Questions & explanations
1. Diazonium salts are important intermediates in organic synthesis because they:
- can be converted into a variety of substituted aromatic compounds
- are stable at high temperatures
- can be prepared from aliphatic amines
- are used only for dye synthesis
Answer: can be converted into a variety of substituted aromatic compounds
Through replacement reactions (Sandmeyer, Gattermann, Balz–Schiemann, etc.), diazonium salts allow the introduction of many different functional groups (Cl, Br, I, F, CN, OH, H, etc.) into the aromatic ring.
2. Aromatic diazonium salts are more stable than aliphatic diazonium salts because:
- The positive charge on the diazonium group is delocalized into the aromatic ring
- They are covalent compounds
- They do not decompose even at high temperature
- They are soluble in water
Answer: The positive charge on the diazonium group is delocalized into the aromatic ring
The stability of aromatic diazonium salts arises from resonance delocalization of the positive charge into the π‑system of the aromatic ring, which is not possible in aliphatic diazonium salts.
3. Which of the following reagents is used to introduce a fluorine atom into the benzene ring via a diazonium salt?
- HBF4, then heat
- KI
- H2O
- CuCl
Answer: HBF4, then heat
The Balz–Schiemann reaction involves converting the diazonium salt into a diazonium tetrafluoroborate, which upon heating decomposes to yield aryl fluoride.
4. The preparation of benzene diazonium chloride from aniline involves diazotisation. Which of the following is the correct condition for this reaction?
- Aniline + NaNO2 + HCl at 0-5°C
- Aniline + HNO3 at 100°C
- Aniline + NaOH at room temperature
- Aniline + H2SO4 at 150°C
Answer: Aniline + NaNO2 + HCl at 0-5°C
Diazotisation requires nitrous acid generated in situ from NaNO2 and HCl, and low temperature (0-5°C) to prevent decomposition of the diazonium salt.
5. In the Sandmeyer reaction, which reagent is used to replace the diazonium group by chlorine?
- CuCl
- Cu
- CuSO4
- CuCl2
Answer: CuCl
The Sandmeyer reaction uses cuprous chloride (CuCl) in the presence of HCl to convert a diazonium salt into the corresponding aryl chloride.
6. The product obtained when benzene diazonium chloride reacts with phenol in alkaline medium is:
- p-Hydroxyazobenzene
- p-Aminophenol
- Chlorobenzene
- Benzene
Answer: p-Hydroxyazobenzene
In coupling reactions, the diazonium salt couples with phenol at the para position, forming a coloured azo dye (p-hydroxyazobenzene).