ChemistryChapter 10: Hydroxy Compounds
10.2 Preparation of Phenol
- Phenol:
Explanation level
In the previous section, we learned about the preparation of esters. Now, we turn our attention to another important class of hydroxy compounds: phenols. Phenols are compounds in which a hydroxyl (-OH) group is directly attached to an aromatic ring. This section describes a specific method for preparing phenol from aniline via a diazonium salt intermediate.
Definition
- Phenol: An organic compound with a hydroxyl group (-OH) directly attached to a benzene ring. Its formula is C_6H_5OH.
- Diazonium Salt: A compound containing the diazonium group (-N_2^+). An example is benzene diazonium chloride (C_6H_5N_2^+Cl^-).
Simple Explanation: Phenol is a molecule with an -OH group attached to a benzene ring. It can be prepared by converting aniline (a benzene ring with an -NH₂ group) into a diazonium salt, and then hydrolyzing that salt to replace the diazonium group with an -OH group.
Main Idea
The main idea is that phenol can be produced by the reaction of phenyamine (aniline) with nitrous acid (HNO_2) in the presence of hydrochloric acid at a low temperature (below 10°C) to produce a diazonium salt (benzene diazonium chloride). This diazonium salt is unstable and, upon warming (temperature greater than 10°C), undergoes hydrolysis to form phenol.
Core Concept
The core concept is the diazotization reaction followed by hydrolysis. The diazotization reaction converts the amino group (-NH_2) of aniline into a diazonium group (-N_2^+) at low temperatures. The diazonium group is a good leaving group and is subsequently replaced by a hydroxyl group (-OH) upon heating in water, a process called hydrolysis. This is a versatile method for synthesizing substituted benzenes.
Detailed Explanation
The textbook states that phenol can be produced by the reaction of phenyamine with HNO_3 or NaNO_3 and dilute acid below 10°C to produce the diazonium salt.
Step 1: Formation of Benzene Diazonium Chloride
Benzene diazonium chloride is prepared by the reaction of aniline with nitrous acid (HNO_2) in the presence of hydrochloric acid at a low temperature (0–5°C).
The reaction is: 2C_6H_5NH_2 + NaNO_2 + 2HCl 0-5°C→ 2C_6H_5N_2^+Cl^- + NaCl + 4H_2O
- Reagents: Aniline, sodium nitrite (NaNO_2), and hydrochloric acid (HCl).
- Nitrous Acid: The HNO_2 is generated in situ by the reaction of NaNO_2 with HCl.
- Temperature: The reaction must be carried out at low temperature (0–5°C) to prevent the decomposition of the diazonium salt.
- Product: Benzene diazonium chloride (C_6H_5N_2^+Cl^-), which is a stable salt at low temperatures.
Step 2: Hydrolysis to Phenol
The benzene diazonium chloride is then hydrolyzed by heating it with water. If the temperature is greater than 10°C, hydrolysis of benzene diazonium chloride takes place and phenol is formed.
The reaction is: C_6H_5N_2^+Cl^- + H_2O >10°C→ C_6H_5OH + N_2 + HCl
- Condition: Heating (temperature > 10°C) with water.
- Products: Phenol (C_6H_5OH), nitrogen gas (N_2), and hydrochloric acid (HCl).
- Observation: The evolution of nitrogen gas is a visible sign of the reaction.
Summary of the Process
| Step | Reaction | Conditions |
|---|---|---|
| 1. Diazotization | Aniline + NaNO_2 + HCl → Benzene diazonium chloride | 0–5°C |
| 2. Hydrolysis | Benzene diazonium chloride + H_2O → Phenol + N_2 + HCl | > 10°C |
Intermediate Level
The preparation of phenol from aniline involves two key steps: diazotization and hydrolysis. In the first step, aniline reacts with nitrous acid (generated from sodium nitrite and hydrochloric acid) at a low temperature (0–5°C) to form benzene diazonium chloride. This salt is stable only at low temperatures. In the second step, the diazonium salt is warmed with water (above 10°C), causing it to hydrolyze. The diazonium group is replaced by a hydroxyl group, and nitrogen gas is released, yielding phenol. This method is useful because it allows for the conversion of an amino group into a hydroxyl group under mild conditions.
Important Points
- - Starting Material: Aniline (C_6H_5NH_2).
- - Reagents: NaNO_2 and HCl (to generate HNO_2).
- - Step 1 (Diazotization): Forms benzene diazonium chloride at 0–5°C.
