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Such as in the attached question I was taking into account the stability of the carbocations (by the number of resonating structures) formed after removal of Chlorine from and concluded to be the order I>II>III>IV>V the answer states the order to be I>III>IV>V>II

Can conclude what factors they are priority to. Help needed enter image description here

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  • $\begingroup$ Are you familiar with the concept of aromaticity? The Hückel rule can be applied to charged species too! $\endgroup$ – Klaus-Dieter Warzecha Feb 25 '15 at 10:24
  • $\begingroup$ Yes I know Huckel rule. Kindly, enlighten me the procedure. In which way I should apply the rule. $\endgroup$ – Dhruba Banerjee Feb 25 '15 at 10:27
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If you need to consider the stability of the carbocations obtained after chloride leaves, we can, as Klaus commented already, use Hückels rule, which more or less states that fully $\ce{sp^2}$ (i.e. usually $\ce{sp^2}$, as long as there is atleast one full p-orbital) hyberdized compounds with $4n+2$ electrons are particularly stable or aromatic. The same type of compounds that have $4n$ are particularly unstable, anti-aromatic. Of course a carbocation has an empty p-orbital, so compunds I and III will be aromatic and compound II anti-aromatic. Compound I is fully benzenoid and also less strained than compound III and will hence be more stable, so I > III. Then follows the allylic carbocation IV which is obviously more stable than its non-allylic counterpart V. And last but not least the anti-aromatic compound II.

In conclusion I > III > IV > V > II, as the answer indicated.

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  • $\begingroup$ shouldn't it be 4n + 2 $\endgroup$ – bon Feb 25 '15 at 15:15
  • $\begingroup$ Yes, I was about to suggest the edit. $\endgroup$ – Dhruba Banerjee Feb 25 '15 at 18:30

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