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This point (iv) has been given under reasons for extremely low reactivity of aryl and vinyl halides. I cannot understand it. First of all, isn't the positive carbon atom in phenyl as well as vinyl carbocation sp-hybridized. But in the point, the positively charged carbon atom of the phenyl carbocation is referred to as sp2-hybridized. I cannot understand why. Also I cannot understand the "perpendicular part". That the orbitals are perpendicular and so the carbocation is not formed. But how are they perpendicular and how does this hinder the formation of carbocation. I cannot understand the last diagram (for vinyl carbocation). Please help.

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marked as duplicate by Mithoron, andselisk, Todd Minehardt, Mathew Mahindaratne, Nuclear Chemist Feb 10 at 15:24

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ No, I do not think my question is a duplicate. My concerns are a bit different. If you read my question properly, you can understand. My question is mainly about why the perpendicularity of the orbitals destabilizes the carbocation, the hybridization of the carbon atoms and the meaning of the last diagram. $\endgroup$ – MrAP Feb 9 at 19:46
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As depicted, it is unlikely that the halobenzene splits to yield the phenyl cation, because the phenyl cation is of very high energy. It is of very high enery since the then empty C(sp2) orbital is not stabilized by the neighbouring C(p) orbitals (which otherwise create the continous cyclic cloud of electron density above / below the plane of the benzene molecule). Since the empty orbital (formal positive charge) is pointing away from the molecule -- think as pointing into the direction of the formerly bond halogen atom -- there is no stabilization of this orbital possible. A better visualization is the following one:

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(credit: Grossmann book, 2nd edition, p. 109. To match your case, think substitutent R equally stands for a hydrogen .)

While the dissociation drawn in your reference for halogen is unlikely, I would however object the idea that aryl cations can not exist. The intermediate generation of the aryl diazonium ion at low temperatures, typically followed by release of nitrogen gas (triple bond $\ce{N#N}$) is one of rare cases where this high energy situation may be compensated, offering a wide array of subsequent chemistry.

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(credit: source)

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Well here is a simple answer to your question.

Consider the resonating structures of aryl halide and vinyl halides. You will find that the C-X(carbon-halide) bond will have partial double bond character. Now double bonds are more powerful than single bonds. Hence they will not break easily and hence aryl and vinyl halides are less reactive towards neucleophillic substitution reaction.

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