CBSE Class 12 Chemistry Exemplar Chapter 10: Haloalkanes and Haloarenes NCERT Solutions

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NCERT Solutions PDF Class 12 PDF

CBSE Class 12 Chemistry Exemplar, Chapter 10, focuses on Haloalkanes and Haloarenes, exploring the reactivity of alcohols with halogen acids. The NCERT Solutions delve into the fundamental S<sub>N</sub>1 and S<sub>N</sub>2 reaction mechanisms, clarifying how the stability of carbocation intermediates directly impacts reaction rates. Students will find detailed explanations for various questions, including multiple-choice ones, which illuminate why certain alcohols exhibit faster reaction speeds compared to others. This resource is meticulously crafted to solidify students' grasp of core concepts within haloalkanes and haloarenes. By offering clear, sequential problem-solving strategies for pivotal topics, it serves as an invaluable tool for enhancing conceptual understanding and bolstering exam readiness.

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BoardCBSE
ClassClass 12
SubjectChemistry Exemplar
Session2026
LanguageEnglish
TypeNCERT Solutions
ChapterChapter 10

Chapter summary

Chapter 10 of the CBSE Class 12 Chemistry Exemplar focuses on Haloalkanes and Haloarenes. The provided NCERT Solutions cover key concepts such as the reactivity order of alcohols with halogen acids, emphasizing the role of S<sub>N</sub>1 and S<sub>N</sub>2 mechanisms. It explains the stability of carbocation intermediates and their impact on reaction outcomes. The solutions offer detailed explanations for multiple-choice questions, ensuring a thorough understanding of the principles governing these reactions.

Learning outcomes

  • Understand the reactivity order of alcohols with halogen acids.
  • Explain the S<sub>N</sub>1 and S<sub>N</sub>2 reaction mechanisms.
  • Analyze the stability of carbocation intermediates.
  • Solve problems related to the reaction of alcohols with concentrated HCl.
  • Identify factors influencing the rate of reaction between alcohols and halogen acids.

Topics covered

Paper topics

  • Haloalkanes
  • Haloarenes
  • Reactivity of Alcohols
  • Halogen Acids
  • S<sub>N</sub>1 Mechanism
  • S<sub>N</sub>2 Mechanism
  • Carbocation Stability
  • Tertiary Alcohols
  • Secondary Alcohols
  • Primary Alcohols
  • Reaction Mechanisms
  • Alkyl Chloride Formation

Important topics

  • Reactivity order of alcohols with halogen acids
  • S<sub>N</sub>1 and S<sub>N</sub>2 reaction mechanisms
  • Carbocation stability and its effect on reactivity
  • Reaction of alcohols with concentrated HCl at room temperature

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Questions and Solutions

Question 1

The order of reactivity of the following alcohols with halogen acids is ...... .

CH_3CH_2CH_2OH (A)

CH_3CH_2CH(OH)CH_3 (B)

CH_3CH_2C(OH)(CH_3)_2 (C)

(a) (A) > (B) > (C)

(b) (C) > (B) > (A)

(c) (B) > (A) > (C)

(d) (A) > (C) > (B)

Solution: The reaction between alcohols and halogen acids typically follows an SN1 mechanism, especially for secondary and tertiary alcohols. This mechanism involves the formation of carbocation intermediates. The stability of these carbocations dictates the reactivity order. Tertiary carbocations are the most stable, followed by secondary, and then primary carbocations. Therefore, alcohols that form more stable carbocations react faster.

Let's analyze the carbocations formed from the given alcohols:

  • Alcohol (A) is a primary alcohol, forming a primary carbocation: CH_3CH_2CH_2^{\oplus}. This is the least stable.
  • Alcohol (B) is a secondary alcohol, forming a secondary carbocation: CH_3CH_2\stackrel{\oplus}{C}HCH_3. This is more stable than a primary carbocation.
  • Alcohol (C) is a tertiary alcohol, forming a tertiary carbocation: CH_3CH_2C^{\oplus}(CH_3)_2. This is the most stable carbocation.

