At low temperature, phenol reacts with dil. HNO₃ to yield
At low temperature, phenol reacts with dil. HNO₃ to yield
Options
Correct option: (D) ortho- and para-nitrophenol
Phenol with dilute HNO₃ at low temperature (~298 K) undergoes electrophilic substitution to give a mixture of o-nitrophenol and p-nitrophenol.
- The –OH group is a strong activating, o/p-directing group.
- Dilute acid + low temperature = mild condition → only mono-nitration, at both o and p positions.
Marking Scheme
- 11 mark: correct option (D) ortho- and para-nitrophenol.
- 2No partial credit; 'o only' or 'p only' or trinitrophenol are incorrect.
Hint
Dilute acid + low temperature = mild mono-nitration; –OH is ortho/para directing, so both isomers form.
Quick Oral Answer
Because the hydroxyl group activates the ring and directs ortho/para, dilute nitric acid at low temperature gives a mixture of ortho- and para-nitrophenol, later separated by steam distillation.
Analysis & Explanation
Concept
The –OH group donates electron density into the ring (+R effect), making phenol far more reactive than benzene and directing incoming electrophiles to the ortho and para positions.
Reaction (mild condition)
With dilute HNO₃ at low temperature the nitronium ion supply is limited, so substitution stops at the mono-nitro stage, producing both o- and p-nitrophenol.
Separation point (favourite in exams)
The two products are separated by steam distillation: o-nitrophenol is steam-volatile (intramolecular H-bonding, chelation) while p-nitrophenol is not (intermolecular H-bonding).
Why the distractors are wrong
- (A) 2,4,6-Trinitrophenol (picric acid) forms only with concentrated HNO₃ (with H₂SO₄), a much harsher condition.
- (B) & (C) 'only' one isomer is wrong — both o and p form simultaneously.
Common Mistakes
- 1Choosing picric acid (2,4,6-trinitrophenol) — that needs concentrated HNO₃, not dilute.
- 2Assuming only one isomer forms; –OH directs to both ortho and para.
- 3Forgetting that the o- and p-nitrophenols are later separated by steam distillation.
Interesting Facts
o-Nitrophenol boils lower than p-nitrophenol because it forms an intramolecular hydrogen bond (chelation), making it steam-volatile.
Picric acid (2,4,6-trinitrophenol) from concentrated nitration was once used both as a yellow dye and as a military explosive.
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Frequently Asked Questions
How are o-nitrophenol and p-nitrophenol separated?
By steam distillation. o-Nitrophenol has intramolecular hydrogen bonding (chelation), so it is volatile in steam and distils over; p-nitrophenol has intermolecular hydrogen bonding, is less volatile, and remains behind.
What forms if concentrated HNO₃ is used instead?
Concentrated HNO₃ (usually with H₂SO₄) causes complete nitration to give 2,4,6-trinitrophenol, known as picric acid.