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I read on the net that phenylalanine gives a negative result with xanthoproteic test and the reason that was given was that the benzene ring is deactivated and hence the extent of nitration is very less.

What I dont understand is that isn't the benzene ring in phenylalanine actually activated through hyperconjugation from the $\ce{CH2}$ group of $\beta$-carbon and hence shouldn't it give positive results with the xanthoproteic test?

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  • $\begingroup$ Phenylalanine can react with HNO3, but is less reactive then aromatic aminoacids. $\endgroup$ – Mithoron Jul 16 '17 at 16:04
  • $\begingroup$ that's exactly my question... why is it less reactive than other amino acids even though the ring is activated through hyperconjugation? $\endgroup$ – physics123 Jul 16 '17 at 16:09

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