Phenylamine and azo compounds — common mistakes

The temperature condition for diazotisation is critical and frequently tested. If the temperature rises above 10°C, the unstable diazonium salt decomposes to form phenol ($C_6H_5OH$), nitrogen gas ($N_2$), and HCl. You would observe effervescence as nitrogen gas is evolved.

The diazonium salt ($C_6H_5N_2^+$) is thermally unstable. The C-N bond is relatively weak, and at temperatures above 10°C, the molecule readily decomposes. Water in the aqueous solution acts as a nucleophile, displacing the excellent leaving group, nitrogen gas ($N_2$), to form phenol ($C_6H_5OH$).

The benzenediazonium ion is a weak electrophile. Phenol itself is not reactive enough to be attacked. In alkaline conditions (e.g., NaOH solution), phenol is deprotonated to form the phenoxide ion ($C_6H_5O^-$). The negative charge on the oxygen atom is delocalised into the ring, making the ring much more electron-rich and thus highly activated towards electrophilic attack.

Phenylamine is only slightly soluble in water. Although the $-NH_2$ group can form hydrogen bonds with water, the large, non-polar benzene ring disrupts the hydrogen bonding network of water, making it largely insoluble. However, it will dissolve in acidic solutions because it acts as a base, forming the soluble phenylammonium salt, $C_6H_5NH_3^+$.

Several tests can be used. A simple one is to test the pH of their aqueous solutions; phenylamine is weakly basic, while phenol is weakly acidic. Alternatively, adding neutral iron(III) chloride solution will give a violet/purple colour with phenol but no change with phenylamine. Adding aqueous bromine will produce a white precipitate with both, so this is not a distinguishing test.