Case Study Questions for Class 12 Chemistry Chapter 10 Haloalkanes and Haloarenes

There is Case Study Questions in class 12 Chemistry in session 2020-21. For the first time, the board has introduced the case study questions in the board exam. The first two questions in the board exam question paper will be based on Case Study and Assertion & Reason. The first question will have 5 MCQs out of which students will have to attempt any 4 questions. The second question will carry 5 Assertion & Reason type questions with the choice to attempt any four. Here are the questions based on case study.

Case Study Question 1:

Read the passage given below and answer the following questions:

Nucleophilic substitution reactions are of two types; substitution nucleophilic bimolecular (S_N2) and substitution nucleophilic unimolecular (S_N1) depending on molecules taking part in determining the rate of reaction. Reactivity of alkyl halide towards S_N1 and S_N2 reactions depends on various factors such as steric hindrance, stability of intermediate or transition state and polarity of solvent. S_N2 reaction mechanism is favoured mostly by primary alkyl halide or transition state and polarity of solvent, S_N2 reaction mechanism is favoured mostly by primary alkyl halide then secondary and then tertiary. This order is reversed in case of SN1 reactions.

The following questions are multiple choice question. Choose the most appropriate answer:

(i) Which of the following is most reactive towards nucleophilic substitution reaction?
(a) C₆H₅Cl
(b) CH₂=CHCl
(c) ClCH₂CH=CH₂
(d) CH₃CH=CHCl

(ii) Isopropyl chloride undergoes hydrolysis by
(a) S_N1 mechanism
(b) S_N2 mechanism
(c) S_N1 and S_N2 mechanism
(d) neither S_N1 nor S_N2 mechanism

(iii) The most reactive nucleophile among the following is
(a) CH₃O-
(b) C₆H₅O-
(c) (CH₃)₂CHO-
(d) (CH₃)₃CO-

(iv) Tertiary alkyl halides are practically inert to substitution by S_N2 mechanism because of
(a) insolubility
(b) instability
(c) inductive effect
(d) steric hindrance

OR

Which of the following is the correct order of decreasing S_N2 reactivity?
(a) RCH₂X > R₂CHX > R₃CX
(b) R₃CX > R₂CHX >RCH₂X
(c) R₂CHX >R₃CX > RCH₂X
(d) RCH₂X >R₃CX >R₂CHX

Answers

(i) (c) Allylic chlorides are most reactive.
(ii) c
(iii) a
(iv) d
or
(a) Larger the number of alkyl groups at alpha-carbon atom, more is the steric hindrance and hence lesser the reactivity towards SN2 mechanism.