensive approach that can sometimes provide a reasonably accurate alternative to the ESP method. 

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Most organic reactions occur in solution and, naturally, any comprehensive analysis of the mechanism, kinetics, and thermodynamics of a given reaction must take account of effects of the medium, in other words, of the solvent effects. The number of publications is very great devoted to experimental research into the multifarious effects a solvent has on the rate and mechanism of organic reactions which were first detected about one hundred years ago by Menshutkin. How essential the effect of a solvent can be shows, for example, a comparison between the reaction of alkali hydrolysis of alkylhalogenides and esters in water and in the gas phase. The rate of the gas-phase reaction is by 20 orders of magnitude faster than in solution (see Sect. 5.1.1.4). 

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