- - Step 2 (Hydrolysis): Heats the diazonium salt with water (>10°C) to form phenol.
- - By-product: Nitrogen gas (N_2) and HCl.
- - Intermediate: Benzene diazonium chloride (C_6H_5N_2^+Cl^-).
- - Temperature Control: Critical to the success of the reaction.
Remember
- - Phenol preparation: Aniline → Diazonium salt → Phenol.
- - Step 1: Aniline + NaNO_2 + HCl at 0–5°C.
- - Step 2: Diazonium salt + H_2O at > 10°C.
- - Product: Phenol (C_6H_5OH) + N_2 + HCl.
- - Diazonium salt: Benzene diazonium chloride.
- - Nitrogen gas is evolved in the second step.
- - Temperature control is crucial.
Common Mistakes
- - Forgetting the low temperature for the first step. The diazotization reaction must be carried out at 0–5°C. If the temperature is too high, the diazonium salt will decompose before it can be isolated.
- - Not heating the diazonium salt for the second step. The hydrolysis reaction requires heating (above 10°C) to proceed.
- - Confusing the reagents. The first step uses sodium nitrite and hydrochloric acid, not nitric acid directly.
- - Assuming the diazonium salt is stable at room temperature. It is unstable and decomposes to phenol and nitrogen gas.
- - Not recognizing the evolution of nitrogen gas. This is a key observation for the hydrolysis step.
Quick Revision
Phenol can be prepared from aniline via a two-step process. First, aniline is reacted with nitrous acid (generated from sodium nitrite and hydrochloric acid) at a low temperature (0–5°C) to form benzene diazonium chloride. This is the diazotization step. Second, the diazonium salt is heated with water (above 10°C), causing it to hydrolyze. The diazonium group is replaced by a hydroxyl group, and nitrogen gas is released, yielding phenol. The temperature must be carefully controlled in both steps.
60-Second Revision
Phenol is prepared from aniline in two steps. First, aniline reacts with NaNO_2 and HCl at 0–5°C to form benzene diazonium chloride. Second, the diazonium salt is heated with water (>10°C) to hydrolyze it to phenol, releasing nitrogen gas. The overall process is: Aniline → Diazonium salt → Phenol.
Frequently Asked Questions (FAQs)
- Diazotization: Aniline reacts with nitrous acid (from sodium nitrite and hydrochloric acid) at 0–5°C to form benzene diazonium chloride.
- Hydrolysis: The benzene diazonium chloride is heated with water (above 10°C) to form phenol, with the release of nitrogen gas and hydrogen chloride.
Q2. What is the role of temperature in the preparation of phenol from aniline? Answer: Temperature control is crucial. The first step (diazotization) must be carried out at a low temperature (0–5°C) to prevent the diazonium salt from decomposing. The second step (hydrolysis) requires heating (above 10°C) to convert the diazonium salt to phenol.
- Q3. Write the chemical equation for the formation of benzene diazonium chloride.
- Answer: The equation is:
- C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{0-5^\circ\text{C}} C_6H_5N_2^+Cl^- + NaCl + 2H_2O
- Q4. Write the chemical equation for the hydrolysis of benzene diazonium chloride to phenol.
- Answer: The equation is:
- C_6H_5N_2^+Cl^- + H_2O \xrightarrow{>10^\circ\text{C}} C_6H_5OH + N_2 + HCl
Q5. What is a diazonium salt? Answer: A diazonium salt is a compound containing the diazonium group (-N_2^+). In the preparation of phenol, the diazonium salt is benzene diazonium chloride (C_6H_5N_2^+Cl^-). It is an unstable intermediate that decomposes on heating to form phenol and nitrogen gas.
Q6. Why is the diazonium salt unstable at room temperature? Answer: The diazonium group (-N_2^+) is a good leaving group. At room temperature or above, it readily decomposes to form a carbocation (which is then attacked by water to form phenol) and nitrogen gas. Therefore, it must be kept cold (0–5°C) to remain stable.
Q7. What is the observation during the hydrolysis step? Answer: The evolution of nitrogen gas (N_2) is observed as bubbles. The reaction mixture also becomes acidic due to the formation of hydrogen chloride (HCl).
Q8. Can other substituted phenols be prepared using this method? Answer: Yes, by starting with a substituted aniline, various substituted phenols can be prepared. For example, using 4-methylaniline (p-toluidine) would yield 4-methylphenol (p-cresol).
Q9. What is the overall transformation in the preparation of phenol from aniline? Answer: The overall transformation is the replacement of the amino group (-NH_2) of aniline with a hydroxyl group (-OH), resulting in phenol. This is achieved via the diazonium salt intermediate.
