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Steam distillation separates o-nitrophenol from p-nitrophenol even though the two have the same formula, C₆H₅NO₃. The reason is a hydrogen bond that stays inside one molecule. This post explains how an intramolecular hydrogen bond changes melting point, boiling point, solubility and volatility, checks NCERT's explanation against measured data, and points out an acidity question where the same idea needs care.
Where is this in the syllabus?
The NCERT Class 11 chapter Chemical Bonding and Molecular Structure (Reprint 2026-27) defines two types of hydrogen bond, intermolecular and intramolecular, and uses o-nitrophenol as its example of the second (Fig. 4.22). The NCERT Class 12 chapter Alcohols, Phenols and Ethers says the ortho and para nitrophenols "can be separated by steam distillation" because o-nitrophenol is steam volatile due to intramolecular hydrogen bonding. The JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus list "hydrogen bonding and its applications", and the JEE (Advanced) 2026 brochure lists "hydrogen bonding effects". Check your own year's document.
For the basics of hydrogen bonds between molecules, read our post on hydrogen bonding and boiling points first. This post is about the other kind.
What is an intramolecular hydrogen bond?
NCERT describes it as a hydrogen bond formed when a hydrogen atom sits between two highly electronegative atoms in the same molecule. In o-nitrophenol the –OH and –NO₂ groups are on neighbouring ring carbons. The O–H hydrogen can reach an oxygen of the nitro group, closing a six-membered ring of atoms: H, the phenol O, two ring carbons, N and the nitro O.
In p-nitrophenol the two groups are on opposite sides of the ring and cannot reach each other. The O–H hydrogen bonds to a nitro or hydroxyl oxygen on another molecule instead, so the molecules are linked into a network.
How does this show up in measured data?
Measured values from PubChem's compound records for 2-nitrophenol and 4-nitrophenol:
Melting point: about 45 °C (ortho) and 113.8 °C (para). A gap of about 69 °C.
Boiling point: about 216 °C (ortho); about 279 °C for para, which decomposes.
Solubility in water at 20 °C: 2,100 mg/L (ortho) and 11,600 mg/L (para).
All three point the same way. Para molecules are held together more strongly, so it takes more energy to melt or boil them. Para also dissolves about 5.5 times better, because its free O–H can hydrogen bond to water, while the ortho O–H is mostly busy inside its own molecule.
Why does that make o-nitrophenol steam volatile?
In steam distillation, a compound that is volatile in steam distils over with the water vapour. o-Nitrophenol's molecules are not tied to each other or to water through the O–H, so it is carried over with the steam. p-Nitrophenol, held in its hydrogen-bonded network, stays behind in the flask. This is the same separation NCERT describes after the nitration of phenol, which gives a mixture of the two isomers.
Worked example: predict, then check
Apply the same reasoning to the hydroxybenzoic acids. In 2-hydroxybenzoic acid (salicylic acid) the –OH and –COOH are neighbours and can form an intramolecular hydrogen bond. In 4-hydroxybenzoic acid they cannot.
Prediction: the ortho isomer should melt lower and be less soluble in water.
Check with PubChem data: salicylic acid melts at about 159 °C and dissolves to 2,240 mg/L at 25 °C. 4-Hydroxybenzoic acid melts at about 214.5 °C and dissolves to 5,000 mg/L at 25 °C. The prediction holds: about 55 °C lower melting point and less than half the solubility.
Both acids still form intermolecular hydrogen bonds through their –COOH groups, so do not expect salicylic acid to behave like o-nitrophenol in every way. Compare isomers, not unrelated compounds.
What about acidity?
NCERT's Table 7.3 gives pKa values of 7.2 for o-nitrophenol and 7.1 for p-nitrophenol, and PubChem lists 7.23 and 7.15. Both nitrophenols are much more acidic than phenol (pKa 10.0 in NCERT), as NCERT's Intext Question 7.8 expects you to explain through resonance. But the ortho isomer is not the stronger of the two, even though its nitro group is closer. The difference is small: 10^0.1 ≈ 1.26, so p-nitrophenol is about 1.3 times stronger using NCERT's values.
Use this as a check on your reasoning. If you had argued "closer nitro group, stronger acid" you would have the order wrong. An O–H hydrogen held in an intramolecular hydrogen bond is a reasonable factor to weigh, but rely on the data the question gives you.
Which mistakes cost marks?
Saying intramolecular hydrogen bonding raises the boiling point. It lowers it compared with the isomer that bonds between molecules.
Drawing the hydrogen bond to the ring or to a carbon. It goes from the O–H hydrogen to a lone pair on the nitro oxygen.
Claiming p-nitrophenol is steam volatile. It is the ortho isomer that distils with steam.
Assuming every ortho isomer has an intramolecular bond. It needs an H on O, N or F next to an acceptor atom, placed so that a small ring, usually six-membered, can close. o-Cresol has no acceptor next to its O–H.
A practice routine
Draw o-nitrophenol with its six-membered hydrogen-bonded ring, and p-nitrophenol with two molecules linked.
For three ortho/para pairs, predict which melts lower and which dissolves better, then check one pair on PubChem.
Answer NCERT Exercise 7.8 on steam distillation in two sentences.
If you want to see where this idea connects to the rest of chemical bonding, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can trace hydrogen bonding back to electronegativity.
Today, explain in three sentences why o-nitrophenol melts lower than p-nitrophenol, using the word "intermolecular" correctly once.
