Definition
Diazonium salts
Aryl diazonium chloride: ArN₂⁺Cl⁻. Prepared by diazotisation: ArNH₂ + HCl + NaNO₂ at 0-5°C → ArN₂⁺Cl⁻. Highly versatile in synthesis.
-- NCERT Class 12 Chemistry, Ch. 9, p. 274Diazonium Salts
8 MCQs9-step worked example
Source: NCERT AminesPYQ coverage: NEET 2021, 2022, 2025Official key: NTA-verifiedLast updated: 27 Sep 2026
Diazonium Salts, explained for NEET
The diazonium salt question on NEET is a reagent-recall trap. You see a benzenediazonium chloride (C₆H₅N₂⁺Cl⁻) reacting with a copper salt or water, and the answer hinges on whether you remember which copper reagent gives which product. Confuse Sandmeyer with Gattermann and you lose four marks — the wrong option is designed to look right.
What is a diazonium salt? When a primary aromatic amine (like aniline) reacts with nitrous acid (NaNO₂ + HCl) at 273–278 K, the amino group is replaced by the diazonium group (–N₂⁺). The product, benzenediazonium chloride, is the gateway to a wide range of aromatic substitutions that are otherwise difficult or impossible by direct methods (NCERT Class 12 Chemistry Chapter 9, page 274).
Why is this synthetically important? The –N₂⁺ group can be replaced by –Cl, –Br, –CN, –I, –OH, –H, –F, and –NO₂ through specific named reactions. This makes diazonium chemistry the single most versatile route to substituted benzene rings in NCERT organic chemistry (NCERT Class 12 Chemistry Chapter 9, page 276).
The high-frequency trap — Sandmeyer vs. Gattermann reagent confusion. Both reactions replace –N₂⁺ with a halide, but the reagents differ:
- Sandmeyer reaction: ArN₂⁺ + CuCl → ArCl (or CuBr → ArBr, CuCN → ArCN). Reagent is cuprous salt in solution.
- Gattermann reaction: ArN₂⁺ + Cu powder + HCl → ArCl (or HBr → ArBr). Reagent is copper metal powder with the hydrogen halide.
Other key conversions: hydrolysis with warm water gives ArOH (phenol). Reaction with HBF₄ followed by heating gives ArF (Balz–Schiemann). Reaction with H₃PO₂ gives ArH (deamination). Azo coupling with phenol or aniline in alkaline medium gives azo dyes.
Watch out: NEET distractors routinely swap the Sandmeyer reagent (CuX) with the Gattermann reagent (Cu + HX). Anchor the distinction: Sandmeyer = cuprous salt; Gattermann = copper powder + acid.
Can you answer these Diazonium Salts MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
Benzenediazonium chloride is obtained by treating aniline with NaNO₂ and HCl at 273–278 K. This reaction is called:
Show answer and why every option is right or wrong
Answer: C. The conversion of a primary aromatic amine to a diazonium salt using NaNO₂ + HCl at 273–278 K is called diazotisation (NCERT Class 12 Chemistry Chapter 9, page 274).
Why A is wrong: A is wrong because the Sandmeyer reaction replaces the –N₂⁺ group with –Cl, –Br, or –CN using cuprous salts — it does not form the diazonium salt.
Why B is wrong: B is wrong because coupling reactions involve the diazonium salt reacting with electron-rich aromatics (phenol, aniline) to form azo dyes, not the formation of the diazonium salt itself.
Why D is wrong: D is wrong because the Gattermann reaction replaces –N₂⁺ with halides using Cu powder + HX — it is a substitution on an already-formed diazonium salt.
MCQ 2Easy RecallPractice
Which of the following is the correct reagent and product for the Sandmeyer reaction starting from benzenediazonium chloride?
Show answer and why every option is right or wrong
Answer: D. The Sandmeyer reaction uses cuprous salts (CuCl, CuBr, CuCN) to replace –N₂⁺. ArN₂⁺ + CuBr → ArBr is a standard Sandmeyer example (NCERT Class 12 Chemistry Chapter 9, page 276).
Why A is wrong: A is wrong because Cu powder + HCl is the Gattermann reaction, not the Sandmeyer reaction. Sandmeyer uses cuprous salt (CuCl), not metallic copper with acid. This is the common Sandmeyer–Gattermann confusion trap.
Why B is wrong: B is wrong because H₃PO₂ replaces –N₂⁺ with –H (deamination), giving benzene. This is a reduction, not a Sandmeyer-type substitution.
Why C is wrong: C is wrong because HBF₄ followed by heating is the Balz–Schiemann reaction for fluorobenzene — it does not involve a cuprous salt and is not classified as a Sandmeyer reaction.
MCQ 3Easy RecallPractice
Which reagent is used in the Gattermann reaction to convert benzenediazonium chloride to chlorobenzene?
