Transition-state Model for Solution Reactions

In solution, the intimate contact between solute and solvent molecules, constituting as it does a state of constant collision, makes for a rate of energy transfer between solute and solvent as rapid, probably, as that between loosely coupled, normal modes of vibration in a single, large molecule. With the exception of very unusual cases, this will be of the order of magnitude of vibration frequencies (that is, 10 sec ), which is sufficiently rapid that we may expect to find transition-state complexes in nearly good thermodynamic equilibrium with unreacted species. Under these conditions, w e may employ the formalism of any of the transition-state treatments which has been developed earlier. 

For an nth-order reaction between species A, B, C,.. . to produce a transition-state complex whose rate of disappearance may be equated to the rate of appearance of products we may write 

Reaction Solvent Rate constants, X 10 , liters/mole-sec Temp, °C Activation energy, Kcal/mole 

2 polymethyl methacrylate radicals stable products Methyl methacrylate 0.024 25 2.8 

If the over-all specific rate of appearance of products is given by the expression dP jdi = /c (A)(B) , where is the experimentally observed nth order specific rate constant, we can write for hn 

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Since the transition-state model for solution reactions is an equilibrium model, the true testing ground for solution theories will be not kinetic data but rather thermodynamic data. From this point of view, the continuum model of the solvent has had only fair success— for example, in correlating thermodynamic data for ions. Laidler has recently shown that the entropies of aqueous monatomic ions of molecular weight M with reference to = —5.5 e.u. may be repre.sented by the empirical... 

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