Chemistry Class 12 NCERT Solutions Chapter 11 Alcohols, Phenols and Ethers – Important Questions

Chemistry is a fascinating subject that encompasses a wide range of topics and concepts. For Class 12 students, Chapter 11 of the NCERT Chemistry textbook is crucial as it lays the foundation for understanding some fundamental principles in chemistry. This chapter, which deals with "Alcohols, Phenols, and Ethers," is essential for students preparing for their board exams and various competitive tests. This article provides a comprehensive guide to important questions from Chapter 11, focusing on key concepts, important questions, and solutions that are frequently asked in exams.

Introduction to Chapter 11: Alcohols, Phenols, and Ethers

Chapter 11 of the Class 12 NCERT Chemistry textbook focuses on three important classes of organic compounds:

1. Alcohols: These are organic compounds that contain one or more hydroxyl (-OH) groups attached to a carbon atom. They are classified into primary, secondary, and tertiary alcohols based on the number of alkyl groups attached to the carbon bearing the hydroxyl group.

2. Phenols: Phenols are organic compounds where a hydroxyl group is directly bonded to an aromatic hydrocarbon ring. Phenols are classified based on the number of hydroxyl groups attached to the ring.

3. Ethers: Ethers are compounds in which an oxygen atom is bonded to two alkyl or aryl groups. They are classified into symmetrical and asymmetrical ethers based on the nature of the groups attached to the oxygen atom.

Important Questions and Solutions

1. Explain the nomenclature of alcohols with examples.

Solution:

The nomenclature of alcohols is based on the IUPAC system. Here’s a step-by-step explanation:

• Identify the Longest Chain: Find the longest carbon chain that includes the hydroxyl group (-OH). This chain determines the base name of the alcohol.

• Number the Chain: Number the chain starting from the end nearest to the hydroxyl group to give the -OH group the lowest possible number.

• Suffix: Use the suffix “-ol” for alcohols. For example, if the longest chain is a six-carbon chain with the hydroxyl group on the third carbon, the name is "3-hexanol".

• Substituents: If there are substituents, they are named and numbered accordingly. For example, 2-methyl-3-hexanol.

Examples:

• Methanol: CH3OH (one-carbon alcohol)
• Ethanol: C2H5OH (two-carbon alcohol)
• 2-Butanol: CH3CH(OH)CH2CH3 (four-carbon alcohol with -OH on the second carbon)

2. What are the methods of preparation of phenols?

Solution:

Phenols can be prepared by various methods, including:

1. From Benzenesulfonic Acid: Benzenesulfonic acid can be treated with sodium hydroxide to form sodium benzenesulfonate. Hydrolysis of this compound yields phenol.

   $\text{C}_6\text{H}_5\text{SO}_3\text{H} + \text{NaOH} \rightarrow \text{C}_6\text{H}_5\text{SO}_3\text{Na}$ $\text{C}_6\text{H}_5\text{SO}_3\text{Na} + \text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_5\text{OH} + \text{NaHSO}_3$

2. From Haloarenes: Phenol can be synthesized by the reaction of haloarenes with an alkali. For instance, chlorobenzene reacts with sodium hydroxide under high pressure to form phenol.

   $\text{C}_6\text{H}_5\text{Cl} + \text{2NaOH} \rightarrow \text{C}_6\text{H}_5\text{OH} + \text{NaCl} + \text{Na}_2\text{O}$

3. From Cumene: Phenol is also prepared from cumene (isopropylbenzene) by oxidation followed by acid-catalyzed hydrolysis.

   $\text{C}_6\text{H}_5\text{CH}_3 + \text{O}_2 \rightarrow \text{C}_6\text{H}_5\text{CH(OH)CH}_3$ $\text{C}_6\text{H}_5\text{CH(OH)CH}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{C}_6\text{H}_5\text{OH} + \text{CH}_3\text{COOH}$

3. Describe the reactions of alcohols with active metals.

Solution:

Alcohols react with active metals like sodium and potassium to form alkoxides and hydrogen gas. The general reaction is:

$2R-OH + 2Na \rightarrow 2R-O-Na + H_2$

Examples:

• Ethanol with Sodium:

  $2\text{C}_2\text{H}_5\text{OH} + 2\text{Na} \rightarrow 2\text{C}_2\text{H}_5\text{ONa} + \text{H}_2$

• Methanol with Sodium:

  $2\text{CH}_3\text{OH} + 2\text{Na} \rightarrow 2\text{CH}_3\text{ONa} + \text{H}_2$

These reactions are highly exothermic and must be handled carefully.

4. What is the difference between phenol and alcohol in terms of acidity?

Solution:

Phenols are generally more acidic than alcohols. This difference in acidity can be explained as follows:

1. Phenols: The -OH group in phenol is directly attached to an aromatic ring. The resonance stabilization of the phenoxide ion (C6H5O-) formed upon deprotonation increases the acidity of phenols. This resonance stabilization lowers the energy of the phenoxide ion, making phenol more acidic.

