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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?

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

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    $\begingroup$ Do you mean the back bonding due to fluorine, and hence filling up of the the empty d orbitals orbitals, making it pretty stable, and it refusing to add any more electrons? $\endgroup$ – Sujith Sizon Dec 2 '15 at 11:02
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    $\begingroup$ @SujithSizon They don't really fill any d-orbitals and there's no such backbonding $\endgroup$ – Mithoron Dec 2 '15 at 16:52
  • $\begingroup$ @Mithoron But phosphorous is electropositive and lone pair of fluorine is hence more closer to the central phosphorous atom, partially occupying d orbitals, right? $\endgroup$ – Sujith Sizon Dec 2 '15 at 17:08
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    $\begingroup$ @SujithSizon chemistry.stackexchange.com/a/5242/9961 $\endgroup$ – Mithoron Dec 2 '15 at 17:13

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