Polar solvents Menschutkin

The Menschutkin Reaction (1.27) provides an example of a case where in polar solvents the solvation of the activated complex has a major effect on the rate. Hartmann and Schmidt (1969) showed that in a series of 12 solvents of increasing polarity from 1,1,1-trichloroethane (c =7.52, =0.170) to nitrobenzene (e =34.78,... 

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],... 

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.

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].

In addition to the application of SnI reactions as model reactions for the evaluation of solvent polarity, Drougard and Decroocq [48] suggested that the value of Ig kj for the Sn2 Menschutkin reaction of tri-n-propylamine and iodomethane at 20 °C -termed according to Eq. (7-21) - should also be used as a general measure of solvent polarity. 

Z values have been widely used to correlate other solvent-sensitive processes with solvent polarity, e.g. the a absorption of haloalkanes [61], the n n and n n absorption of 4-methyl-3-penten-2-one [62], the n n absorption of phenol blue [62], the CT absorption of tropylium iodide [63], as well as many kinetic data (Menschutkin reactions, Finkelstein reactions, etc. [62]). Copol5mierized pyridinium iodides, embedded in the polymer chain, have also been used as solvatochromic reporter molecules for the determination of microenvironment polarities in synthetic polymers [173]. No correlation was observed between Z values and the relative permittivity e, or functions thereof [317]. Measurement of solvent polarities using empirical parameters such as Z values has already found favour in textbooks for practical courses in physical organic chemistry [64]. 

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... 

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... 

Whereas methyl transfer reactions proceed faster in nonpolar media, the Menschutkin (Sn2) reaction (CH3CH2)3N + CH3CH2I —> (CH3CH2)4N+ + T proceeds faster in polar media. Variations in nt upon a change on solvent polarity control chemical reactivity in methyl transfers, whereas for the above mentioned Sn2 displacement the significant changes of AG ovCTCome the effect of nt and dominate the variations on AG. ... 

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