Multiple Choice Questions (MCQs)
- Q1. What is the starting material for the preparation of phenol in the textbook method?
- A. Benzene
- B. Aniline
- C. Toluene
- D. Benzoic acid
- Correct Answer: B
- Explanation: The textbook describes the preparation of phenol from phenyamine (aniline).
- Q2. What is the intermediate formed in the preparation of phenol from aniline?
- A. Benzene diazonium chloride
- B. Chlorobenzene
- C. Benzoic acid
- D. Phenoxide ion
- Correct Answer: A
- Explanation: The intermediate is benzene diazonium chloride.
- Q3. At what temperature is the first step (diazotization) carried out?
- A. 0–5°C
- B. 25°C
- C. 100°C
- D. 150°C
- Correct Answer: A
- Explanation: The diazotization reaction must be carried out at a low temperature (0–5°C).
- Q4. What is the condition for the hydrolysis of benzene diazonium chloride to phenol?
- A. Cooling to 0°C
- B. Heating above 10°C
- C. Adding a catalyst
- D. Adding a base
- Correct Answer: B
- Explanation: Hydrolysis requires heating above 10°C.
- Q5. What gas is evolved during the hydrolysis of benzene diazonium chloride?
- A. Hydrogen
- B. Oxygen
- C. Nitrogen
- D. Chlorine
- Correct Answer: C
- Explanation: Nitrogen gas (N_2) is evolved.
- Q6. Which reagent is used to generate nitrous acid in the first step?
- A. NaNO_2 and HCl
- B. HNO_3 and H_2SO_4
- C. NaCl and H_2SO_4
- D. NaOH and HCl
- Correct Answer: A
- Explanation: Nitrous acid is generated from sodium nitrite and hydrochloric acid.
- Q7. What is the chemical formula of phenol?
- A. C_6H_5OH
- B. C_6H_6
- C. C_6H_5NH_2
- D. C_6H_5COOH
- Correct Answer: A
- Explanation: Phenol is C_6H_5OH.
- Q8. What is the chemical formula of aniline?
- A. C_6H_5OH
- B. C_6H_6
- C. C_6H_5NH_2
- D. C_6H_5COOH
- Correct Answer: C
- Explanation: Aniline is C_6H_5NH_2.
- Q9. Why must the diazotization reaction be kept cold?
- A. To speed up the reaction.
- B. To prevent the diazonium salt from decomposing.
- C. To increase the yield of aniline.
- D. To avoid the formation of phenol.
- Correct Answer: B
- Explanation: The diazonium salt is unstable and decomposes at higher temperatures, so the reaction must be kept cold.
- Q10. What is the overall transformation in the preparation of phenol from aniline?
- A. -NH_2 replaced by -OH.
- B. -OH replaced by -NH_2.
- C. -NH_2 replaced by -Cl.
- D. -OH replaced by -Cl.
- Correct Answer: A
- Explanation: The amino group is replaced by a hydroxyl group.
Multiple Choice Questions (MCQs)
- Q1. What is the starting material for the preparation of phenol in the textbook method?
- A. Benzene
- B. Aniline
- C. Toluene
- D. Benzoic acid
- Correct Answer: B
- Explanation: The starting material is aniline.
- Q2. What is the intermediate formed in the preparation of phenol from aniline?
- A. Benzene diazonium chloride
- B. Chlorobenzene
- C. Benzoic acid
- D. Phenoxide ion
- Correct Answer: A
- Explanation: The intermediate is benzene diazonium chloride.
- Q3. At what temperature is the first step (diazotization) carried out?
- A. 0–5°C
- B. 25°C
- C. 100°C
- D. 150°C
- Correct Answer: A
- Explanation: The diazotization reaction must be carried out at 0–5°C.
- Q4. What is the condition for the hydrolysis of benzene diazonium chloride to phenol?
- A. Cooling to 0°C
- B. Heating above 10°C
- C. Adding a catalyst
- D. Adding a base
- Correct Answer: B
- Explanation: Hydrolysis requires heating above 10°C.
- Q5. What gas is evolved during the hydrolysis of benzene diazonium chloride?
- A. Hydrogen
- B. Oxygen
- C. Nitrogen
- D. Chlorine
- Correct Answer: C
- Explanation: Nitrogen gas is evolved.
- Q6. Which reagent is used to generate nitrous acid in the first step?
