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I learned that the order of $+M$ effect (mesomeric effect) for halogens is as follows: $\ce{F} > \ce{Cl} > \ce{Br} > \ce I$.

I wonder why this is so?

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    $\begingroup$ Can you briefly describe what should than be observable result of such effect? $\endgroup$
    – ssavec
    Dec 13, 2013 at 20:44
  • $\begingroup$ I am confused why would Florine being more EN and smallest will have the highest tendency to donate lone pair than Iodine. $\endgroup$ Dec 14, 2013 at 8:06

3 Answers 3

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$\ce{2p-2p}$ orbitals overlaps better than $\ce{2p-3p}$, $\ce{2p-4p}$, $\ce{2p-5p}$. Although $\ce{F}$ is more electronegative, but it is also smaller in size. Both $\ce{F}$ and $\ce{C}$ (next to it) have $\ce{2p}$ orbitals to overlap.

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Found the answer, it is because of size difference that smaller halogens are able to make more effective p orbital overlap with carbon.

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  • $\begingroup$ But shouldn't fluorine have a lower urge to use its electron pair for resonance due to smaller radius and higher electronegativity and won't the bond me more polarized in case of fluorine as compared to that of iodine, leading to a weaker covalent bond...? $\endgroup$
    – oshhh
    Feb 19, 2017 at 15:20
  • $\begingroup$ In general, mesomeric effects are greater than inductive effects, as mesomeric effects change the nature of a bond, while mesomeric effects only polarize them. $\endgroup$
    – Mäßige
    Jun 27, 2023 at 10:51
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Basically electronegativity is related to ability of atom to attract BOND PAIR electron, so donation of Lone Pair is not concerned with electronegativity. This leaves us to look for overlapping, generally we talk about donating lone pair to carbon atom which has 2p orbital and fluorine too have lone pair in 2p orbital as 2p - 2p overlapping in better than 2p -3p and so on this +M of F>Cl>Br>I.

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