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Here, in $\ce{CH3CONH2 -}$ group, the nitrogen has a lone pair and can thus activate the benzene ring along with having an ortho-para directive effect in the following manner:

enter image description here

However at the same time, both nitrogen and oxygen are electronegative. Hence, they should attract (roughly speaking) the electron cloud of the ring to deactivate the ring and may result in a meta directive effect in the following manner:

enter image description here

So which effect is applicable here and why?

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  • $\begingroup$ What is chlorine , fluorine ? What sort of director. $\endgroup$ – Avnish Kabaj Feb 24 '18 at 14:32
  • $\begingroup$ Halogens are, as far as i know, ortho para directing even though they are deactivating $\endgroup$ – Rajdeep Dutta Feb 24 '18 at 14:42
  • $\begingroup$ I believe the same principle is applicable here but not a deactivator as as halogens have the strongest $\pu{-I}$. $\endgroup$ – Avnish Kabaj Feb 24 '18 at 15:10
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First of all, your second diagram concerning canonical structures are INCORRECT. There $\ce{N}$ has formed five bonds. $\ce{N}$ can't form five bonds because of unavailability of valance $\ce{d}$ subshells. It can only have the maximum covalency upto $4$ and how do you even think of keeping $\ce N$ atom neutral after forming five bonds which is anyway wrong. So, please make that correction.

There will be only one resonance structure:
enter image description here (source)

Due to that delocalisation with carbonyl carbon, $\ce N$ will acquire a $\delta$+ charge which will increase its electron withdrawing effect (similar to $\ce{-I}$ effect) to some extent. Still, due to large no. of canonical structures of delocalisation in the benzene ring from the $\ce N$ atom, its $\ce{+R}$ effect is much stronger (strong $\ce{2p$\pi$-2p$\pi$} $ overlap) than its $\ce{-I}$ effect and thus the $\ce{-NHCOCH3}$ group still acts as an ortho-para directing group. But, its activating ability is slightly decreased from $\ce{-NH_2}$ group due to that $\ce{-I}$ effect.

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Procedure for the nitration of acetanilide, noting that the products are primarily p with a small percentage o.

http://jscienceclass.blogspot.co.uk/2012/01/nitration-of-acetanilide-lab-report.html

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