Reaction in Solution and the Transition-State Theory

FIGURE 1.8 Effect of solvation on activation energy. Potential energy V versus the reaction coordinate. Solid curves represent the energy profile in the absence of solvation, (a) Solvation of the reactant (increased activation energy), (b) Solvation of the activated complex (reduced activation energy). 

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, 

The overall effect on the reaction rate thus depends on the free energies of the initial and transition states. The various possibilities, in terms of the free energy, ate summarized qualitatively in Table 1.3. Reinforcement occurs if the transfer free energies of reactants and transition state have opposite signs. If they have the same sign, partial or complete balancing is expected. 

For reactions in solntion an additional thermodynamic property that can be helpful is available. The effect of pressure on the equilibrium constant of a reaction yields the volume change of reaction, AT, given by Equation 1.28. 

TABLE 1.3 TYansfer Free Energies of Reactants and IVansition States and Solvent Effects on Reaction Rates. Classification of Reaction Types 

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