Kinetics and Mechanism of 1,4-Cycloaddition between Tetracyanoethylene and Styrenes. III. Effects of Solvent Polarity and Temperature

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Abstract
Experimental evaluation was made of rate constants for foward and backward steps of 1,4-cycloaddition between TCNE and α-methylstyrene and of constants for their complex formations in 1,2-dichloroethane, dichloromethane, chloroform, and carbon tetrachloride at a few temperatures around 25 °C. Kirkwood’s dipole model for solvation is applied to analyzing kinetic data obtained. The dipole moment of the transition state is estimated to be 8.5±0.5 Debye, which is larger than either that of the complex (assumed as 1 Debye) or that of the cycloadduct (6±1 Debye). Major cause for the high dipole moment of the transition state is breakdown of the high molecular symmetry of TCNE and minor one is a certain contribution of a zwitterionic structure. The activation entropy for the cycloreversion is negative and in the range of −21 to −48 J K−1 mol−1, corresponding to the high polarity of the transition state.

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