For comparing the stability of different compounds that undergo resonance, I was taught to first check for equivalent resonance, then the number of atoms on which the delocalization was taking place and so on(e.g phenolate ion vs acetate ion)

But comparing benzyl carbocation with $\ce{CH2=CH-CH2+}$, even though $\ce{CH2=CH-CH2+}$ has equivalent resonating structures(and hence a greater extent of delocalization of electrons), it is less stable than benzyl carbocation. Why?

  • 3
    $\begingroup$ Benzyl carbocation has more number of resonating structures than this carbocation. $\endgroup$
    – Arishta
    Mar 26 '17 at 10:32
  • $\begingroup$ @cotton phenolate ion has more resonating structures than acetate ion too but because of equivalent resonating structures, the acetate ion is more stable. $\endgroup$
    – xasthor
    Mar 26 '17 at 10:39
  • $\begingroup$ @xasthor, see when a negative charge is delocalised on a set of electronegative elements, it can be considered stable, and similarly for a positive charge on electropositive ones, but comparing the resonance of acetate ion,with that of an allyl carbocation, doesn't seem fit. I agree with what cotton said, the no of resonating structures determines the resonance energy, hence stability. $\endgroup$
    – SubZero
    Mar 26 '17 at 10:43
  • 2
    $\begingroup$ @Cotton (very first comment) Not to forget that beside "simple resonance" structures drawn on paper, benzyl cations rearrange into Hueckel aromatic tropylium cations, as seen in mass spectroscopy at $m/z = 92$, too. $\endgroup$
    – Buttonwood
    Mar 26 '17 at 11:05
  • 3
    $\begingroup$ More resonance structures does not necessarily mean more stable. Maybe here it does, but not in general. $\endgroup$
    – orthocresol
    Mar 26 '17 at 11:39

What all the comments boil down to is we are comparing different things:

Allyl vs benzyl cations: Both contain onlybhydrogen and carbon, with the positive charge formally distributed over two carbon atoms in allyl versus four carbon atoms. Putting the positive charge on more carbon atoms makes the benzyl cation more stable.

Acetate vs phenolate anions: Here, unlike the hydrocarbon actions mentioned above, oxygen enters the picture. The negative charge in the acetate ion is distributed over two oxygen atoms, versus phenolate having its negative charge on one oxygen plus multiple carbons. Having the second oxygen atom wins out because of the superior electronegaytivity of oxygen, which is a nonfactor for the cation case mentioned above.

Note that the "rules" above are not absolute laws. You have to consider other factors such as aromatic or antiaromatic coupling that involves the charge; e.g. cyclopropenyl cation is much more stable than benzyl cation because of the positive charge being inherent to an aromatic ring, even though cyclopropenyl has only three instead of four carbons to take up the charge.


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