Menschutkin reactions, transition states

Figure 8-6. Ploi according to Fig. 8-5 of transfer free energies of the transition state (ordinate) and reactant state (abscissa) for the Menschutkin reaction of triethylamine and ethyl iodide. The reference solvent is N, Af-dimethylformamide (No. 27). Data are from Table 8-10, where the solvents are identified by number. Closed circles are polychlorinated solvents.

Reactions that take place through dipolar transition states Menschutkin reaction (10-44), electrophilic aromatic substitution. 

Information as to the nature of the transition state in reaction (71) may be obtained through a comparison of AG f(Tr) values with those for other transition states and those for various model solutes. Data are available62 on solvent effects on the transition state in the Menschutkin reaction... 

Fig. 10.2. Predictions of the logarithmic relative rate constants of the Menschutkin reaction in various solvents [C32]. The COSMO polarization charge densities a of the transition state are visualized in the inset.

Secondly, we note that if we take the charge development from the values of Z in Table 18 then again the transition states are much too reactant-like. Hence we prefer to use the values of JZ. This means that the transition states for the Menschutkin reactions of methyl and ethyl iodide are fairly loose 0.65). This inference may seem to conflict with the good agreement... 

Fig. 27 Map of the transition states. The transition states are located as follows A, hydrolysis of methyl halides B, isopropyl transition states C, hydrolysis of t butyl chloride (or at D in Fig. 3) M, Menschutkin reactions...

Solvent effect on rate constants. In this section, the rate constant will be predicted qualitatively in CO2 for the Diels-Alder cycloaddition of isoprene and maleic anhydride, a reaction which has been well-characterized in the liquid state (23,24). In a previous paper, we used E data for phenol blue in ethylene to predict the rate constant of the Menschutkin reaction of tripropylamine and methyliodide (19). The reaction mechanisms are quite different, yet the solvent effect on the rate constant of both reactions can be correlated with E of phenol blue in liquid solvents. The dipole moment increases in the Menschutkin reaction going from the reactant state to the transition state and in phenol blue during electronic excitation, so that the two phenomena are correlated. In the above Diels-Alder reaction, the reaction coordinate is isopolar with a negative activation volume (8,23),... 

There is no need for the transition states to be charged for these effects to be observed. As noted in Section 2, dipolar aprotic solvents also interact strongly with polarizable neutral species. The data for Menschutkin reactions on transfer from methanol to DMF (Table 12 ... 

Fig. 37 More O Ferrall-Jencks diagram for the Menschutkin reactions of 1-phenylethy] and benzyl chlorides with pyridine. The structures of transition states were optimized by ab initio MO calculation (RHF/b-Sf G ). O, substituted 1-phenylethyl chlorides with pyridine , benzyl chlorides with pyrindine , with 4-nitropyridine O, methyl and A, ethyl chlorides with pyridine (Fujio et al, unpublished).

Contrary to reactions going through isopolar transition states, reactions of types 3 to 8 in Table 5-25, which involve formation, dispersal or destruction of charge, should exhibit large solvent effects on their activation volumes. This is shown in Table 5-27 for the Sn2 substitution reaction between triethylamine and iodoethane [441], an example of the well-known Menschutkin reaction, the pressure dependence of which has been investigated thoroughly [439-445, 755],... 

The Menschutkin reaction (Menschutkin, 1890 Hinshelwood et al., 1936) between tertiary amines and alkyl halides, is a classical one in terms of solvent effects on rate. It is of the same charge type as the back reaction (14) and shows reasonable correlation with Kosower s Z values, for a series of protic solvents (Kosower, 1958). Many rate data are available, so that a meaningful discussion of solvent effects on bimolecular reactions between molecules might evolve, if the appropriate solvent activity coefficients for reactants and transition states of Menschutkin reactions were known. 

R—X, of reaction (15). It is then interesting to compare this solvent activity coefficient with those of species which may act as models for the transition state, such as ion pairs, dissociated ions, and polar 8 2 transition states (e.g. of Menschutkin reactions and of reaction (14)). In this way, some estimate of the nature of the 1 transition state can... 

A-liphatic nucleophilic substitution played a central role in the recent debate on variable transition states (TS) (1-20). In previous papers (1-4), we successfully interpreted the variation in the TS structure of the Menschutkin-type reaction of benzyl benzenesulfonates with N,N-dimethylanilines (equation 1), by the extensive use of carbon-14 and tritium isotope effects. 

As pointed out by Taft and co-workers (27) on the basis of linear free energy relationships, highly polar transition states are prime candidates to undergo specific solvation by polar or polarizable species. This concept was already set forth in 1935 by Wynne-Jones and Eyring (35) on an entirely different basis, namely, the interpretation of kinetic results in mixed solvents (36). The study of reaction rates in mixed solvents is an almost untapped source of information on the solvation of transition states, Recently, Drougard and Decroocq (37) have studied the kinetics of the Menschutkin reaction between EtsN and Mel in binary and ternary solvent systems. From their experimental data it appears that... 

The first systematic investigation on the influence of solvent on reaction rates was reported by Menschutkin(l) as long ago as 1890. Quite soon after this study, chemists began to consider whether or not solvent influences on reaction rates were connected with the effect of solvents on the reactants (i.e. with initial-state effects). However, a careful and extensive investigation by Von Halban(2) in 1913 showed conclusively that for the reaction of trimethylamine with p-nitrobenzyl chloride, solvent effects on the reactants could not account quantitatively for the overall influence of solvent on the reaction rate constant. Little further progress was made on these lines until the advent of transition state theory, when it then became clear that in principle it was possible to dissect the influence of solvent on rate constants into initial-state and transition-state contributions(3-5). 

In A we have two neutral reactant molecules which are not appreciably solvated, hence will not be appreciably affected by the change in medium from protic to dipolar aprotic, so that the initial state is coincident in the two media. However, the polarizable transition state will be more solvated in a dipolar aprotic medium such as DMF, so there should normally be a rate enhancement on changing to a dipolar aprotic medium (6h is negative). This is the Menschutkin type reaction which is discussed in detail by Professor Abraham(36), who has observed cases in which the realation-ship of the coincident initial states in the two media does not hold(31). However this relationship does hold for this particular reaction and we selected it so as to contrast with the other reaction types shown in this Figure, in which the initial state is not coincident in the two media. It may perhaps be emphasized that this is the only case, of the systems illustrated here, in which changes in the transition state solvation are solely responsible for the medium effect on the reaction rate. In terms of the classification in Table I we can describe this as a "positive transition state control" reaction type. 

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