Show answer and why every option is right or wrong
Answer: B. The Gattermann reaction uses copper powder with hydrogen halide (Cu + HCl) to replace the diazonium group with a halide (NCERT Class 12 Chemistry Chapter 9, page 276).
Why A is wrong: A is wrong because CuCl (cuprous chloride) in solution is the Sandmeyer reagent, not the Gattermann reagent. The distinction is cuprous salt (Sandmeyer) vs. copper metal powder + acid (Gattermann).
Why C is wrong: C is wrong because CuCN is used in the Sandmeyer reaction to form ArCN (cyanobenzene), and NaCl plays no role in Gattermann chemistry.
Why D is wrong: D is wrong because CuCl₂ (cupric chloride) is not the reagent for either Sandmeyer or Gattermann. Sandmeyer uses Cu⁺ (cuprous) salts; Gattermann uses Cu⁰ (metallic copper) + HX.
MCQ 4Direct ApplicationPractice
Benzenediazonium chloride on treatment with CuCN gives product X. What is X?
Show answer and why every option is right or wrong
Answer: C. ArN₂⁺ + CuCN → ArCN (cyanobenzene / benzonitrile). This is the Sandmeyer reaction with CuCN replacing the diazonium group with –CN (NCERT Class 12 Chemistry Chapter 9, page 276).
Why A is wrong: A is wrong because nitrobenzene is formed by nitration of benzene (HNO₃/H₂SO₄), not from diazonium salt reactions with CuCN.
Why B is wrong: B is wrong because aniline is the starting material for making diazonium salts. CuCN does not regenerate the amine; it replaces –N₂⁺ with –CN.
Why D is wrong: D is wrong because phenol is obtained by hydrolysis of the diazonium salt with warm water, not by reaction with CuCN.
MCQ 5Direct ApplicationPractice
Benzenediazonium chloride reacts with warm water to form:
Show answer and why every option is right or wrong
Answer: B. Warming benzenediazonium chloride with water replaces –N₂⁺ with –OH, producing phenol with loss of N₂ gas (NCERT Class 12 Chemistry Chapter 9, page 276).
Why A is wrong: A is wrong because chlorobenzene is formed via Sandmeyer (CuCl) or Gattermann (Cu + HCl), not by aqueous hydrolysis.
Why C is wrong: C is wrong because benzene is formed by reduction with H₃PO₂ (deamination), not by reaction with water.
Why D is wrong: D is wrong because aniline is the precursor used to prepare the diazonium salt. Hydrolysis does not regenerate the amine group; it replaces –N₂⁺ with –OH.
MCQ 6Direct ApplicationPractice
Which of the following reactions of benzenediazonium chloride follows the Balz–Schiemann pathway?
Show answer and why every option is right or wrong
Answer: D. The Balz–Schiemann reaction converts the diazonium salt to aryl fluoride via the tetrafluoroborate intermediate: ArN₂⁺BF₄⁻ → ArF + N₂ + BF₃ on heating (NCERT Class 12 Chemistry Chapter 9, page 276).
Why A is wrong: A is wrong because ArN₂⁺ + CuCl → ArCl is the Sandmeyer reaction, not Balz–Schiemann. Sandmeyer gives chloro, bromo, or cyano products via cuprous salts.
Why B is wrong: B is wrong because H₃PO₂ reduces the diazonium group to –H (deamination to ArH), which is distinct from the fluoride-forming Balz–Schiemann reaction.
Why C is wrong: C is wrong because Cu powder + HBr is the Gattermann reaction for ArBr, not the Balz–Schiemann pathway. Balz–Schiemann specifically introduces fluorine.
MCQ 7Concept TrapPractice
Benzenediazonium chloride couples with phenol in alkaline medium. The site of coupling on phenol is preferentially at the:
Show answer and why every option is right or wrong
Answer: A. Azo coupling is an electrophilic substitution. The diazonium ion (a weak electrophile) attacks the activated aromatic ring of phenol preferentially at the para position, which is less sterically hindered than ortho (NCERT Class 12 Chemistry Chapter 9, page 276).
Why B is wrong: B is wrong because while ortho is also activated, the para position is favoured due to lower steric hindrance around the bulky diazonium electrophile. The major product is the para-azo compound.
Why C is wrong: C is wrong because the –OH group is ortho/para-directing. Electrophilic substitution on phenol does not occur at the meta position under normal coupling conditions.
Why D is wrong: D is wrong because ipso substitution (at the carbon bearing –OH) would require displacement of the hydroxyl group, which does not occur under standard azo-coupling conditions.
MCQ 8CalculationPractice
A student needs to convert aniline to iodobenzene. The correct sequence is:
(i) NaNO₂ + HCl at 273–278 K
(ii) Reagent X
Identify Reagent X.
