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background :

As proved by rate of EAS, mesomeric or simply conjugated resonance is weaker for halogens than inductive and make them electron taking, with fluorine almost doing nothing due to strongest mesomeric and strongest inductive among halogens, So, halogens should take electrons away from any system if they do it in a specially conjugated system known for many resonating structures

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If we have simple carbocation as shown in the chlorine and carbocation, being right next to chlorine Inductive effect must overpower resonance/mesomeric but why the opposite happens?

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Simply put, the energy costs for a sextet carbocation are too high to allow chlorine's strong inductive effects to dominate the situation.

The driving for for a carbon octet is very high.

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If we have a simple carbocation as shown in the chlorine and carbocation example, being right next to chlorine the inductive effect should overpower resonance/mesomeric but why does the opposite happen?

No, it shouldn’t. Remember that chlorine’s electronegativity — a measure for the strength of its inductive effect — is less than both oxygen’s and nitrogen’s. Therefore, the inductive effect is weakest for chlorine.

All things considered, resonance effects usually overpower inductive effects, especially at short distances.

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  • $\begingroup$ I'm with you on the weak inductive effect, but if you look at the inductive effect order, halogens beat NH2 and OH by a good margin, and 3p being so bad for resonance and having empty d orbital, back bonding strength is quite bad $\endgroup$
    – Mrigank
    Feb 5 '16 at 0:09
  • $\begingroup$ Aren't inductive effects supposed to fade out quickly with distance and mesomeric or conjugative effect remains constant? if we look at a lot of problems, almost in everyone , inductive effect wins over mesomeric FCOOH (pKa=1) $\endgroup$
    – Mrigank
    Feb 5 '16 at 0:19
  • $\begingroup$ @ELiT Stop any discussion with d-orbitals immediately. Main group elements’ d-orbitals are too far removed to take part in bonding. Yes, inductive effects are only relevant at short distances, but where mesomeric effects are helpful, they generally win. Note that in your carbonic acid fluoride example, the mesomeric effects are not helpful, so we have to consider inductive effects. $\endgroup$
    – Jan
    Feb 5 '16 at 0:20
  • $\begingroup$ In EAS, oxygen and nitrogen are both activating substituents (i.e. their resonance effect wins out over their inductive effect). Chlorine, however, is weakly deactivating. Chlorine, having a lower electronegativity than oxygen, should be activating if all resonance factors are equal. This suggests that chlorine's poor overlap with the aromatic LUMO causes such poor resonance to occur that its lower inductive effect still dominates. Perhaps the increased electronegativity of the carbocation reduces the chlorine's inductive effect, rather than forming a π bond? $\endgroup$
    – SendersReagent
    Feb 5 '16 at 0:45
  • $\begingroup$ @DGS At least I can fully agree with the aromatic discussion: chlorine is bad at overlapping with $\ce{C=C}$ double bonds in general. I think that could be the reason why the extremely weak inductive effect actually plays a role. While if there is an empty p-orbital next to chlorine, it doesn’t have to interact with a pi-bond making the entire thing easier. $\endgroup$
    – Jan
    Feb 5 '16 at 0:48

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