Solvent Effects in Methyl Group Transfers

The mechanistic complexity of 8 2 reactions is removed in aqueous solutions. The solvation of the attacking nucleophile reduces the energy of the reactants below that of the complex, and the reaction occurs in a single step, with concerted bond breaking and bond making. 

Energies of activation for symmetrical exchange reactions in different media, in kJ moR.  

The comparison between gas phase and solution becomes more convoluted when crossreactions are considered. For example, the rate of the Cl + CHjBr reaction, at 298 K (in M see ), decreases from 8x 10 in the gas phase, to 0.4 in DMF, 6x 10 in methanol, and reaches 5 x 10 in water [23], We have seen that this reaction is exothermic in the gas phase. The difference in free energies of formation in aqueous solution, at 298 K in kJ/mol, of CHjCl (-57) and CHjBr (-24), almost compensates those of Cl (-131) and Br (-104), and the exothermicity of this reaction in aqueous solution, ACP = -6 kJ mol , is comparable with the value given previously for the gas phase [22]. Clearly, the minor decrease in exothermicity for the gas phase to solution cannot justify the 15 orders of mag-nimde change in the methyl transfer rate. This reactivity difference must also be related to the differential solvation of the TS and the reactants. 

In addition to the striking difference in absolute rates in different media, there are differences and reversals in relative rates. For instance, the halogen nucleophilic order is changed from F Cl Br I in the gas phase and polar aprotic solvents such as acetone and DMF, to I Br Cl F in protic solvents such as methanol and water [23]. This difference can also be understood in terms of differential solvation of the TS and the reactants. The enthalpies of transfer from dimethylsulphoxide (polar aprotic solvent) to methanol are, at 298 K in kJ/mol, Cl (—10.4), Br (—0.4), and I (11.3). The increase in the relative solvation of the smaller halides in methanol is enough to reverse their order of reactivity, since the solvation of the TSs is not as sensitive to solvent changes as the solvation of the reactant anions [23]. 

The fact that methyl transfers in water are elementary reactions suggests that they should be a better ground to test the applicability of free-energy relationships in 8 2 reactions. Albery and Kreevoy made an extensive smdy of such reactions and attempted to interpret their free-energy dependence using the Marcus cross-reaction scheme. According to the Marcus cross-relation, the free energy of activation of the cross-reaction 

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