An Overview of Chapter 11 - Alcohols, Phenols and Ethers of Class 12 Chemistry
FAQs on NCERT Books Free Download for Class 12 Chemistry Chapter 11 - Alcohols, Phenols, and Ethers
1. What is the expected weightage for 'Alcohols, Phenols, and Ethers' in the CBSE Class 12 Chemistry board exam for 2025-26?
For the CBSE 2025-26 board exams, the unit containing 'Alcohols, Phenols, and Ethers' is typically allocated around 6-8 marks. Students should expect a mix of questions, including Multiple Choice Questions (MCQs), 2-mark reasoning questions (e.g., comparing acidity or boiling points), 3-mark questions on reaction mechanisms or conversions, and potentially a part of a 5-mark or case-based question involving distinction tests.
2. Which named reactions from this chapter are considered most important for the board exams?
From an examination perspective, certain named reactions from this chapter are frequently asked and carry high marks. The most important ones to master are:
- Kolbe's Reaction: The synthesis of salicylic acid from phenol.
- Reimer-Tiemann Reaction: The ortho-formylation of phenol to produce salicylaldehyde.
- Williamson's Ether Synthesis: A versatile method for preparing both symmetrical and unsymmetrical ethers.
3. What are the important chemical tests for distinguishing between primary, secondary, and tertiary alcohols?
A very important question for both theory and practical exams is distinguishing between the three classes of alcohols. The primary test to use is the Lucas test.
- Reagent: A solution of anhydrous zinc chloride in concentrated HCl.
- Observation: Tertiary alcohols give immediate turbidity. Secondary alcohols give turbidity after 5-10 minutes. Primary alcohols do not produce turbidity at room temperature.
4. Why is phenol more acidic than ethanol? How is this concept tested in exams?
This is a fundamental and frequently asked conceptual question. Phenol is more acidic than ethanol because its conjugate base, the phenoxide ion, is highly stabilised by resonance. The negative charge is delocalised over the benzene ring. In contrast, the conjugate base of ethanol, the ethoxide ion, is destabilised because the ethyl group exerts a positive inductive effect (+I effect), which intensifies the negative charge on the oxygen atom. In exams, this is typically a 2-mark 'give reason' question.
5. From an exam point of view, which industrial preparation of Phenol is most significant?
The most important commercial method for manufacturing phenol, often highlighted in exams, is the Cumene process. It starts with the hydrocarbon cumene (isopropylbenzene). The process is significant because it is economical and produces a valuable by-product, acetone. Questions related to this process may involve writing the full reaction or identifying the reactants and products.
6. How do reaction conditions determine the product when ethanol is dehydrated using concentrated sulphuric acid?
This is a classic example of a temperature-controlled reaction, making it an important topic for conceptual questions. The dehydration of ethanol can lead to two different products:
- At a high temperature of 443 K, an intramolecular dehydration (elimination) occurs, forming ethene.
- At a lower temperature of 413 K, an intermolecular dehydration (nucleophilic substitution) takes place, resulting in the formation of ethoxyethane (an ether).
7. Why does the boiling point of isomeric alcohols decrease with an increase in branching?
This is a common reasoning question in exams. While isomeric alcohols have the same molecular mass, their boiling points differ due to their structure. With increased branching, the molecule becomes more compact or spherical. This decreases the surface area available for intermolecular contact. As a result, the strength of the van der Waals forces decreases, leading to a lower boiling point. For example, butan-1-ol has a higher boiling point than 2-methylpropan-2-ol.
8. What are the expected products when anisole (methoxybenzene) is treated with excess hydroiodic acid (HI)?
The cleavage of ethers with HI is a very important reaction. In anisole, the oxygen atom is attached to both a methyl group and a phenyl group. The bond between oxygen and the phenyl group (O–C₆H₅) has a partial double-bond character due to resonance, making it stronger and harder to break. Therefore, the weaker O–CH₃ bond cleaves. The final products are Phenol and Methyl iodide. This is a frequently tested reaction for 1 or 2 marks.
