Problem 182 - A Complete Resource Book in Chemistry for JEE Main

Question

The organic chloro compound, which shows complete stereochemical inversion during a \(\mathrm{SN}^{2}\) reaction, is \([2008]\) (a) \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{CHCl}\) (b) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}\) (d) \(\mathrm{CH}_{3} \mathrm{Cl}\)

Step-by-Step Solution

Answer

The correct answer is (d) \(\mathrm{CH}_3 \mathrm{Cl}\).

Step 1: Understand SN2 Reactions

In an \(\mathrm{SN}^{2}\) reaction, a nucleophile attacks the carbon atom bonded to a leaving group, resulting in inversion of stereochemistry at the reaction site.

Step 2: Analyze Each Compound

We must evaluate each option to determine steric hindrance, which affects the \(\mathrm{SN^2}\) mechanism: (a) \((\mathrm{C}_{2} \mathrm{H}_{5})_{2} \mathrm{CHCl}\) is a secondary chloride, offering moderate steric hindrance.(b) \((\mathrm{CH}_{3})_{3} \mathrm{CCl}\) is a tertiary chloride, which causes high steric hindrance, making \(\mathrm{SN^2}\) unlikely.(c) \((\mathrm{CH}_{3})_{2} \mathrm{CHCl}\) is a secondary chloride with moderate steric hindrance.(d) \(\mathrm{CH}_{3} \mathrm{Cl}\) is a primary chloride with the least steric hindrance, favoring \(\mathrm{SN^2}\) reactions.

Step 3: Assess Steric Effects

Primary alkyl halides like \(\mathrm{CH}_{3} \mathrm{Cl}\) undergo \(\mathrm{SN^2}\) reactions easily due to minimal steric hindrance. Both secondary and tertiary are less favorable for \(\mathrm{SN^2}\). With this, \(\mathrm{CH}_{3} \mathrm{Cl}\) facilitates complete stereochemical inversion efficiently.

Step 4: Identify the Correct Answer

Given the necessity for complete stereochemical inversion through an \(\mathrm{SN^2}\) mechanism, the organic chloro compound that allows this is \(\mathrm{CH}_{3} \mathrm{Cl}\). It is primary and thus most suitable for \(\mathrm{SN^2}\) reactions.

Key Concepts

Stereochemical Inversion

In an SN2 reaction, stereochemical inversion is a key hallmark. This means that the configuration of the molecule flips as the reaction proceeds.
A simple way to think about it is how an umbrella turns inside out during a gust of wind.
During an SN2 reaction, a nucleophile, which is a molecule with a lone pair ready to bond, approaches the carbon atom from the opposite side of the leaving group.

• Think of this as a backdoor entrance.
• This attack happens in one swift step, ejecting the leaving group while the nucleophile bonds.

The result is a change in configuration, where the spatial arrangement of atoms is flipped, achieving what is known as Walden inversion.

Steric Hindrance

Steric hindrance is like traffic in a crowded city; it can slow things down. In chemistry, it refers to the bulk of groups around an atom blocking access.
For an SN2 reaction to happen efficiently, the nucleophile needs clear access to the carbon.
A bulky environment causes steric hindrance, which can prevent or slow down reactions.

• Primary alkyl halides, with less congestion, have low steric hindrance.
• Secondary alkyl halides present moderate hindrance.
• Tertiary alkyl halides are heavily crowded, creating high steric hindrance.

Reducing steric hindrance is crucial for SN2 reactions, as it allows quick and easy nucleophilic attacks.

Primary Alkyl Halides

Primary alkyl halides are your go-to choice for SN2 reactions.
They have a simple structure, basically a carbon with a halogen (like chlorine) and three hydrogens.
This setup leaves the carbon open and available for nucleophilic attack, minimizing steric hindrance.

• In the context of SN2 reactions, primary alkyl halides like methyl chloride (\(\mathrm{CH}_{3}\mathrm{Cl}\)) excel due to their minimal complexity.
• The lack of additional carbon branches prevents unnecessary roadblocks.

This makes the reaction smooth and promotes efficient stereochemical inversion.

Nucleophilic Substitution

Nucleophilic substitution is like a carefully choreographed dance.
It's an exchange process where one dancer (the nucleophile) takes the place of another (the leaving group) at the carbon center.
In an SN2 reaction, this happens in a single, concerted step without intermediates.

• The nucleophile must be strong enough to displace the leaving group.
• The leaving group must be stable enough to depart with ease.

The beauty of the SN2 mechanism lies in this simplicity and the precision of the substitution process, often leading to complete inversion of configuration.

Organic Chloro Compounds

Organic chloro compounds involve the carbon-halogen bond, with chlorine as the halogen.These compounds are central in SN2 reactions due to the modest leaving ability of chlorine. Compare these scenarios:

• In a complex environment, the leaving group's ease of departure can impact the reaction's efficiency.
• Chlorine, being an effective leave-taker, facilitates successful substitution.

Focusing on primary chloro compounds like \(\mathrm{CH}_{3}\mathrm{Cl}\), they provide the ideal condition for SN2 reactions: low steric hindrance and seamless nucleophilic attack.
Thus, these organic chloro compounds stand as exemplary models for understanding nucleophilic substitution.