The increasing order of boiling points of the following compounds is: I. 4-methylphenol (p-cresol) II. 4-nitrophenol…

🧠 Boiling point reflects the strength of intermolecular forces. For para-substituted phenols, the key factor is how the substituent affects the polarity of the O–H bond (H-bond donor strength).

🗺️ Analyse each compound:

I. 4-methylphenol (p-cresol): –CH₃ is weakly electron-donating → slightly decreases O–H polarity → weakest H-bond donor → lowest bp (bp ≈ 202 °C)

III. 4-aminophenol: –NH₂ is strongly electron-donating via resonance → reduces O–H polarity; N–H H-bonds (N–H···N) are weaker than O–H···O H-bonds → moderate overall bp

IV. 4-methoxyphenol: –OCH₃ is inductive donor but mesomeric EW through O; –OCH₃ itself can act as H-bond acceptor; overall moderate–high bp (bp ≈ 243 °C)

II. 4-nitrophenol: –NO₂ is strongly electron-withdrawing → increases O–H polarity significantly → strongest H-bond donor → highest bp (bp ≈ 279 °C)

Increasing order:

$\text{I (p-cresol)} < \text{III (4-aminophenol)} < \text{IV (4-methoxyphenol)} < \text{II (4-nitrophenol)}$

$\boxed{\text{Answer: I} < \text{III} < \text{IV} < \text{II}}$