Problem 4 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

In a nucleophilic substitution reaction: $$ \mathrm{R}-\mathrm{Br}+\mathrm{Cl}^{-} \stackrel{\mathrm{DMF}}{\longrightarrow} \mathrm{R}-\mathrm{Cl}+\mathrm{Br}^{-} $$ which one of the following undergoes complete inversion of configuration? (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHC}_{6} \mathrm{H}_{5} \mathrm{Br}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{Br}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHCH}_{3} \mathrm{Br}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CCH}_{3} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\)

Step-by-Step Solution

Answer

Option (b) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{Br} \) undergoes complete inversion of configuration.

Step 1: Understanding Substitution Reactions

A nucleophilic substitution reaction involves the replacement of a leaving group (such as a halogen) by a nucleophile. There are two main types of nucleophilic substitutions: \( S_N1 \) and \( S_N2 \). \( S_N2 \) reactions occur with a single step mechanism and typically result in inversion of configuration at the carbon center due to the backside attack by the nucleophile.

Step 2: Determine the Reaction Type

The given reaction occurs in DMF (Dimethylformamide), a polar aprotic solvent, which favors \( S_N2 \) reactions. In \( S_N2 \) reactions, a nucleophile attacks the substrate from the opposite side of the leaving group, causing an inversion of the stereochemistry.

Step 3: Analyze Each Option's Structure

Each compound given has different structural characteristics, which affect the rate and type of nucleophilic substitution: (a) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHC}_{6} \mathrm{H}_{5} \mathrm{Br} \) (a secondary benzylic positions), (b) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{Br} \) (a primary benzylic position), (c) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHCH}_{3} \mathrm{Br} \) (a secondary benzylic position with a methyl group), (d) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CCH}_{3} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br} \) (a tertiary benzylic position). \( S_N2 \) reactions occur best at primary centers due to less steric hindrance.

Step 4: Select the Most Suitable Compound

Among all options, the compound with the least steric hindrance at the reactive site, favoring an \( S_N2 \) mechanism, is \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{Br} \) (option b). It has a primary benzyl position, which is optimal for \( S_N2 \) reactions, leading to complete inversion of configuration.

Key Concepts

S_N2 Reaction

The S_N2 reaction, or bimolecular nucleophilic substitution, is a fundamental mechanism in organic chemistry. This type of reaction occurs in a single step.
The nucleophile approaches the substrate from the opposite side of the leaving group, leading to a **backside attack**. This simultaneous bond-breaking and bond-forming process results in the departure of the leaving group and the attachment of the nucleophile.
S_N2 reactions are characterized by a second-order kinetics, meaning the rate depends on both the concentration of the substrate and the nucleophile:

• The reaction rate is given by the formula: \[ ext{Rate} = k [ ext{Nu}^-][ ext{R-LG}] \]
• Polar aprotic solvents, like DMF (Dimethylformamide), favor these reactions as they do not solvate the nucleophile strongly, allowing it to remain reactive.

This mechanism is most efficient in substrates with minimal steric bulk, as we will discuss in the **Steric Hindrance** section, making primary substrates ideal candidates for S_N2 reactions.

Inversion of Configuration

One distinct feature of S_N2 reactions is the **inversion of configuration**. This occurs because the nucleophile attacks the substrate from the back, where the leaving group is attached.
As a result, this attack flips the configuration of the carbon molecule involved.
The inversion is often likened to an umbrella flipping inside out in a windstorm, completely switching the orientation of the atoms around the stereocenter:

• In chemical terms, if the substrate's configuration is originally R, it will change to S, or vice versa, in the substitution reaction.
• This inversion is only guaranteed when the carbon atom undergoing substitution is **chiral**, meaning it has four different substituents.

In the exercise, compound (b), \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{Br} \), undergoes complete inversion because it is primed for S_N2 attack, with minimal steric hindrance making the backside attack possible and effective.

Steric Hindrance

Steric hindrance refers to the obstruction to a chemical reaction caused by the spatial arrangement of atoms within a molecule. In an S_N2 reaction, this factor is crucial as it affects the nucleophile's ability to access the reactive carbon.
Simply put, larger groups surrounding the carbon make it challenging for the nucleophile to reach and attack it.
Key considerations include:

• The reaction occurs most easily at a **primary carbon center** as these have fewer attached carbon groups causing less blockage or hindrance.
• Secondary centers are less reactive in S_N2 reactions due to increased steric bulk.
• Tertiary centers are typically unreactive via S_N2 because the nucleophile cannot effectively approach such a crowded region.

For instance, in the exercise options, compound (b), \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{Br} \), was selected due to its primary carbon position, leading to successful inversion of configuration and a more favorable S_N2 pathway.