Menschutkin reaction solvent effects

Table 8-10. Solvent Effects and Transfer Free Energies for the Menschutkin Reaction of Triethylamine and Ethyl Iodide" ...

In this contribution, we describe and illustrate the latest generalizations and developments[1]-[3] of a theory of recent formulation[4]-[6] for the study of chemical reactions in solution. This theory combines the powerful interpretive framework of Valence Bond (VB) theory [7] — so well known to chemists — with a dielectric continuum description of the solvent. The latter includes the quantization of the solvent electronic polarization[5, 6] and also accounts for nonequilibrium solvation effects. Compared to earlier, related efforts[4]-[6], [8]-[10], the theory [l]-[3] includes the boundary conditions on the solute cavity in a fashion related to that of Tomasi[ll] for equilibrium problems, and can be applied to reaction systems which require more than two VB states for their description, namely bimolecular Sjy2 reactions ],[8](b),[12],[13] X + RY XR + Y, acid ionizations[8](a),[14] HA +B —> A + HB+, and Menschutkin reactions[7](b), among other reactions. Compared to the various reaction field theories in use[ll],[15]-[21] (some of which are discussed in the present volume), the theory is distinguished by its quantization of the solvent electronic polarization (which in general leads to deviations from a Self-consistent limiting behavior), the inclusion of nonequilibrium solvation — so important for chemical reactions, and the VB perspective. Further historical perspective and discussion of connections to other work may be found in Ref.[l],... 

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... 

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),... 

The role of solvent effects in quaternization is one of the first physical organic studies and this is due to Menschutkin (1879LA334). It shows an increase in relative rate from 1 to 742 on going from benzene to benzyl alcohol, which suggests no simple explanation. Typical ranges of solvent-dependent rate ratios are 15,700/1 (nitromethane/cyclohexane) in the alkylation of triethylamine by methyl iodide (68BSF2678), 1660/1 [dimethylsulfoxide (DMSO)/carbon tetrachloride] in the reaction of l,4-diazabicyclo[2,2,2]-octane (DABCO) (5) with (2-bromoethyl)benzene (75JA7433) (Scheme 5),... 

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 ... 

For many physical organic chemists, the Menschutkin reaction was a kind of guinea pig , which has been extensively used for the study of solvent effects on chemical reactivity. A comprehensive review of this reaction has been given by Abboud el al. [786], More recent theoretical treatments of the solvent influence on Menschutkin reactions can be found in references [787-789]. 

Although Eq. (5-87) is often qualitatively obeyed, as has been frequently mentioned, there is no exact linear correlation between the rate of Menschutkin reactions and the functions of relative permittivity as in the case of Fig. 5-12 [246, 247]. A complete absence of a regular effect of changes in the dielectric properties of the solvent on the reaction rate has also been observed [248, 249]. Sometimes a satisfactory correlation has been obtained because the reaction under consideration was studied in only a limited... 

As the data for the Menschutkin reactions indicate, the character of the solute-solvent interactions is more complex than described by Eq. (5-87). It is evident that functions of relative permittivity alone, as given in Eq. (5-87), are not useful for describing the solvent effect on reactions between dipolar reactants, except in certain special cases, such as when a mixture of two solvents is used. In addition to electrostatic forces, non-electrostatic interactions, such as dispersion forces and hydrogen-bonding, must also be involved in Menschutkin reactions. 

Similar results are found for the above-mentioned Menschutkin reaction (c). The added pro tic solvent can also combine with the main solvent. Such solvent/solvent association leads to a diminution of the specific inhibitory and catalytic effect of protic solvents on the Sn2 reaction (c) [584], The basicity of the main solvent determines the extent of deactivation of the protic solvent through H-bond association this is analogous to Eq. (5-107). 

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],... 

Table 5-27. Effect of external pressure and solvent polarity on reaction rate and activation volume of the Menschutkin reaction between triethylamine and iodoethane at 50 °C [441] cf. also Table 5-5 in Section 5.3.1 [59].

It is expected that the excellence of a correlation should diminish as the correlated process and the model process are made increasingly different in their mechanistic character. Thus, we should not expect great generality from univariate correlations on the other hand, from the quality of the correlation we may be able to leam something about the correlated process. It is not surprising that the rate of one Menschutkin reaction is well correlated with the rate of another Menschutkin reaction, for their mechanisms should be very similar. The solvent effect data (8mAG ) in Table 8-10 are poorly correlated with the Kirkwood dielectric constant function, but a plot against Er (30) shows some improvement. - In this... 

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. 

Cerveny et al. (69), attempted to apply the Drougard-Decroocq equation (34) to the data obtained in the hydrogenation of cyclohexene and 1-hexene on the catalyst 5% Pt on silica gel in 19 solvents. Since correlation of the reaction rates with the parameters of solvents x, originally obtained for the homogeneous Menschutkin reaction (88) between methyl iodide and tri-propylamine was unsuccessful, an analogous definition was used for the parameters x, which were to characterize the solvent with respect to its effect in heterogeneously catalyzed reactions ... 

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