# Which factor should I take into priority to determine reactivity of of Halides?

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

• Are you familiar with the concept of aromaticity? The Hückel rule can be applied to charged species too! – Klaus-Dieter Warzecha Feb 25 '15 at 10:24
• Yes I know Huckel rule. Kindly, enlighten me the procedure. In which way I should apply the rule. – Dhruba Banerjee Feb 25 '15 at 10:27

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.