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Is it because, due to the eletronegativity of Fluorine, his mesomeric effect will be less pronounced, which means the electronic density will be lower, than for the clorobenzes, in the ortho positions?

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    $\begingroup$ Mesomeric effect of fluorine is stronger. However, flourine has a stronger inductive effect too and its ortho position is more severly deactivated than chlorine's. $\endgroup$ – Sawarnik Apr 5 '17 at 6:31
  • $\begingroup$ Halogens — though deactivating groups — are ortho-para directing. The inductive effect reduces the electron density at ortho position more than in the para position (inductive effect varies with distance). As fluorine is more electronegative than chlorine, you will get less ortho products with fluorine than with chlorine. $\endgroup$ – Yashas Apr 5 '17 at 14:39
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Halogens are deactivating groups even though they show $+M$ mesomeric effect. The inductive effect is stronger than mesomeric effect. As the halogens show $-I$ inductive effect, they are deactivating groups.

When it comes to selectivity, the mesomeric effect plays a role. As the halogens show $+M$ mesomeric effect, the resonance structure has higher electron density at ortho and para positions. Therefore, halogens are ortho-para directing.

When you take inductive effect into account for its directing properties, the para position is more favorable for electrophilic reagents because it has a higher electron density compared to the ortho position. The strength of the inductive effect decreases with distance. As the ortho position is closer to the halogen, the inductive is stronger at this position. Therefore, the major product for electrophilic substitution reactions for haloarenes is the para-substituted product.

Fluorine is more electronegative than chlorine and shows a higher tendency to withdraw electrons towards itself than chlorine. Therefore, fluorine shows a stronger inductive effect than chlorine. As fluorine reduces the electron density at the ortho position more than chlorine, you get a lesser amount of ortho substituted product with fluorine.

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