Entropy pericyclic reactions

The cycloaddition reaction rate revealed activation parameters which are characteristic of a pericyclic reaction. The entropy of activation is large (ASt = -176.95 Kr1 mor1), and the enthalpy of activation is small (A//t = 30.3 kJ mol-1)- These data imply a highly ordered, early transition state with concerted bond making and breaking. Solvent polarity had only a small effect on the rate of the reaction, indicating little separation of charge in the transition state, consistent with a concerted mechanism. 

The Diels—Alder reaction is a pericyclic reaction, which takes place in a concerted single step by redistribution of bonding electrons. The two reactants simply join through a cyclic transition state in which the two new carbon—carbon a-bonds are formed at the cost of the two rr-bonds of the diene and dienophile. The high syn stereospeciflcity of the reaction, the low solvent effect on the reaction rate, and the large negative values of both activation entropy and activation volume comprise the evidence usually given in favor of a pericyclic Diels—Alder reaction. 

Fxirther evidence for a highly ordered transition state conies from the observation that entropies of activation (-125 to -170 J/moUK) are comparable to that of the Diels-Alder reaction or other pericyclic processes. Additionally, high pressure chemistry studied on Alder-Ene reactions with mesoxalate showed a large negative volume of activation (-36 to -48 mL/mol), similar to the Diels-Alder reaction. One would expect the biradical process to be about 10 mL/mol less than the pericyclic process. 

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