Problem 132 - A Complete Resource Book in Chemistry for JEE Main

Question

Arrange the following in the order of decreasing tendency towards \(\mathrm{S}_{\mathrm{N}}^{2}\) reaction 1\. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Cl}\) 2\. \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{CHCl}-\mathrm{CH}_{3}\) 3\. \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCH}_{2} \mathrm{Cl}\) 4\. \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{Cl}\) (a) \(1>3>2>4\) (b) \(4>4>3>1\) (c) \(2>1>3>4\) (d) \(1>2>4>3\)

Step-by-Step Solution

Answer

The correct order is (a) 1 > 3 > 2 > 4.

Step 1: Understanding SN2 Reaction

In an SN2 reaction, a nucleophile attacks a carbon atom, and a leaving group is displaced. The reaction primarily occurs for substrates that are less sterically hindered because the nucleophile needs to access the reaction site for attack.

Step 2: Evaluate Substrate Steric Hindrance

Analyze the given compounds based on steric hindrance. Less hindered substrates will show greater reactivity towards SN2 reactions. 1. Propyl chloride, \ \( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{Cl} \ \), is a primary alkyl halide, leading to lower steric hindrance. 2. 2-Chlorobutane, \ \( \mathrm{CH}_{3}\mathrm{CH}_{2}-\mathrm{CHCl}-\mathrm{CH}_{3} \ \), is secondary. 3. 2-Chloropropane, \ \( \left(\mathrm{CH}_{3}\right)_{2}\mathrm{CHCH}_{2}\mathrm{Cl} \ \), is also secondary. 4. Tert-butyl chloride or \ \( \left(\mathrm{CH}_{3}\right)_{3}\mathrm{C}-\mathrm{Cl} \ \) is tertiary and has the highest steric hindrance.

Step 3: Rank Compounds in Decreasing SN2 Reactivity

Arrange the compounds based on steric hindrance and SN2 tendency. Less hindered compounds react faster. From least to most hindered for SN2: 1. \ \( 1 \) (primary) > 2. \ \( 3 \) (secondary) > 3. \ \( 2 \) (secondary) > 4. \ \( 4 \) (tertiary).

Step 4: Match the Ranking with Given Options

Compare the derived order, \ \( 1 > 3 > 2 > 4 \ \), with the provided options. Option (a) \ \( 1 > 3 > 2 > 4 \ \) matches perfectly.

Key Concepts

Nucleophilic substitution

In organic chemistry, nucleophilic substitution is a fundamental reaction type where a nucleophile, which is a species rich in electrons, replaces a leaving group in a molecule. The SN2 (bimolecular nucleophilic substitution) reaction is one subtype of such substitutions.
In an SN2 reaction, the nucleophile makes a direct attack on the carbon atom that is bonded to the leaving group. This process occurs in a single concerted step, which means the bond between the nucleophile and the carbon forms at the same time that the bond between the carbon and the leaving group breaks. The result is an inversion of configuration at the carbon center, essentially flipping the arrangement of groups around the carbon atom.
The efficiency of an SN2 reaction largely depends on the accessibility of the carbon atom under attack, which is influenced by the surrounding groups attached to the carbon.

Steric hindrance

Steric hindrance is a concept that describes how the size of groups attached to a carbon center can block the attack of a nucleophile. In simpler terms, the more cluttered a carbon is with bulky groups, the harder it becomes for a nucleophile to get close and react.
In the context of SN2 reactions, steric hindrance greatly affects the reaction rate. Primary carbon centers, which are attached to only one other carbon, are less hindered and more accessible to nucleophiles compared to secondary or tertiary centers.
Here's how steric hindrance affects the reaction:

• Primary alkyl halides: Less steric hindrance; more reactive towards SN2.
• Secondary alkyl halides: Moderate steric hindrance; moderately reactive.
• Tertiary alkyl halides: High steric hindrance; least reactive, often not reacting via SN2.

Alkyl halides

Alkyl halides are compounds that contain a carbon-halogen bond. They play a crucial role in SN2 reactions, serving as the substrate from which the leaving group detaches.
In the exercises we're dealing with, the halogens are chlorine atoms, and the carbon atoms vary in how many other carbon groups they are attached to. This classification gives rise to primary, secondary, and tertiary alkyl halides:

• Primary alkyl halides: Only one carbon group attached to the carbon with the halogen (e.g., Propyl chloride).
• Secondary alkyl halides: Two carbon groups attached (e.g., 2-Chlorobutane, 2-Chloropropane).
• Tertiary alkyl halides: Three carbon groups attached (e.g., Tert-butyl chloride).

Alkyl halides undergo nucleophilic substitution reactions where their reactivity is closely linked to the type of carbon center they possess and the nature of the leaving group.

Reaction kinetics

In the world of chemistry, reaction kinetics refers to the speed or rate at which a chemical reaction proceeds. For SN2 reactions, several factors influence these kinetics.
An SN2 reaction is bimolecular, meaning its rate depends on the concentration of both the nucleophile and the substrate (usually the alkyl halide). The rate equation for an SN2 reaction can be represented as:\[ ext{Rate} = k [ ext{Nucleophile}][ ext{Substrate}] \]where \( k \) is the rate constant.
This means that as the concentration of either reactant increases, the rate of reaction will also increase. Due to the direct attack mechanism in SN2 reactions, the substrate must be relatively unhindered by bulky groups to allow quick and efficient reaction. Thus, primary alkyl halides, with less steric hindrance, tend to have faster reaction kinetics in SN2 reactions compared to secondary or tertiary counterparts.