Question

The order of reactivity towards \( \text{S}_\text{N}2 \) reaction among the following is
\[ (a)\ \text{CH}_3\text{CH}_2\text{Cl} \quad (b)\ \text{CH}_3\text{CH(Cl)}\text{CH}_3 \quad (c)\ \text{(CH}_3\text{)}_3\text{CCI} \quad (d)\ \text{CH}_3\text{CH}_2\text{Cl} \]

• A. \( a > d > b > c \)
• B. \( a > b > c > d \)
• C. \( a > c > b > d \)
• D. \( b > a > c > d \)
• E. \( c > b > d > a \)

Hint:

In \( \text{S}_\text{N}2 \) reactions, primary carbons are most reactive due to less steric hindrance, while tertiary carbons are least reactive.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The question asks for the order of reactivity of different alkyl chlorides towards the S\(_N\)2 (bimolecular nucleophilic substitution) reaction. The rate of an S\(_N\)2 reaction is primarily governed by steric hindrance.
Step 2: Key Formula or Approach:
The S\(_N\)2 mechanism involves a backside attack by the nucleophile on the carbon atom bearing the leaving group. This attack is hindered by the presence of bulky alkyl groups attached to that carbon. Therefore, the reactivity of alkyl halides in an S\(_N\)2 reaction decreases as steric hindrance increases.
The order of reactivity is:
Methyl halide>Primary (1\(^\circ\)) halide>Secondary (2\(^\circ\)) halide>Tertiary (3\(^\circ\)) halide
\[ \text{CH}_3\text{-X}>\text{RCH}_2\text{-X}>\text{R}_2\text{CH-X}>\text{R}_3\text{C-X} \] Step 3: Detailed Explanation:
Let's classify each of the given alkyl chlorides:
(a) CH\(_3\)Cl (Methyl chloride): This is a methyl halide. It has the least steric hindrance.
(b) CH\(_3\)CH(Cl)CH\(_3\) (Isopropyl chloride or 2-chloropropane): The carbon atom attached to the chlorine is bonded to two other carbon atoms. This is a secondary (2\(^\circ\)) alkyl halide.
(c) (CH\(_3\))\(_3\)CCl (tert-Butyl chloride or 2-chloro-2-methylpropane): The carbon atom attached to the chlorine is bonded to three other carbon atoms. This is a tertiary (3\(^\circ\)) alkyl halide. It is the most sterically hindered.
(d) CH\(_3\)CH\(_2\)Cl (Ethyl chloride): The carbon atom attached to the chlorine is bonded to one other carbon atom. This is a primary (1\(^\circ\)) alkyl halide.
Now, let's arrange them in decreasing order of reactivity towards S\(_N\)2:
1. Methyl halide (a) - Most reactive.
2. Primary halide (d) - Second most reactive.
3. Secondary halide (b) - Less reactive.
4. Tertiary halide (c) - Least reactive (practically unreactive via S\(_N\)2).
The order is: a>d>b>c.
Step 4: Final Answer:
The correct order of reactivity towards S\(_N\)2 reaction is CH\(_3\)Cl>CH\(_3\)CH\(_2\)Cl>CH\(_3\)CH(Cl)CH\(_3\)>(CH\(_3\))\(_3\)CCl, which corresponds to a>d>b>c. This matches option (A).