Phenol on reaction with dilute nitric acid, gives two products. Which method will be most efficient for large scale…

Step 1: Identify the Products of Phenol + Dilute HNO₃

Phenol reacts with dilute $\ce{HNO3}$ (cold, dilute) to produce two structural isomers:

• o-nitrophenol (ortho-nitrophenol)
• p-nitrophenol (para-nitrophenol)

These two compounds are structural isomers with very similar properties, making simple separation methods ineffective.

Step 2: Evaluate Each Separation Method

(a) Chromatographic separation ✅ Chromatography (column chromatography) separates compounds based on differential adsorption. Despite having similar structures, o-nitrophenol and p-nitrophenol have different polarities due to intramolecular vs intermolecular hydrogen bonding:

• o-nitrophenol: Forms intramolecular H-bond (less polar, lower bp = 214°C)
• p-nitrophenol: Forms intermolecular H-bond (more polar, higher bp = 279°C) This polarity difference allows chromatographic separation. For large-scale use, column chromatography is most efficient.

(b) Fractional crystallisation ❌ Both isomers have different melting points (o: 45°C, p: 113°C) — crystallisation is possible but NOT the most efficient for large scale separation of closely related compounds.

(c) Steam distillation ❌ While o-nitrophenol is steam-volatile (due to intramolecular H-bonding, low intermolecular association), this technique is NOT suitable for large-scale production.

(d) Sublimation ❌ Neither compound sublimes significantly at convenient conditions.

Step 3: Conclusion

Answer: (a) Chromatographic separation

Key Points to Remember:

• o-nitrophenol: intramolecular H-bond → less polar → lower bp → more volatile → higher $R_f$
• p-nitrophenol: intermolecular H-bond → more polar → higher bp → less volatile → lower $R_f$
• Chromatography separates based on polarity differences — ideal for structural isomers
• Note: steam distillation CAN separate them but NOT on large scale