In the previous post[1] I followed up on an article published on the theme “Physical Organic Chemistry: Never Out of Style“.[2] Paul Rablen presented the case that the amount of o (ortho) product in electrophilic substitution of a phenyl ring bearing an EWG (electron withdrawing group) is often large enough to merit changing the long held rule-of-thumb for EWGs from being just meta directors into being ortho and meta-directors, with a preference for meta. I showed how Paul’s elegant insight could be complemented by an NBO7 analysis of the donor-acceptor interactions in the σ-complex formed by protonating the phenyl ring bearing the EWG. Both the o– and m– isomers showed similar NBO orbital patterns and associated E(2) donor/acceptor interaction energies and also matched the observation that the proportion of meta is modestly greater than ortho substitution (steric effects not modelled). These interactions were both very different from those calculated for the para isomer.
References
- H. Rzepa, ""Typical Electron-Withdrawing Groups Are o, m-Directors Rather than m-Directors in Electrophilic Aromatic Substitution"", 2025. https://doi.org/10.59350/rzepa.28993
- P.R. Rablen, "Typical Electron-Withdrawing Groups Are <i>ortho</i>, <i>meta</i>-Directors Rather than <i>meta</i>-Directors in Electrophilic Aromatic Substitution", The Journal of Organic Chemistry, vol. 90, pp. 6090-6093, 2025. https://doi.org/10.1021/acs.joc.5c00426