Problem 186 - A Complete Resource Book in Chemistry for JEE Main

Question

In \(\mathrm{S}_{\mathrm{N}}^{2}\) reactions, the correct order of reactivity for the following compounds: \(\mathrm{CH}_{3} \mathrm{Cl}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\), \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}\) and \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\) is: |2014] (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}>\mathrm{CH}_{3} \mathrm{Cl}>\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}>\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}>\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}>\mathrm{CH}_{3} \mathrm{Cl}>\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\) (c) \(\mathrm{CH}_{3} \mathrm{Cl}>\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}>\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}>\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\) (d) \(\mathrm{CH}_{3} \mathrm{Cl}>\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}>\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}>\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\)

Step-by-Step Solution

Answer

Option (d) is the correct order: \( \text{CH}_3\text{Cl} > \text{CH}_3\text{CH}_2\text{Cl} > (\text{CH}_3)_2\text{CHCl} > (\text{CH}_3)_3\text{CCl} \).

Step 1: Understanding SN2 Reactions

In an SN2 reaction, a nucleophile attacks the electrophilic carbon at the same time as the leaving group departs. This mechanism involves a backside attack, resulting in an inversion of configuration at the carbon atom. The rate depends on the steric hindrance around the carbon atom.

Step 2: Analyze Steric Hindrance

Steric hindrance increases as the number of alkyl groups attached to the central carbon increases. Therefore, primary alkyl halides are more reactive than secondary and tertiary ones in SN2 reactions.

Step 3: Evaluate the Reactivity of Provided Compounds

The compounds given are, in order of increasing bulkiness around the carbon atom bearing the chlorine:- \( \text{CH}_3\text{Cl} \) (methyl chloride, no carbon groups attached)- \( \text{CH}_3\text{CH}_2\text{Cl} \) (ethyl chloride, one carbon group attached)- \( (\text{CH}_3)_2\text{CHCl} \) (isopropyl chloride, two carbon groups attached)- \( (\text{CH}_3)_3\text{CCl} \) (tert-butyl chloride, three carbon groups attached).

Step 4: Determine the Correct Order

The reactivity order is based on the steric hindrance around the carbon atom. Therefore, the order from most reactive to least reactive is as follows: \( \text{CH}_3\text{Cl} > \text{CH}_3\text{CH}_2\text{Cl} > (\text{CH}_3)_2\text{CHCl} > (\text{CH}_3)_3\text{CCl} \).

Step 5: Match with Given Options

Comparing this order with the options provided: - Option (d) \( \text{CH}_3\text{Cl} > \text{CH}_3\text{CH}_2\text{Cl} > (\text{CH}_3)_2\text{CHCl} > (\text{CH}_3)_3\text{CCl} \) matches the determined order.

Key Concepts

Nucleophilic Substitution

Nucleophilic substitution is a fundamental reaction mechanism in organic chemistry. It involves the replacement of a leaving group by a nucleophile. A nucleophile, rich in electrons, searches for an electrophilic center, usually a carbon atom, to attack. The most common settings are where a negative or neutral nucleophile targets a positive or partially positive carbon atom.
In an SN2 reaction, the nucleophile approaches the electrophilic carbon from the opposite side (backside) of the leaving group. This specific type of nucleophilic substitution involves a single step mechanism without any intermediate formation. The attack results in the inversion of the molecule's stereochemistry, resembling an umbrella turning inside out during inversion. Due to the direct nature of this attack, the factors like steric hindrance greatly influence these reactions.

Steric Hindrance

Steric hindrance plays a crucial role in the speed and probability of a successful nucleophilic attack in SN2 reactions. It refers to the effect the physical size and shape of molecules have on the chemical reactions they undergo. When larger groups are attached to the electrophilic carbon, they physically block the nucleophile from effectively reaching the carbon for a successful reaction.
The fewer large groups around the target carbon, the less steric hindrance there is. For example:

• Methyl chloride (_3Cl) has no additional carbon groups attached, resulting in minimal steric hindrance.
• Ethyl chloride (_3CH1_2Cl) has one carbon group attached, offering slightly more hindrance.
• Isopropyl chloride ((_3)C_2CHCl) and tert-butyl chloride ((_3)C_3CCl) have increasingly bulky groups, further hindering the approach.

As steric hindrance increases, the reaction rate decreases, making primary alkyl halides generally more reactive in SN2 reactions compared to secondary or tertiary ones.

Reactivity Order

In SN2 reactions, the reactivity order of molecules is mainly dictated by steric hindrance. Primary alkyl halides, like methyl chloride, are typically the most reactive due to the lack of steric hindrance. As we introduce more substituents or larger groups around the carbon bonded to the halide, reactivity decreases due to increased steric interference.
Let's look at the specific cases:

• Chloromethane (1_3Cl) is the most reactive because there's minimal hindrance to block the nucleophilic attack.
• Chloroethane (1_3CH1_2Cl) follows, slightly less reactive due to having one more carbon, but still accessible for the nucleophile.
• Isopropyl chloride, with two additional methyl groups, exhibits substantial steric hindrance, reducing its reactivity further.
• Finally, tert-butyl chloride is the least reactive due to its bulky surrounding groups, almost entirely preventing a back-side nucleophilic attack.

By understanding these trends, one can predict the relative speeds and outcomes of potential SN2 reactions, helping to deduce the correct order of reactivity among different compounds.