Show answer and why every option is right or wrong
Answer: A. Step (i) forms benzenediazonium chloride. For iodobenzene, the Sandmeyer route does not work (CuI is not used). Instead, ArN₂⁺ is treated with KI directly: ArN₂⁺ + KI → ArI + N₂ + KCl. This is a key distinction from chloro/bromo conversions (NCERT Class 12 Chemistry Chapter 9, page 276).
Why B is wrong: B is wrong because the Sandmeyer reaction with CuI is not a standard method. Sandmeyer works with CuCl, CuBr, and CuCN. Iodo substitution uses KI directly without a copper catalyst — this is a common confusion in diazonium chemistry.
Why C is wrong: C is wrong because I₂ + Cu is not a recognised reagent combination for diazonium reactions. Iodobenzene is obtained simply by treating the diazonium salt with KI.
Why D is wrong: D is wrong because HI + Cu powder mimics the Gattermann pattern (Cu + HX), but the Gattermann reaction is documented for HCl and HBr, not HI. The correct reagent for ArI is KI alone.
Free NEET study resources
Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.
How do you solve a Diazonium Salts question? A worked example
- 1
Given
• Starting material: Aniline (C₆H₅NH₂)• Reagent 1: NaNO₂ + HCl, 273–278 K• Reagent 2: CuBr
- 2
Required
Final organic product and reaction name.
- 3
Concept
Primary aromatic amines undergo diazotisation with NaNO₂ + HCl at low temperature to form diazonium salts. The diazonium group can then be replaced by various nucleophiles. Cuprous salts (CuX) drive the Sandmeyer reaction.
- 4
Formula / Reaction scheme
• Step 1: C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O (at 273–278 K)• Step 2: C₆H₅N₂⁺Cl⁻ + CuBr → C₆H₅Br + N₂ + CuCl
- 5
Substitution
Aniline → benzenediazonium chloride → bromobenzene. The cuprous bromide replaces –N₂⁺ with –Br.
- 6
Calculation
No numerical calculation. This is a product-identification problem. The key reasoning step is recognising CuBr as a Sandmeyer reagent (cuprous salt) rather than Gattermann (which would require Cu powder + HBr).
- 7
Final answer
The product is bromobenzene (C₆H₅Br). The second step is the Sandmeyer reaction.
- 8
Common trap
Students confuse the Sandmeyer reagent (CuBr) with the Gattermann reagent (Cu + HBr). Both give bromobenzene, but the question asks you to identify the reaction by the reagent. If the reagent is a cuprous salt → Sandmeyer. If the reagent is copper powder + hydrogen halide → Gattermann. Picking "Gattermann" when CuBr is given loses you 4 marks.
- 9
Similar NEET-style question
"Aniline is diazotised and then treated with CuCN. Name the reaction and the product." (Answer: Sandmeyer reaction; product = cyanobenzene / benzonitrile.)
---
What to remember before solving Diazonium Salts questions
Key Fact
Reactions of diazonium salts
Sandmeyer: ArN₂⁺Cl⁻ + Cu/HX → ArX (X = Cl, Br, CN). Gattermann (Cu powder), Hofmann (KI for ArI). Hydrolysis: ArN₂⁺ + H₂O → ArOH. Coupling with phenols/anilines → azo dyes (orange/red).
-- NCERT Class 12 Chemistry, Ch. 9, p. 275Where do students lose marks on Diazonium Salts?
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Root cause: concept gap
Correction
Sandmeyer: ArN₂⁺ + CuCl/CuBr/CuCN → ArCl/ArBr/ArCN. Gattermann: Cu powder + HX. Hydrolysis: H₂O → ArOH.
More in Amines: 9 exam traps and mistakes · 1 formula · 2 question patterns from its other lessons.
Diazonium Salts questions from past NEET papers
3 questions from NEET 2021, 2022, 2025. Answers verified against NTA official keys.
NEET 2025
1Statement I is incorrect but Statement II is correct
2Both Statement I and Statement II are correct
3Both Statement I and Statement II are incorrect
4Statement I is correct but Statement II is incorrect
NTA Answer: Option 2(final)
NEET 2022
1Statement I is incorrect but Statement II is correct.
2Both Statement I and Statement II are correct.
3Both Statement I and Statement II are incorrect.
4Statement I is correct but Statement II is incorrect.
NTA Answer: Option 4(final)
NEET 2021
1CuCN/KCN
2H₂O
3CH₃CH₂OH
4HI
NTA Answer: Option 3(final)
How does NEET ask about Diazonium Salts?
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
Predict product of diazonium reactions: Sandmeyer (CuX), hydrolysis (H2O), coupling (phenol/aniline).
RecallMedium
Common distractors
confuses sandmeyer gattermann
Mixes up Cu reagents
Sources
NCERT refs: Class 12 Chemistry Chapter 9, p.274 | Class 12 Chemistry Chapter 9, p.276
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
Test yourself on this topic with real past-paper questions:
Practice this topic →