[danyrs]

in the xanthoproteic test, my book says it only reacts with 2 of the 3 aromatic aminoacids, my question is why?, why is deactivated the ring?, bc i understand that an alkyl group (the closest CH2 the ring has) is an activating group. Thanks.

[AWK]

This is a problem of reactivity. Phenyl ring in Phe is not deactivated, it is insufficiently activated.
Phenol is nitrated by pure HNO₃ (sometimes even diluted), where us benzene or toluene - by a nitrating mixture (conc. HNO₃ + conc. H₂SO₄)