- A. NaNO_2 and HCl
- B. HNO_3 and H_2SO_4
- C. NaCl and H_2SO_4
- D. NaOH and HCl
- Correct Answer: A
- Explanation: Nitrous acid is generated from sodium nitrite and hydrochloric acid.
- Q7. What is the chemical formula of phenol?
- A. C_6H_5OH
- B. C_6H_6
- C. C_6H_5NH_2
- D. C_6H_5COOH
- Correct Answer: A
- Explanation: Phenol is C_6H_5OH.
- Q8. What is the chemical formula of aniline?
- A. C_6H_5OH
- B. C_6H_6
- C. C_6H_5NH_2
- D. C_6H_5COOH
- Correct Answer: C
- Explanation: Aniline is C_6H_5NH_2.
- Q9. Why must the diazotization reaction be kept cold?
- A. To speed up the reaction.
- B. To prevent the diazonium salt from decomposing.
- C. To increase the yield of aniline.
- D. To avoid the formation of phenol.
- Correct Answer: B
- Explanation: The diazonium salt decomposes at higher temperatures.
- Q10. What is the overall transformation in the preparation of phenol from aniline?
- A. -NH_2 replaced by -OH.
- B. -OH replaced by -NH_2.
- C. -NH_2 replaced by -Cl.
- D. -OH replaced by -Cl.
- Correct Answer: A
- Explanation: The amino group is replaced by a hydroxyl group.
Multiple Choice Questions (MCQs)
- Q1. What is the role of NaNO_2 in the preparation of phenol?
- A. It provides the nitro group.
- B. It generates nitrous acid (HNO_2).
- C. It acts as a catalyst.
- D. It acts as a solvent.
- Correct Answer: B
- Explanation: NaNO_2 reacts with HCl to generate nitrous acid.
- Q2. What is the role of HCl in the first step?
- A. It acts as a solvent.
- B. It provides the chloride ion for the diazonium salt.
- C. It generates nitrous acid with NaNO_2.
- D. It acts as a catalyst.
- Correct Answer: C
- Explanation: HCl reacts with NaNO_2 to generate nitrous acid.
- Q3. What is the by-product of the diazotization reaction?
- A. N_2 gas
- B. NaCl and H_2O
- C. Phenol
- D. H_2 gas
- Correct Answer: B
- Explanation: The by-products are NaCl and water.
- Q4. Why is the diazonium salt not isolated in most cases?
- A. It is too stable.
- B. It is explosive.
- C. It decomposes easily.
- D. It is insoluble.
- Correct Answer: C
- Explanation: The diazonium salt is unstable and decomposes easily, so it is usually used immediately.
- Q5. What is the product of the hydrolysis of benzene diazonium chloride?
- A. Chlorobenzene
- B. Phenol
- C. Aniline
- D. Benzene
- Correct Answer: B
- Explanation: The product is phenol.
- Q6. What is the observation when the diazonium salt is heated with water?
- A. A white precipitate forms.
- B. Nitrogen gas bubbles are evolved.
- C. A color change occurs.
- D. No visible change.
- Correct Answer: B
- Explanation: Nitrogen gas is evolved.
- Q7. What is the chemical name for aniline?
- A. Phenyamine
- B. Phenol
- C. Benzene
- D. Toluene
- Correct Answer: A
- Explanation: Aniline is also called phenyamine.
- Q8. What is the chemical name for benzene diazonium chloride?
- A. C_6H_5N_2^+Cl^-
- B. C_6H_5NH_2
- C. C_6H_5OH
- D. C_6H_5Cl
- Correct Answer: A
- Explanation: The formula is C_6H_5N_2^+Cl^-.
- Q9. What is the overall reaction for the preparation of phenol from aniline?
- A. C_6H_5NH_2 + H_2O \rightarrow C_6H_5OH + NH_3
- B. C_6H_5NH_2 + HNO_2 \rightarrow C_6H_5OH + N_2 + H_2O
- C. C_6H_5NH_2 + HCl \rightarrow C_6H_5Cl + NH_3
- D. C_6H_5NH_2 + O_2 \rightarrow C_6H_5OH + NO_2
- Correct Answer: B
- Explanation: The overall reaction involves the conversion of aniline to phenol via the diazonium salt.
- Q10. What is the function of water in the second step?
- A. It acts as a solvent.
- B. It acts as a nucleophile, attacking the diazonium carbon.
- C. It acts as a catalyst.
- D. It acts as a base.
- Correct Answer: B
- Explanation: Water acts as a nucleophile in the hydrolysis step.
