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I was wondering in this problem that if chlorine will show positive mesomeric effect as it is at para-position and strength of inductive effect decreases with distance from carbon atom and chlorine can expand its octet due to presence of $\mathrm{d}$ orbitals so will this structure be more stable than toluene carbocation I mean if we remove $\ce{Cl}$ from the para-position of same carbocation?

p-Chlorotoluene cation

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Cloro substituent in an aryl ring has two effects combined:

  1. I- effect, owing to its electronegative character and tendency to pull electrons towards itself from the ring and thus deactivating the ring.

  2. M+ effect to donate electron to the ring by resonance.

Generally the I- effect is stronger than the M+ effect, leading to an overall decrease in carbocation stability and deactivation of the ring towards attack by an electrophile.

The M+ effect occurs owing to the formation of strong pπ-pπ lateral bond due to relatively similar sized, 2p orbital of carbon of ring and 3p orbital of chlorine. Since a number of lone pares occur in chlorine these may be donated to the ring by resonance.

However due to the large size and highly diffused nature of the 3d orbitals of chlorine the overlap with 2p of carbon in ring is not very efficient and therefore back bonding or reverse conjugation is not observed.

The above 4-chlorocarbocation is less stable than benzyl carbocation (formed by removing a proton from toluene).

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    $\begingroup$ "large size and highly diffused nature of the 3d orbitals" The biggest factor is simply that the 3d orbitals are just very high in energy. Backbonding into the 3d orbitals is just not energetically feasible. $\endgroup$ Commented May 1, 2021 at 15:30

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