Dramatic solvent effect on the volume of the Diels-Alder reaction between tetracyanoethylene and cyclopentadiene

The partial molar volumes (V) and the enthalpies of dissolution (Delta(dis) H) for tetracyanoethylene, cyclopentadiene, and their Diels-Alder adduct we re determined at 25 degrees C. Eleven solvents of the pi- and n-donor type were used. The use of alkylbenzenes as solvents for tetracyanoethylene indu ces pronounced changes in the enthalpy of dissolution (up to 26 kJ mol(-1)) and in the partial molar volume (up to 11 cm(3) mol(-1)), whereas these pa rameters for the adduct change slightly. The V and Delta(dis)H values for c yclopentadiene virtually do not depend on the nature of the solvent. In the case of tetracyanoethylene and the adduct in n-donor solvents, considerabl e variations of the V and Delta(dis)H values are observed; they are not lin ear functions of the change in the partial molar volume of the adduct. Ther efore, the reaction volumes in acetonitrile (-40.69) and ethyl acetate (-45 .56) differ sharply from those in o-xylene (-24.28) and mesitylene (-21.76 cm(3) mol(-1)).