# Why can't phosphorus trifluoride be used as a Lewis acid catalyst in electrophilic substitution of benzene?

Which of the following cannot be used as Lewis acid in the electrophilic substitution of benzene?

1. $\ce{BF3}$
2. $\ce{SbCl3}$
3. $\ce{FeCl3}$
4. $\ce{PF3}$

My Attempt:

1. $\ce{BF3}$ and $\ce{FeCl3}$ are Lewis acids.
2. $\ce{PF3}$ though it has a filled s and p orbitals it can accept electrons from donors and expand its coordinate number up to six due to vacant d orbitals. It acts as a mild Lewis acid.
3. $\ce{SbCl3}$ has a completely filled s,p,d orbitals so it can no longer accept electrons, hence doesn't act as a Lewis acid.

My answer is option (2) but the answer provided is (4), where am I going wrong?

• This is indeed confusing. I expect both B and D not to be Lewis acids (they are both Lewis bases). – Jan Dec 1 '15 at 16:56
• @Jan They can both act as acids or bases depending on situation. en.wikipedia.org/wiki/Antimony_trichloride, en.wikipedia.org/wiki/Phosphorus_trifluoride – Mithoron Dec 1 '15 at 17:30
• @Jan perhaps the real question might be, will all Lewis acids catalyse EAS reactions? – Sujith Sizon Dec 1 '15 at 17:49
• Yes, probably PF3 is simply too weak to do it. – Mithoron Dec 2 '15 at 16:48

The best answer I can give is that there is enough space on $\ce{Sb}$ in $\ce{SbCl3}$ for an additional Lewis Base to interact with a nonbonding orbital on the $\ce{Sb}$ atom. While $\ce{P}$ can be hypervalent, the LUMO in this case would be $\ce{P-F}$ $\sigma^\ast$, which is really high in energy (meaning that it doesn't easily accept electron density from other species in solution).