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