Based on carbocation stability (Tertiary > Secondary > Primary), the reactivity order of the alcohols with halogen acids is (C) > (B) > (A). This corresponds to option (b).

Answer: (b)

Question 2

Which of the following alcohols will yield the corresponding alkyl chloride on reaction with concentrated HCl at room temperature?

CH_3CH_2CH_2OH (a)

CH_3CH_2CH(OH)CH_3 (b)

CH_3CH_2CH(CH_3)CH_2OH (c)

CH_3CH_2C(OH)(CH_3)_2 (d)

Solution: The reaction of alcohols with concentrated HCl at room temperature to form alkyl chlorides proceeds via the SN1 mechanism. This mechanism is favored when a stable carbocation intermediate can be formed. The stability of carbocations follows the order: tertiary > secondary > primary.

Let's examine the carbocations formed from each alcohol:

  • Alcohol (a) is a primary alcohol. It would form a primary carbocation, which is unstable and reacts slowly or via SN2.
  • Alcohol (b) is a secondary alcohol. It forms a secondary carbocation, which is more stable than a primary carbocation and reacts at a moderate rate.
  • Alcohol (c) is a primary alcohol with branching. It forms a primary carbocation, similar to alcohol (a).
  • Alcohol (d) is a tertiary alcohol. It forms a tertiary carbocation (CH_3CH_2C^{\oplus}(CH_3)_2) upon protonation and loss of water. Tertiary carbocations are highly stable and are readily formed, allowing the reaction with Cl- to occur efficiently at room temperature.

Therefore, the tertiary alcohol (d) will yield the corresponding alkyl chloride most readily at room temperature.

Answer: (d)

Common mistakes

  • Confusing S<sub>N</sub>1 and S<sub>N</sub>2 mechanisms.
  • Incorrectly assessing carbocation stability.
  • Misinterpreting reactivity trends based on alcohol structure.
  • Not considering reaction conditions (e.g., room temperature).

Revision tips

  • Review the stability order of primary, secondary, and tertiary carbocations.
  • Understand the conditions under which S<sub>N</sub>1 and S<sub>N</sub>2 mechanisms are favored.
  • Practice identifying the type of alcohol (1°, 2°, 3°) in reaction problems.
  • Focus on how reaction conditions affect the outcome.

Practice MCQs

Q1. What is the order of reactivity of the following alcohols with halogen acids?

Q2. Which of the following alcohols will yield the corresponding alkyl chloride on reaction with concentrated HCl at room temperature?

Frequently asked questions

What is the primary mechanism for the reaction between alcohols and halogen acids?

The reaction between alcohols and halogen acids primarily follows the S<sub>N</sub>1 mechanism, especially with secondary and tertiary alcohols, due to the formation of carbocation intermediates. Primary alcohols may react via S<sub>N</sub>2.

Why is the stability of carbocations important in these reactions?

The stability of carbocations is crucial because the S<sub>N</sub>1 mechanism involves their formation as intermediates. More stable carbocations are formed more readily, leading to a faster reaction rate and higher reactivity of the corresponding alcohol.

Which type of alcohol is most reactive towards halogen acids?

Tertiary alcohols are the most reactive towards halogen acids because they form the most stable tertiary carbocations, which are key intermediates in the S<sub>N</sub>1 reaction pathway.

How does the structure of the alcohol affect its reactivity with concentrated HCl at room temperature?

The structure affects reactivity based on the stability of the carbocation formed. Tertiary alcohols form stable tertiary carbocations and react readily at room temperature. Secondary alcohols react slower, and primary alcohols react even slower or require different conditions.

What is the difference in reactivity between primary, secondary, and tertiary alcohols with halogen acids?

Tertiary alcohols are most reactive, followed by secondary alcohols, and then primary alcohols. This order is due to the increasing stability of carbocations formed: tertiary > secondary > primary.

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