# Why is this carbocation more stable?

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?

• Benzyl carbocation has more number of resonating structures than this carbocation. – Arishta Mar 26 '17 at 10:32
• @cotton phenolate ion has more resonating structures than acetate ion too but because of equivalent resonating structures, the acetate ion is more stable. – xasthor Mar 26 '17 at 10:39
• @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. – Supernova Mar 26 '17 at 10:43
• @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. – Buttonwood Mar 26 '17 at 11:05
• More resonance structures does not necessarily mean more stable. Maybe here it does, but not in general. – orthocresol Mar 26 '17 at 11:39