Q.Which one of the following compounds contains azo linkage ?
a. Hydrazine b. p-Hydroxyazobenzene c. N-Nitrosodiethylamine d. Ethylenediamine

Step 1. Recall the azo linkage (section 13.7.2). An azo compound carries a -N=N- linkage joining two aromatic rings, formed by coupling an arene diazonium salt with a reactive aromatic compound such as a phenol or an aromatic amine.

Step 2. Check each option. (a) Hydrazine, H2N-NH2, has an N-N single bond, not the N=N double-bond azo linkage, and is not aromatic.

(b) p-Hydroxyazobenzene is exactly the chapter's own worked example (section 13.7.2), formed by coupling benzenediazonium chloride with phenol -- it genuinely carries the -N=N- azo linkage joining two benzene rings. …

Checking each option's nitrogen-nitrogen bonding pattern against section 13.7.2's d …

• Confusing 'contains nitrogen-nitrogen bonds' in general (true of hydrazine and N-nitrosodiethylamine too) with specifically containing the DOUBLE-bonded azo (-N=N-) linkage, which only the azo compound genuinely has. …