Solution reactions Eyring plot

Modification of the reaction medium corresponds to a change in the intimate microscopic solute to solution interactions. Hence, the influence of the solvent on the selectivity is a function of enthalpy and entropy contributions522. Thus, Eyring plots (diastereomeric excess vs. temperature) for the reaction of the lithium enolate of /-butylacetate with racemic 2-phenylpropanal gave AAH = —0.70 kcalmol-1 and AAS = —0.10 cal mol 1 K in //-hexane and AA= —0.59 kcalmol-1 and AA.S, = +1.7 calmol-1 K in THF. The... 

This equation is known as the Eyring equation. When In fe is plotted versus 1 /T, a line is obtained having a slope of —AH /R. Once AH is known, AS can be calculated by means of this equation. The entropy of activation is a useful property that is based on the choice of standard states. For a gas phase reaction in which a molecule X—Y dissociates, AS would be expected to be positive. However, if the reaction takes place in solution and if the solvent is polar, dissociation of X—Y into X and Y followed by solvation of the ions could result in AS being negative. It should be noted that Eq. (2.194) appHes strictly only to first-order processes. For appHcations to other reaction orders, see R. Schmid and V. N. Sapunov, Non-formal Kinetia, Verlag Chemie, Weinheim, 1982, p. 110. We will have more to say in later chapters about the effects of temperature and solvation on reaction rates. 

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