Conversion of Diazonium Salt to Phenol (Hydroxyl Replacement)
Chemistry · Amines · NEET
When benzenediazonium chloride is warmed with water, the -N2+ group is replaced by -OH and phenol is formed, releasing nitrogen gas. Memory hook: "warm water washes off nitrogen and leaves OH." The diazonium group (-N2+) leaves as N2 gas, and water supplies the -OH.
Benzenediazonium chloride warmed with water: the -N2+ group leaves as N2 gas and is replaced by -OH, giving phenol. Using H3PO2 or ethanol instead would give benzene.
Your doubts, answered
What exactly happens when benzenediazonium chloride is warmed with water?
The -N2+ group leaves as nitrogen gas (N2) and is replaced by -OH from water. The product is phenol (C6H5OH). Equation: C6H5N2+Cl- + H2O -> C6H5OH + N2 + HCl. This is a substitution: one group (-N2+) is swapped for another (-OH).
Why do we warm the solution instead of keeping it cold?
The diazonium salt is made and kept cold (273-278 K, 0-5 C) so it stays stable. To convert it to phenol you must warm it. Warming supplies energy so the C-N bond breaks, N2 leaves, and water attacks the ring carbon to give -OH. Cold keeps the salt; warm destroys it into phenol.
Does warming a diazonium salt with water give phenol or benzene? I get confused.
With plain water (and warming) an aromatic diazonium salt gives PHENOL, not benzene. You get benzene only when you use hypophosphorous acid (H3PO2) or ethanol, which replace -N2+ with -H (deamination). So: water -> -OH (phenol); H3PO2 -> -H (benzene). Read the reagent carefully.
Why does an aliphatic amine give an alcohol instead of a stable salt?
Aliphatic diazonium salts (like C2H5N2+) have no benzene ring to spread out the charge, so they are very unstable and break apart the moment they form. With water present they lose N2 and give the alcohol (for example ethylamine gives ethanol). This is why aliphatic amines with NaNO2/HCl in water do not give a usable diazonium salt.
What is the role of the ring in stabilising the aromatic diazonium salt?
In benzenediazonium chloride the positive charge on -N2+ is spread over the benzene ring by resonance. This delocalisation makes the aromatic salt stable enough to isolate at low temperature. The aliphatic version has no ring, so it cannot be stabilised and decomposes at once.
⚠️ The NEET trap ✗ Warming benzenediazonium chloride with water gives benzene. ✓ Warming with water gives phenol (-N2+ replaced by -OH). Benzene comes only from H3PO2 or ethanol (replaced by -H). 🧠 Water writes OH. Only H3PO2 or ethanol writes H.
Real NEET questions
NEET 2025
Which one of the following reactions does NOT give benzene as the product?
A · HC≡CH passed through red-hot iron tube at 873 K
B · Benzenediazonium chloride (C6H5N2+Cl-) warmed with H2O ✓
C · Sodium benzoate (C6H5COONa) heated with soda lime
D · n-hexane heated over Mo2O3 at 773 K, 10-20 atm
Solution: Benzenediazonium chloride warmed with water gives phenol (C6H5OH), not benzene, because -N2+ is replaced by -OH. The other three options each give benzene (cyclic trimerisation of ethyne, decarboxylation of sodium benzoate, and aromatisation of n-hexane). To get benzene from the diazonium salt you would need H3PO2 or ethanol, not water.
NEET 2026
The major product Z in: C2H6 →[Cl2/UV] X →[NH3] Y →[(i) NaNO2/HCl, (ii) H2O] Z
A · C2H5NO2
B · C2H5-N=N-OH
C · C2H5OH ✓
D · C2H5NH2
Solution: Chlorination gives chloroethane (X); ammonolysis gives ethylamine (Y). A primary aliphatic amine with NaNO2/HCl forms an unstable aliphatic diazonium ion (no ring to stabilise it) that loses N2 at once, and with water present is converted to the alcohol. So Z is ethanol, C2H5OH. This is the aliphatic parallel of the diazonium-to-hydroxyl replacement.
Solved Amines NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What are the reagents and conditions to convert a diazonium salt to phenol?
Warm benzenediazonium chloride with water (dilute acidic solution, warming above about 283 K). The -N2+ group is replaced by -OH and phenol forms with release of N2 gas.
Why is nitrogen gas released in this reaction?
The diazonium group -N2+ is a very good leaving group. When it leaves the ring carbon it goes away as stable N2 gas, which drives the reaction forward. Water then supplies -OH in its place.
Can this reaction be used to put -OH on a benzene ring at a chosen position?
Yes. This is why diazonium salts are important in synthesis. You first fix an -NH2 group at the right position, convert it to -N2+, then warm with water to get -OH exactly where you want it. It is a controlled way to make substituted phenols.
What is the difference between using water and using H3PO2 on a diazonium salt?
Water replaces -N2+ with -OH (gives phenol). Hypophosphorous acid H3PO2 (or ethanol) replaces -N2+ with -H (gives benzene, called deamination). Same starting salt, different product depending on the reagent.
