I was reading about electrophilic aromatic substitution reactions, and in every book, I found that the only example given for ortho-para directors - which deactivate the ring - were halogens. So are there any other substituents, other than halogens, which are ortho-para directors but ring deactivators? If so, do they do it like halogens?


1 Answer 1


The nitroso $\ce{-NO}$ group is deactivating as well as ortho para directing just like the halogens in electrophilic aromatic substitution.

For reasons of this behaviour, refer to this question on the website.

Here are more examples from Peter Skyes' Guidebook to Mechanism in Organic Chemistry: (least deactivating to most deactivating)

$\ce{-F}<\ce{-Cl,-Br, -I}<\ce{CH2Cl < -CH=CH-COOH < -CH=CH-NO2}$

For all these groups $\ce{-I}$ effect is more dominating than $\ce{+M}$ effect. Hence they deactivate the ring. However, when the arenium ion is formed they stabilise the ring through the $\ce{+M}$ effect (hyperconjugation in case of $\ce{CH2-Cl}$)

The book doesn't mention the nitroso group in the order but since it has very weak +M we should expect it to be after $\ce{CH2Cl}$

$\ce{Cl-, Br- , I-}$ are almost equally deactivating therefore they have been separated by a comma.


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