1
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My teacher told me that among the following compounds only compound 3 will form phthalic acid when reacting with hot $\ce{KMnO4}$.

I do understand why the other compound will not form phthalic acid.

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3
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Looking at the substituents; NH2- activates the ring, making it more reactive than dibenzene ring system (easily oxidised), and the NO2- substituent is very deactivating - (strongly electron withdrawing), making it less reactive (not easily oxidised).

Although a powerful oxidising, optimised oxidising agent (KMnO4) is being used, it is only able to oxidise the 2-naphthylamine compound to produce phthalic acid.

and 2-nitronaphthalene is very unlikely or at least takes longer to completion with very little yield.

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  • $\begingroup$ Could the nitro compound oxidize on the other ring, maybe? $\endgroup$ – Oscar Lanzi Mar 29 '17 at 1:30
  • $\begingroup$ @OscarLanzi even the second ring is already deprived of electrons (they are delocalised but spend most of the time towards the NO2- substitiuent due to its strong electron withdrawing nature, besides directly cleaving the arene bond and oxidising it is very difficult , which is the reason why reactions inlvoving arenes are determined by substiuents and **not ** on the arene ring itself. I hope it makes sense. $\endgroup$ – xavier_fakerat Mar 29 '17 at 9:12

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