. 'X' is:

Reaction Mechanism:

1. Diazotization: When an aromatic primary amine reacts with HNO2 it undergoes diazotization to form a highly reactive diazonium ion.
2. Intramolecular Cyclization:

   Since o-phenylenediamine has two amino groups ortho to each other, one of the $$-\text{NH}_2$$ groups is converted into a diazonium group ($$-\text{N}_2^+$$).

3. Ring Closure:

   The adjacent, unreacted primary amino group ($$-\text{NH}_2$$) acts as an internal nucleophile. The lone pair on its nitrogen atom attacks the neighboring diazonium group, causing a rapid intramolecular cyclization to close a stable 5-membered heterocyclic ring.

4. Deprotonation:

   After losing a proton ($$\text{H}^+$$), the stable heterocyclic compound Benzotriazole is formed.

Structure of the Product:

The resulting compound contains a benzene ring fused to a triazole ring (a 5-membered ring containing three nitrogen atoms), with the structure:

• A benzene core.
• Fused to a ring containing $$-\text{N}=\text{N}-\text{NH}-$$.