Problem 67 - A Complete Resource Book in Chemistry for JEE Main

Question

The reaction of the formation of bromobenzyl chloride with \(\mathrm{NaCN}\) in ethanol leads to (a) 4-bromo-2-cyanobenzyl chloride (b) 4 -cyanobenzyl cyanide (c) 4 -cyanobenzyl chloride (d) 4 -bromobenzyl cyanide

Step-by-Step Solution

Answer

The reaction leads to (d) 4-bromobenzyl cyanide.

Step 1: Identify the Reaction Type

The reaction of bromobenzyl chloride with \(\mathrm{NaCN}\) in ethanol is a nucleophilic substitution reaction. In this type of reaction, the cyanide ion (\(\mathrm{CN}^-\)) from \(\mathrm{NaCN}\) replaces the leaving group in the substrate molecule.

Step 2: Analyze the Substrate

The substrate is bromobenzyl chloride, where a chlorine atom is directly attached to a benzyl carbon connected to a benzene ring. Typically, \(\mathrm{Cl}^-\) can act as a good leaving group in substitution reactions.

Step 3: Determine the Product Formation

In the presence of \(\mathrm{NaCN}\), the \(\mathrm{CN}^-\) ion will attack the carbon atom bonded to the chlorine, substituting it, and forming a cyanide group (-CN) instead of the chloride group.

Step 4: Identify the Specific Product

Since the \(\mathrm{CN}^-\) group replaces the \(\mathrm{Cl}^-\) group on the benzyl carbon, the product will have the same structure except with a CN group where the Cl was. This leads to the formation of 4-bromobenzyl cyanide.

Key Concepts

Bromobenzyl Chloride

Bromobenzyl chloride is a fascinating compound categorized as an organic molecule, which is often used in various chemical reactions. In its structure, bromobenzyl chloride has a benzene ring, a bromine atom, and a chloromethyl group attached to the benzene. The benzene ring is an aromatic hydrocarbon, characterized by its stability and distinct odor.
Bromobenzyl chloride specifically refers to the placement of the bromine and chloride atoms on the benzene ring and its side chain. The bromine is in the para position relative to the chloromethyl group; this means it is placed opposite to the methylene (CH2) group connected to the chlorine.
In chemical reactions, bromobenzyl chloride can participate in nucleophilic substitution reactions, where the chloride ion can be replaced by other nucleophiles due to its good leaving ability. This makes bromobenzyl chloride a versatile substrate in organic synthesis, facilitating the introduction of various functional groups.

Cyanide Reaction Mechanism

The cyanide reaction mechanism is a crucial aspect of nucleophilic substitution reactions, particularly when \( \text{NaCN} \) is involved. The primary role of the cyanide ion \( \text{CN}^- \) in these reactions is to act as a nucleophile. It is attracted to positive centers within molecules, thanks to its negative charge.
When bromobenzyl chloride meets cyanide ions in a solvent like ethanol, a nucleophilic substitution occurs. Here's how this happens:

• The cyanide ion approaches the carbon atom bonded to the chlorine in bromobenzyl chloride.
• As a nucleophile, the \( \text{CN}^- \) displaces the chloride ion \( \text{Cl}^- \), effectively replacing it.
• This substitution creates a new carbon-nitrogen bond, forming a cyano group \( -\text{CN} \).

This transformation is important in many organic syntheses due to the strong reactivity and versatility of the cyano group. The reaction is usually swift and efficient, making cyanide an effective species for substitutions.

Leaving Group

The leaving group is a fundamental concept in the study of nucleophilic substitution reactions. A leaving group is usually an atom or a group of atoms that detaches from the substrate molecule during the reaction process. It facilitates the replacement by another group, often a nucleophile.
In the case of bromobenzyl chloride, the leaving group is the chloride ion \( \text{Cl}^- \). A good leaving group should stabilize the negative charge after detaching. Chloride, thanks to its relatively large size and the ability to delocalize charge, is typically a good leaving group.
For a nucleophilic substitution to occur effectively, the ease with which the leaving group departs is crucial. Chloride ion quickly dissociates due to the low energy required to break the bond with the carbon atom. Once it leaves, it paves the way for a nucleophile like the cyanide ion to attach, facilitating reactions like the formation of bromobenzyl cyanide.
Understanding the nature of leaving groups can aid in predicting the behavior and outcomes of chemical reactions, essential for planning in organic synthesis.