What should be the order of electron density in monosubstituted halogen derivatives of benzene?

I am aware that fluorobenzene is more ring activating as compared to chlorobenzene due to the more effective +M effect of F over Cl, but I would like to know the full order among all the monohalogenated benzene compounds.

Is there any significant +M effect shown by Br, I with the benzene ring? If yes, then how strong is it as compared to their –I effect?

Please let me know the correct final order of electron density in the benzene ring among Ph—F, Ph—Cl, Ph—Br, Ph—I, as well.

  • $\begingroup$ In the case of electrophilic nitration of the halobenzenes the differences between the 4 compounds is less than a factor of 10; if we include benzene in the series, then the differences are up to a factor of 50 - still small. See this earlier answer for the correct order. At least in the case of nitration @Adiboy's ordering doesn't match the experimental result. $\endgroup$
    – ron
    Nov 29 '20 at 15:27

Firstly, I would like to talk about the order of withdrawing effect among halogens. Since all the four halogens are electron-rich, they tend to donate electrons (+M effect). However, the high electronegativity of these elements hinders the donation by applying an even greater electron-withdrawing effect (-I effect) that overcomes the +M (resonance) effect.

In the case of halogens, the -I effect varies as -$\ce{F}$ > -$\ce{Cl}$ > -$\ce{Br}$ > -$\ce{I}$ due to the decreasing electronegative character as we move from fluorine to iodine.

The +M effect also varies as -$\ce{F}$ > -$\ce{Cl}$ > -$\ce{Br}$ > -$\ce{I}$ due to the decreasing overlap between the 2p orbitals of carbon and the 2p, 3p, 4p and 5p orbitals of the halogens respectively. Since the -I effect is stronger than the +M effect as mentioned earlier, the final order of electron-withdrawing capability is

-$\ce{F}$ > -$\ce{Cl}$ > -$\ce{Br}$ > -$\ce{I}$

Equipped with this knowledge, we come back to the question of comparing the electron densities of mono halogen benzene derivatives. From the above-mentioned trend, we can infer that the order of electron density is

$\ce{Ph-F}$ < $\ce{Ph-Cl}$ < $\ce{Ph-Br}$ < $\ce{Ph-I}$

This trend is validated by the order of reactivity of these compounds towards electrophilic substitution reactions.

Hope it helps!

  • $\begingroup$ Interesting. But, do you mean "electrophilic substitution reactions", instead of "nucleophilic", in the last line? Because, I think, an increase in electron density would mean an increase in electrophilic substitution reactions. $\endgroup$ Nov 29 '20 at 5:51
  • $\begingroup$ Also, due to the absence of an EWG group, nucleophilic aromatic substitution seems unlikely to me. $\endgroup$ Nov 29 '20 at 5:52
  • $\begingroup$ Yes, you are correct. I have edited my answer. Thanks for pointing out the mistake. $\endgroup$
    – Adiboy
    Nov 29 '20 at 6:00
  • $\begingroup$ No problem, brother. Also, if possible, could you provide the reference for the order of reactivity of the halobenzenes towards electrophilic aromatic substitution? $\endgroup$ Nov 29 '20 at 6:03
  • $\begingroup$ Will definitely let you know when I find one $\endgroup$
    – Adiboy
    Nov 29 '20 at 6:07

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