Rotational Barrier and Quantification of Electron-Donating Substituent Effects: a Computational Study of para-Substituted Benzaldehydes

The rotational barrier around the phenyl-formyl bond between the minimum and transition states of para-substituted benzaldehydes was computationally studied for 34 electron-donating substituents. The rotational barrier exhibited very good correlation with shortening of the phenyl-formyl bond, lengthening of carbonyl bond, increase of electron density at the formyl group, increase of stabilization energy, lowering of chemical shift in the 13C NMR of the formyl carbon, and with the values of empirical Hammett σp+ constants. Therefore, rotational barrier is a useful quantum mechanical parameter for quantifying the electron-donating substituent effect and π-conjugation in parasubstituted benzaldehydes. Based upon the rotational barrier a scale has been set in this work to judge the electron donating effect of substituents. Moreover, a canonical structure has been proposed for stronger electron-donating substituents. The results of this study reveal that simultaneous presence of electron acceptor formyl group and electron-donating groups is mandatory for the extension of resonance stabilization.