Eyring parameters inversion

The idea that an activated complex or transition state controls the progress of a chemical reaction between the reactant state and the product state goes back to the study of the inversion of sucrose by S. Arrhenius, who found that the temperature dependence of the rate of reaction could be expressed as k = A exp (—AE /RT), a form now referred to as the Arrhenius equation. In the Arrhenius equation k is the forward rate constant, AE is an energy parameter, and A is a constant specific to the particular reaction under study. Arrhenius postulated thermal equilibrium between inert and active molecules and reasoned that only active molecules (i.e. those of energy Eo + AE ) could react. For the full development of the theory which is only sketched here, the reader is referred to the classic work by Glasstone, Laidler and Eyring cited at the end of this chapter. It was Eyring who carried out many of the... 

The diastereofacial selectivity of the addition of lithioacetonitrile to 2-phenylpro-panol has been studied over a wide range of temperatures, solvents, and bases.256 Eyring plots [In(dr) vs 1 IT], activation parameters, and inversion temperatures have been characterized. In some cases, the differential entropy of activation, AAS, plays an exclusive role in determining anti -selectivity. 

Diastereofacial selectivity in the addition of lithioacetonitrile to 2-phenylpropanal has been found to be temperature and solvent dependent.42 Each solvent studied (g (benzene, toluene, w-hcxanc, cyclohexane, methylcyclohexane, THF, and diethyl ether) showed a different Eyring plot of In(anti/syn) versus 1 IT with specific differential activation parameters AA H and A AS, and disclosed the presence of inversion temperatures (7]nvs). 

We find two linear regions in the Eyring diagram for almost all the systems studied. The characteristic point where a change is observed in the slopes is called the inversion point, and the corresponding temperature is the inversion temperature (7] ,). From the two linear correlations determined for each aminoalcohol, two different sets of discrimination parameters can be observed AAHf and AASf) and defined for T> 7] v when AAH and AAS are obtained for T < ] . The two regions correspond to the dominance of enthalpy or entropy factors in the selection [46] (see Table 4). 

---