In phenyl carbanion, one carbon atom has a lone pair on it:

Structure of Benzene Anion

This answer claims that the anion is aromatic, because "the lone pair on the benzene is in the plane of the ring and hence orthogonal to the conjugated $\pi$-system so that it cannot interact with it."

Why is the lone pair perpendicular to the plane of the molecule? It seems clear to me that the carbon in question has three $\mathrm{sp^2}$ orbitals, perpendicular to which is a pure $\mathrm{p}$-orbital containing two electrons. Assigning one extra electron (i.e., a negative charge) to an uncharged carbon in a benzene ring shouldn't rotate its $\mathrm{p}$-orbital by $90^\circ$.

The $\mathrm{p}$-orbitals in carbon are, of course, always perpendicular to each other. The $\mathrm{p}$-orbital containing the lone pair should, therefore, be perpendicular to the hybridized orbitals.

  • 1
    $\begingroup$ No, the lone pair is in one of the three sp2 orbitals, not the p orbital. $\endgroup$ – orthocresol Mar 20 at 14:58
  • $\begingroup$ @orthocresol Whoops, the hydrogen was still tacked on to the carbon in my mental picture. Thanks. $\endgroup$ – Ray Bradbury Mar 20 at 15:03

The lone pair of electrons will be in a $\mathrm{sp^2}$ hybridized orbital and not in a pure p-orbital. Thus, it wil be in the same plane as the molecule. Only the lone pairs which are in an orbital perpendicular to the molecular plane can take part in resonance, so those $\mathrm{sp^2}$ electrons will not be counted in Hückel's rule.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.