Menshutkin reactions

Figure A3.6.3. Solvent polarity dependence of the rate constant for the Menshutkin reaction (data from [14]).

Caste]on H and Wiberg K B 1999 Solvent effects on methyl transfer reactions. 1. The Menshutkin reaction J. Am. Ohem. Soc. 121 2139-46... 

Figure 11 Potential energy and potential of mean force of the Menshutkin reaction. The dashed line is for reaction in the gas phase, and the solid line for reaction in aqueous solution.

There is evidence, both experimental and theoretical, that there are intermediates in at least some Sn2 reactions in the gas phase, in charge type I reactions, where a negative ion nucleophile attacks a neutral substrate. Two energy minima, one before and one after the transition state, appear in the reaction coordinate (Fig. 10.1). The energy surface for the Sn2 Menshutkin reaction (p. 499) has been examined and it was shown that charge separation was promoted by the solvent.An ab initio study of the Sn2 reaction at primary and secondary carbon centers has looked at the energy barrier (at the transition state) to the reaction. These minima correspond to unsymmetrical ion-dipole complexes. Theoretical calculations also show such minima in certain solvents, (e.g., DMF), but not in water. "... 

The combined QM/MM model can be used along with Statistical Perturbation Theory to carry out a Monte Carlo simulation of a chemical reaction in solution, with the advantage of allowing solute electronic structure relaxation in solution. Particularly, the combined AM1/TIP3P force field has recently been applied to simulate several chemical processes in solution. We will refer here briefly to the Claisen rearrangement and to the Menshutkin reaction. 

Maran, U., Pakkanen, T. A. and Karelson, M. A semiempirical study of the solvent effect on the Menshutkin reaction, J.Chem.Soc.PerkinII, submitted (1995). 

The discussion in the section on the origin of secondary a-deuterium KIEs demonstrated the power of combining theoretical calculations with experimental measurements for determining the origin and developing an understanding of the secondary a-deuterium KIEs in SN2 reactions. One SN2 reaction where a combination of experiment and theory has been used to elucidate the structure of the transition state is the Menshutkin reaction (19) (Abboud et al., 1993)... 

In the first experimental study where KIEs were determined for Menshutkin reactions, Leffek and MacLean (1965) measured the secondary a-... 

While it is difficult to rule out an inductive contribution to the secondary a-deuterium KIEs in these Menshutkin SN2 reactions, the steric origin of these KIEs is clearly indicated by the inverse secondary a-deuterium KIEs found in an extensive study of the SN2 Menshutkin reactions between substituted... 

Table 17 Secondary deuterium KIEs for the Menshutkin reactions of deuterated pyridines and alkyl iodides in nitrobenzene at 25°C.a...

Menshutkin reaction between jV,Ar-dimethyl-d6-aniline and methyl tosylate in nitrobenzene at 51.3°C (21) is consistent with an inductive KIE resulting from the increased electron density on the nitrogen (the nucleophilicity), a steric explanation was preferred for three reasons. 

Table 19 The incoming nucleophile nitrogen KIEs found for Menshutkin reactions with various amines in several solvents.

Matsson and co-workers (Axelsson et al., 1987) measured the carbon-ll/carbon-14 a-carbon KIEs for the Menshutkin reaction between N,N-dimethyl-p-toluidine and labelled methyl iodide in methanol at 30°C. The 1 -labelled methyl iodide required for this study was prepared in three steps [reaction (23)] from the "C atoms produced in a cyclotron (L ngstrom etal., 1987). 

Large (near the theoretical maximum) a-carbon KIEs for the Menshutkin reactions between 3,5-disubstituted pyridines and methyl iodide [reaction (22) Table 20] have also been reported by Yamataka and co-workers (Ando et al., 1987). Although the a-carbon KIEs increase slightly as more electron-withdrawing substituents are added to the nucleophile, they are all large and... 

Table 20 The a-carbon-12/carbon-13 KIEs for the Menshutkin reaction between 3,5-disubstituted pyridines and methyl iodide in 2-nitropropane at 25°C. ...

The newest type of KIE that has been used to characterize the transition state of the Menshutkin reaction is a secondary incoming nucleophile deuterium KIE. These KIEs are measured by using primary amines labelled with deuterium at the nitrogen of the nucleophile [reaction (25)]. 

In fact, Lee and collaborators (Lee et al., 1991) measured both the secondary a-deuterium and the secondary incoming nucleophile deuterium KIEs for four different Menshutkin reactions (Table 22). The secondary... 

Table 24 The calculated and experimental incoming nucleophile nitrogen, a-carbon and secondary a-deuterium KIEs for the SN2 Menshutkin reactions between p-substituted iV.jV-dimethylanilines and methyl iodide at 298 K.°...

---