Relation between Structure and Dissociation Constant in the Hexaarylethane Series

Relation between Structure and Dissociation Constant in the Hexaarylethane Series 

Because of the scarcity of electronic paramagnetic resonance data, and because of the frequent unreliability of the data from paramagnetism, boiling point elevation, spectrophotometry, and ortho-para hydrogen conversion, most published radical dissociation constants can be accepted only with reservations. An error of 50 % is not at all improbable in many cases. We are therefore not yet in a position to explain, or rather to test our explanations of, small differences in dissociation constants. Table I shows the values of K corresponding to various hexaarylethanes in benzene at 25°. Because of the order of magnitude differences in Table I, however, it is likely that some of the expected large effects, such as steric and resonance effects, exist. 

There also seems to be an unexpected effect of meta substituents. Lichtin and Glazer have suggested, since purely radical structures can not involve meta substituents, that combined radical and dipole structures are important 20 

No similar meta substituent effect exists for ions since resonance between a singlet and triplet (diradical) state is prohibited. The dipolar ion theory would suggest that suitably disubstituted radicals should be especially stable, particularly in polar solvents, because of structures like III. 

It has been reported that the degree of dissociation of unsymmetri-cal hexaarylethanes is usually somewhat less than the average of the related symmetrical ethanes.21-22 Unfortunately, the available ethanes do not include any with extremely electronegative and electropositive substituents in opposite parts of the ethane molecule. A marked difference in electronegativity of the two radicals might increase the strength of the central bond by increasing its ionic character. 

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