I've read that Tropylium carbocation is formed by the ring expansion of benzyl carbocation. But in benzyl carbocation, the $\ce{C}$ atoms in the benzene ring are $sp^2$ hybridized i.e. $120^\circ$ is the required angle for stability. And in the benzene ring of the benzyl carbocation, the angle is already 120 degrees. Therefore benzyl ring need not expand for stability. Therefore why does benzyl rearrange to give tropylium?

  • $\begingroup$ Note that such rearrangement is typical for mass spectrometry, not "normal" reactions. $\endgroup$
    – Mithoron
    Apr 18 '17 at 19:29

More stable products are lower in energy. At first glance the bonding arrangements might seem equivalent in both cases in terms of number of $\ce{C-C}$ and $\ce{C-H}$ bonds from $\ce{sp^2}$ carbons, which would result in no net change. However, the change in molecular structure leads to a change in the molecular orbitals and the resulting energies of the molecule for the aromatic/resonance component to the energy. Using the Frost diagram technique we want to compare the orbital energies, nicely pictured here:

enter image description here

The key point to notice is the filled orbitals for seven member ring are slightly lower in energy than the filled orbitals for the six-member ring. This means there is an increase in resonance stabilization energy for making a fully aromatic system rather than just a cation in resonance with benzene ring.

  • $\begingroup$ It does not work that simply. In benzyl cation the non-ring carbon is conjugated with the ring and that alters the Frost circle. $\endgroup$ May 4 '17 at 0:59
  • $\begingroup$ This comment is less helpful than one might assume at first glance. Do you have, for example, some QM calculations to back this up? $\endgroup$
    – Lighthart
    May 4 '17 at 15:43
  • 1
    $\begingroup$ if we are using Frost circles then we can generalize it to the Huckel model. The pi electron stabilization I get from that model = $8.00\beta $ (vs nonbonding $p $ orbitals) for benzene but $8.72\beta $ for benzyl cation, versus $8.99\beta $ for tropylium. The predicted driving force for rearrangement is still positive but much less than if you ignored the conjugation of the benzylic carbon. So, I would expect the benzylic conjugation to be fairly important in more advanced models too. $\endgroup$ May 5 '17 at 0:03
  • $\begingroup$ That gets a little tricky, since benzylic conjugation would disrupt the aromaticity, ya? $\endgroup$
    – Lighthart
    May 5 '17 at 16:42
  • $\begingroup$ That's what the more general model is for. For an all carbon system you just set up the connectivity matrix and get the eigenvalues in units of $\beta $, and those are your energy levels (positive= bonding, negative =antibonding). The Frost circle is just the pattern of eigenvalues specific to a single ring with no pendant pi bonds. $\endgroup$ May 5 '17 at 18:05

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