Decreasing order of electron density in benzene ring among the halobenzenes 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.
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 -$\ce{F}$ &gt; -$\ce{Cl}$ &gt; -$\ce{Br}$ &gt; -$\ce{I}$ due to the decreasing electronegative character as we move from fluorine to iodine.
The +M effect also varies as -$\ce{F}$ &gt; -$\ce{Cl}$ &gt; -$\ce{Br}$ &gt; -$\ce{I}$ 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
-$\ce{F}$ &gt; -$\ce{Cl}$ &gt; -$\ce{Br}$ &gt; -$\ce{I}$
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
$\ce{Ph-F}$ &lt; $\ce{Ph-Cl}$ &lt; $\ce{Ph-Br}$ &lt; $\ce{Ph-I}$
This trend is validated by the order of reactivity of these compounds towards electrophilic substitution reactions.
Hope it helps!