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I came across a question asking for the major product on chlorination of m-nitrophenol. In my opinion the deactivating nature of nitro group should control the regioselectivity, but I found out the answer to be 4-chloro-3-nitrophenol. But why is the activating nature of the hydroxyl group dominating over the deactivating nature of the nitro group?

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But why is the activating nature of hydroxyl group dominating over the deactivating nature of Nitro group?

This is the precise reason why Picric acid is formed. ( Check here )
The activating nature of the $\ce{-OH}$ group is strong enough to counter the deactivating nature of the nitro groups.
Look at Ron's comment on the question over here.
Even methyl group sustains deactivating nature of nitro group in the formation of TNT. So no wonder phenolic group, a better activator, dominates.

The phenolic group increases electron density on ortho para (making it o-p director) while nitro groups decrease e-density on the o-p positions (making it a meta director). When both these effects act simultaneously, the question is who wins and is the density on o-p carbons less than corresponding meta ones? (as that would decide where the electrophile will attack)

Resonace structures of the groups gives the solution.
In nitro group:

enter image description here

There are three charged atoms bonded together in the $\ce{-NO_2}$ molecule. This is less table than the positive charge bearing oxygen atom in phenolic group's resonance structure:

enter image description here

Hence phenolic group wins!!

Coming back to the given starting reactant:

enter image description here

So now we consider the dominating ortho-para effect of phenolic group over the meta-directing effect of nitro groups to get 4-chloro-3-nitrophenol:

enter image description here

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  • $\begingroup$ Well, I had already understood that OH is dominating more an activating group over the deactivating nature of nitro but I still didnt get my answer "why". Anyways, thanks for coming up to help :-) My question was "why is it a better activator" $\endgroup$ – Dhruba Banerjee Mar 1 '15 at 11:59
  • $\begingroup$ @DhrubaBanerjee I edited it to include the reason. $\endgroup$ – Del Pate Mar 1 '15 at 12:48
  • $\begingroup$ @DhrubaBanerjee This reasoning is a theoretical approach but experimental values confirm it . $\endgroup$ – Del Pate Mar 1 '15 at 13:42

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