Multiple Choice Questions (MCQs)
- Q1. What is the role of NaNO_2 in the preparation of phenol?
- A. It provides the nitro group.
- B. It generates nitrous acid (HNO_2).
- C. It acts as a catalyst.
- D. It acts as a solvent.
- Correct Answer: B
- Explanation: NaNO_2 generates nitrous acid.
- Q2. What is the role of HCl in the first step?
- A. It acts as a solvent.
- B. It provides the chloride ion for the diazonium salt.
- C. It generates nitrous acid with NaNO_2.
- D. It acts as a catalyst.
- Correct Answer: C
- Explanation: HCl generates nitrous acid with NaNO_2.
- Q3. What is the by-product of the diazotization reaction?
- A. N_2 gas
- B. NaCl and H_2O
- C. Phenol
- D. H_2 gas
- Correct Answer: B
- Explanation: The by-products are NaCl and water.
- Q4. Why is the diazonium salt not isolated in most cases?
- A. It is too stable.
- B. It is explosive.
- C. It decomposes easily.
- D. It is insoluble.
- Correct Answer: C
- Explanation: The diazonium salt decomposes easily.
- Q5. What is the product of the hydrolysis of benzene diazonium chloride?
- A. Chlorobenzene
- B. Phenol
- C. Aniline
- D. Benzene
- Correct Answer: B
- Explanation: The product is phenol.
- Q6. What is the observation when the diazonium salt is heated with water?
- A. A white precipitate forms.
- B. Nitrogen gas bubbles are evolved.
- C. A color change occurs.
- D. No visible change.
- Correct Answer: B
- Explanation: Nitrogen gas is evolved.
- Q7. What is the chemical name for aniline?
- A. Phenyamine
- B. Phenol
- C. Benzene
- D. Toluene
- Correct Answer: A
- Explanation: Aniline is phenyamine.
- Q8. What is the chemical name for benzene diazonium chloride?
- A. C_6H_5N_2^+Cl^-
- B. C_6H_5NH_2
- C. C_6H_5OH
- D. C_6H_5Cl
- Correct Answer: A
- Explanation: The formula is C_6H_5N_2^+Cl^-.
- Q9. What is the overall reaction for the preparation of phenol from aniline?
- A. C_6H_5NH_2 + H_2O \rightarrow C_6H_5OH + NH_3
- B. C_6H_5NH_2 + HNO_2 \rightarrow C_6H_5OH + N_2 + H_2O
- C. C_6H_5NH_2 + HCl \rightarrow C_6H_5Cl + NH_3
- D. C_6H_5NH_2 + O_2 \rightarrow C_6H_5OH + NO_2
- Correct Answer: B
- Explanation: The overall reaction involves the conversion of aniline to phenol via the diazonium salt.
- Q10. What is the function of water in the second step?
- A. It acts as a solvent.
- B. It acts as a nucleophile, attacking the diazonium carbon.
- C. It acts as a catalyst.
- D. It acts as a base.
- Correct Answer: B
- Explanation: Water acts as a nucleophile.
Multiple Choice Questions (MCQs)
- Q1. What is the correct order of steps in the preparation of phenol from aniline?
- A. Hydrolysis then diazotization.
- B. Diazotization then hydrolysis.
- C. Oxidation then reduction.
- D. Reduction then oxidation.
- Correct Answer: B
- Explanation: Diazotization occurs first, followed by hydrolysis.
- Q2. What is the temperature required for diazotization?
- A. 0–5°C
- B. 10–20°C
- C. 25–30°C
- D. 50–60°C
- Correct Answer: A
- Explanation: Diazotization requires 0–5°C.
- Q3. What is the temperature required for hydrolysis?
- A. 0–5°C
- B. >10°C
- C. 25°C
- D. 100°C
- Correct Answer: B
- Explanation: Hydrolysis requires heating above 10°C.
- Q4. What is the intermediate in the preparation of phenol?
- A. Benzene diazonium chloride
- B. Chlorobenzene
- C. Benzoic acid
- D. Phenoxide ion
- Correct Answer: A
- Explanation: The intermediate is benzene diazonium chloride.
- Q5. What is the by-product of the hydrolysis step?
- A. N_2 and HCl
- B. NaCl and H_2O
- C. H_2 and O_2
- D. CO_2 and H_2O
- Correct Answer: A
- Explanation: The by-products are N_2 and HCl.
- Q6. What is the starting material for the preparation of phenol?
- A. Aniline
- B. Benzene
- C. Toluene
- D. Benzoic acid
- Correct Answer: A
- Explanation: The starting material is aniline.
