# Xanthoproteic test for phenylalanine

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?

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