See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Step 1: Identify A and B. Phenol treated with dilute HNO3 undergoes electrophilic aromatic nitration to give a mixture of ortho-nitrophenol (A, more volatile due to intramolecular hydrogen bonding, lower boiling point) and para-nitrophenol (B, less volatile due to intermolecular hydrogen bonding, higher boiling point). Step 2: Identify C. Treatment of the mixture A + B with (1) Sn/HCl (reduction of nitro group to amino group) and (2) NaNO2/HCl (diazotization) converts the amino groups to diazonium salts. Since A is o-nitrophenol and B is p-nitrophenol, their reduction gives o-aminophenol and p-aminophenol, and diazotization gives o-hydroxyphenyl diazonium chloride and p-hydroxyphenyl diazonium chloride respectively. The diagram shows C as a hydroxybenzene (phenol ring) with a diazonium group, specifically the para isomer (p-hydroxybenzenediazonium chloride) based on context, but both isomers are diazonium salts of hydroxybenzene. Step 3: Identify D. C (a hydroxy diazonium salt, specifically p-hydroxybenzenediazonium chloride) under mild basic conditions reacts with phenol (the benzene ring with OH shown before the arrow to D) via azo coupling reaction. In mild basic conditions, the diazonium salt couples with phenol at the para position of phenol to give an azo dye. The product D is p-hydroxyazobenzene coupled with phenol: HO-C6H4-N=N-C6H4-OH, which is 4-[(4-hydroxyphenyl)diazenyl]phenol, also known as p,p'-dihydroxyazobenzene (Bismarck Brown type or specifically this is para-hydroxyazophenol). Step 4: Determine the molecular formula of D. D = HO-C6H4-N=N-C6H4-OH. Molecular formula: C12H10N2O2. Step 5: Calculate the Double Bond Equivalent (DBE). DBE = (2C + 2 + N - H) / 2 for a molecule with formula CcHhNnOo (oxygen doesn't affect DBE). DBE = (2×12 + 2 + 2 - 10) / 2 = (24 + 2 + 2 - 10) / 2 = 18/2 = 9. The two benzene rings contribute 4 DBE each (3 double bonds + 1 ring = 4 each, total 8), and the azo group -N=N- contributes 1 DBE, giving total DBE = 8 + 1 = 9. Therefore, the correct answer is 9.