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I have read that halogens have a stonger -I (inductive effect) than +M (mesomeric effect),then why do they stabilise a carbocation instead of destabilising it?

I asked this doubt to my teacher and he said that halogens like fluorine do not exert -I on a carbocation and hence they stabilise a carbocation through +M,is this true?

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Fluorine belongs to the same period as carbon. Carbocation is electron deficient which has its two $\mathrm{2p}$ orbitals vacant. One electron of carbon in $\mathrm{2p}$ orbital goes into the formation of $\ce{C-F}$ σ bond. $\ce{F}$ has $\mathrm{2p}$ orbital containing a lone pair which can be donated to the $\mathrm{2p}$ vacant orbitals of carbocation. Same size $\mathrm{2p}$ orbitals of carbon and fluorine overlap effectively to form a π bond.

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Carbocations are stabilized by neighboring atoms with lone pairs. The key stabilizing influence is that the neighboring atom can donate a pair of electrons to the electron-poor carbocation. In this way halogenes, nitrogen or oxygen can actually be an electron-donor group and stabilize the carbocation.

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  • $\begingroup$ This is stated in the question. The actual question is about why the inductive effect does not dominate the mesomeric effect in carbocations, whereas it usually does for fluorine. $\endgroup$
    – bon
    Jan 14 '16 at 19:12
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Most of the times the question that comes in the examinations is that why CH2F+ is more stable than CH2Br+ or CH2I+. The reason behind it is that the p orbital of fluorine overlaps more effectively than that of Br or I with the carbon atom and therefore disperses the charge on it. refer to K L Chugh for the details.

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