- Q7. What is the role of NaNO_2 in the first step?
- A. It generates nitrous acid.
- B. It acts as a catalyst.
- C. It acts as a solvent.
- D. It provides the nitro group.
- Correct Answer: A
- Explanation: NaNO_2 generates nitrous acid.
- Q8. What is the role of HCl in the first step?
- A. It acts as a catalyst.
- B. It generates nitrous acid.
- C. It acts as a solvent.
- D. It provides the chloride ion.
- Correct Answer: B
- Explanation: HCl generates nitrous acid.
- Q9. What is the overall transformation in the preparation of phenol?
- A. -NH_2 replaced by -OH.
- B. -OH replaced by -NH_2.
- C. -NH_2 replaced by -Cl.
- D. -OH replaced by -Cl.
- Correct Answer: A
- Explanation: The amino group is replaced by a hydroxyl group.
- Q10. Why is the diazonium salt unstable at room temperature?
- A. It is a good leaving group.
- B. It is a strong acid.
- C. It is a strong base.
- D. It is a reducing agent.
- Correct Answer: A
- Explanation: The diazonium group is a good leaving group, making the salt unstable.
Multiple Choice Questions (MCQs)
- Q1. What is the correct order of steps in the preparation of phenol from aniline?
- A. Hydrolysis then diazotization.
- B. Diazotization then hydrolysis.
- C. Oxidation then reduction.
- D. Reduction then oxidation.
- Correct Answer: B
- Explanation: Diazotization occurs first, followed by hydrolysis.
- Q2. What is the temperature required for diazotization?
- A. 0–5°C
- B. 10–20°C
- C. 25–30°C
- D. 50–60°C
- Correct Answer: A
- Explanation: Diazotization requires 0–5°C.
- Q3. What is the temperature required for hydrolysis?
- A. 0–5°C
- B. >10°C
- C. 25°C
- D. 100°C
- Correct Answer: B
- Explanation: Hydrolysis requires heating above 10°C.
- Q4. What is the intermediate in the preparation of phenol?
- A. Benzene diazonium chloride
- B. Chlorobenzene
- C. Benzoic acid
- D. Phenoxide ion
- Correct Answer: A
- Explanation: The intermediate is benzene diazonium chloride.
- Q5. What is the by-product of the hydrolysis step?
- A. N_2 and HCl
- B. NaCl and H_2O
- C. H_2 and O_2
- D. CO_2 and H_2O
- Correct Answer: A
- Explanation: The by-products are N_2 and HCl.
- Q6. What is the starting material for the preparation of phenol?
- A. Aniline
- B. Benzene
- C. Toluene
- D. Benzoic acid
- Correct Answer: A
- Explanation: The starting material is aniline.
- Q7. What is the role of NaNO_2 in the first step?
- A. It generates nitrous acid.
- B. It acts as a catalyst.
- C. It acts as a solvent.
- D. It provides the nitro group.
- Correct Answer: A
- Explanation: NaNO_2 generates nitrous acid.
- Q8. What is the role of HCl in the first step?
- A. It acts as a catalyst.
- B. It generates nitrous acid.
- C. It acts as a solvent.
- D. It provides the chloride ion.
- Correct Answer: B
- Explanation: HCl generates nitrous acid.
- Q9. What is the overall transformation in the preparation of phenol?
- A. -NH_2 replaced by -OH.
- B. -OH replaced by -NH_2.
- C. -NH_2 replaced by -Cl.
- D. -OH replaced by -Cl.
- Correct Answer: A
- Explanation: The amino group is replaced by a hydroxyl group.
- Q10. Why is the diazonium salt unstable at room temperature?
- A. It is a good leaving group.
- B. It is a strong acid.
- C. It is a strong base.
- D. It is a reducing agent.
- Correct Answer: A
- Explanation: The diazonium group is a good leaving group, making the salt unstable.
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Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.10.2 Preparation of Phenol
Sticky Notes · Quick Revision
Key Point 1
- Phenol: C_6H_5OH.
Key Point 2
- Aniline: C_6H_5NH_2.
Key Point 3
- Step 1: Diazotization (0–5°C).
Key Point 4
- Step 2: Hydrolysis (>10°C).
Key Point 5
- Reagents: NaNO_2 + HCl.
Key Point 6
- Intermediate: C_6H_5N_2^+Cl^-.
Key Point 7
- By-product: N_2 gas.
Key Point 8
- Overall: Aniline → Phenol.
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