   $\text{C}_6\text{H}_5\text{OH} \leftrightarrow \text{C}_6\text{H}_5\text{O}^- + \text{H}^+$

2. Alcohols: In alcohols, the hydroxyl group is attached to an alkyl group. The alkoxide ion (R-O-) formed after deprotonation does not have such resonance stabilization. Therefore, alcohols are less acidic compared to phenols.

   $\text{R-OH} \leftrightarrow \text{R-O}^- + \text{H}^+$

5. Explain the preparation and reactions of ethers.

Solution:

Preparation of Ethers:

1. Williamson’s Synthesis: This method involves the reaction of an alkoxide ion with a primary alkyl halide. The general reaction is:

   $\text{R-O}^- \text{Na}^+ + \text{R'-X} \rightarrow \text{R-O-R'} + \text{NaX}$

   Example:

   $\text{C}_2\text{H}_5\text{ONa} + \text{C}_2\text{H}_5\text{Br} \rightarrow \text{C}_2\text{H}_5\text{O-C}_2\text{H}_5 + \text{NaBr}$

2. From Alcohols: Ethers can be prepared by the dehydration of alcohols in the presence of an acid catalyst:

   $2\text{R-OH} \xrightarrow{\text{H}_2\text{SO}_4} \text{R-O-R'} + \text{H}_2\text{O}$

   Example:

   $2\text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{H}_2\text{SO}_4} \text{C}_2\text{H}_5\text{O-C}_2\text{H}_5 + \text{H}_2\text{O}$

Reactions of Ethers:

1. Cleavage by Halogens: Ethers can be cleaved by halogens in the presence of an acid to form alcohols and alkyl halides:

   $\text{R-O-R'} + \text{HBr} \rightarrow \text{R-Br} + \text{R'-OH}$

2. Reaction with Oxygen: Ethers can react with oxygen at high temperatures to form peroxides:

   $\text{R-O-R'} + \text{O}_2 \rightarrow \text{R-O-O-R'}$

6. Discuss the mechanism of esterification and hydrolysis reactions.

Solution:

Esterification: This is the reaction between a carboxylic acid and an alcohol to form an ester and water. The reaction is catalyzed by an acid (usually sulfuric acid).

$\text{R-COOH} + \text{R'-OH} \xrightarrow{\text{H}_2\text{SO}_4} \text{R-COOR'} + \text{H}_2\text{O}$

Mechanism:

1. Protonation: The carbonyl oxygen of the carboxylic acid gets protonated by the acid catalyst.
2. Nucleophilic Attack: The alcohol’s hydroxyl group attacks the carbonyl carbon of the protonated carboxylic acid.
3. Formation of Tetrahedral Intermediate: This leads to the formation of a tetrahedral intermediate.
4. Proton Transfer: The intermediate undergoes proton transfer and loses water to form the ester.

Hydrolysis: The process of ester hydrolysis is the reverse of esterification. It can be carried out under acidic or basic conditions.

Acidic Hydrolysis:

$\text{R-COOR'} + \text{H}_2\text{O} \xrightarrow{\text{HCl}} \text{R-COOH} + \text{R'-OH}$

Basic Hydrolysis (Saponification):

$\text{R-COOR'} + \text{NaOH} \rightarrow \text{R-COONa} + \text{R'-OH}$

Mechanism:

1. Nucleophilic Attack: In the case of saponification, the hydroxide ion attacks the carbonyl carbon of the ester.
2. Formation of Tetrahedral Intermediate: This intermediate then collapses, resulting in the formation of carboxylate ion and alcohol.

7. What are the physical properties of alcohols, phenols, and ethers?

Solution:

1. Alcohols:

   • Boiling Points: Alcohols have higher boiling points than hydrocarbons of similar molecular weight due to hydrogen bonding.
   • Solubility: Lower alcohols are soluble in water due to their ability to form hydrogen bonds.

2. Phenols:

   • Boiling Points: Phenols have higher boiling points compared to alkanes and lower compared to alcohols due to hydrogen bonding.
   • Solubility: Phenols are less soluble in water compared to alcohols but soluble in organic solvents.

3. Ethers:

   • Boiling Points: Ethers have lower boiling points compared to alcohols and phenols of similar molecular weight due to the absence of hydrogen bonding.
   • Solubility: Ethers are generally less soluble in water compared to alcohols and phenols.

Conclusion

Chapter 11 of the Class 12 NCERT Chemistry textbook covers essential concepts about alcohols, phenols, and ethers. Understanding the nomenclature, preparation methods, reactions, and physical properties of these compounds is crucial for mastering this chapter. This guide provides a detailed overview of important questions and solutions, helping students to grasp key concepts and prepare effectively for their exams. Mastery of these topics will not only aid in academic success but also provide a strong foundation for further studies in organic chemistry.