Problem 12 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Butanenitrile may be prepared by heating :(a) Propyl alcohol with \(\mathrm{KCN}\) (b) Butyl alcohol with \(\mathrm{KCN}\) (c) Butyl chloride with \(\mathrm{KCN}\)(d) Propyl chloride with \(\mathrm{KCN}\)

Step-by-Step Solution

Answer

Butyl chloride with \( \mathrm{KCN} \) is correct.

Step 1: Identify the Functional Requirement

To form butanenitrile, we need a precursor molecule that can be converted into a four-carbon nitrile. This means we need a starting compound which, on reaction with \( \mathrm{KCN} \), can yield a nitrile group \(-CN\).

Step 2: Analyze Each Option

Analyze each given compound: (a) Propyl alcohol: \( \mathrm{C_3H_7OH} \) - It yields an alcohol group, not suitable for easy conversion to a nitrile.(b) Butyl alcohol: \( \mathrm{C_4H_9OH} \) - Similar issue as Propyl alcohol.(c) Butyl chloride: \( \mathrm{C_4H_9Cl} \) - A good candidate as the chloride ion \( \mathrm{Cl^-} \) can be displaced by \( \mathrm{CN^-} \).(d) Propyl chloride: \( \mathrm{C_3H_7Cl} \) - Yields a three-carbon chain, unsuitable for forming butanenitrile.

Step 3: Determine the Correct Option

From the analysis, butyl chloride readily reacts with \( \mathrm{KCN} \) to undergo a nucleophilic substitution reaction, resulting in the replacement of the chloride ion with the cyanide ion \( \mathrm{CN^-} \), forming butanenitrile (a four-carbon nitrile).

Step 4: Confirm the Reaction

The reaction is:\[\mathrm{C_4H_9Cl + KCN \rightarrow C_4H_9CN + KCl}\]This confirms that option (c), butyl chloride with \( \mathrm{KCN} \), is the correct choice to prepare butanenitrile.

Key Concepts

Nucleophilic Substitution

The nucleophilic substitution reaction is a fundamental concept in organic chemistry where one functional group in a molecule is replaced by another. This process occurs when a nucleophile, an electron-rich species, attacks an electrophile, an electron-poor site, in a molecule. For example, in the reaction between butyl chloride and potassium cyanide (\(\mathrm{KCN}\)), the nucleophile is the cyanide ion \(\mathrm{CN^-}\), and the electrophile is the carbon atom bonded to the chloride ion \(\mathrm{Cl^-}\).
The cyanide ion attacks the carbon, displacing the chloride ion. This results in the formation of butanenitrile.
This type of substitution is important for creating new bonds and integrating functional groups. Nucleophilic substitution is broadly classified into two types:

• \(\mathrm{S_N1}\) - Unimolecular substitution occurs in two steps with a carbocation intermediate.
• \(\mathrm{S_N2}\) - Bimolecular substitution happens in a single concerted step where the nucleophile attacks as the leaving group departs.

In our exercise, we observe an \(\mathrm{S_N2}\) reaction where butyl chloride undergoes a direct exchange of the chloride for a cyanide ion, creating a nitrile group.

Nitrile Formation

Nitrile formation is a critical reaction in organic synthesis that involves the introduction of a \(-\mathrm{CN}\) group into a molecule, resulting in a nitrile. A nitrile is characterized by a carbon triple-bonded to a nitrogen atom, forming a \(-\mathrm{C\equiv N}\) bond.
This bond is highly polar due to the electronegativity difference between carbon and nitrogen, which plays a crucial role in the reactivity of nitriles. The strength of the triple bond makes nitriles stable yet reactive in further synthesis reactions.
In the provided exercise, nitrile formation is demonstrated through the reaction of butyl chloride with \(\mathrm{KCN}\), showcasing how effective nucleophilic substitution with cyanide can be for synthesizing nitriles. This transformation introduces a new functional group into the molecule, diversifying its reactive capabilities and utility in further chemical transformations or product syntheses.

Functional Group Transformation

Functional group transformation is a cornerstone of synthetic organic chemistry, focusing on converting one functional group into another to achieve desired molecular architectures. Through these transformations, chemists can manipulate molecular properties and reactivity to create complex molecules from simpler ones.
In this context, the reaction of butyl chloride with \(\mathrm{KCN}\) illustrates a classic functional group transformation where an alkyl halide is converted into a nitrile. The process involves the substitution of the halide group with a cyanide ion, altering the molecule's reactive profile.
Such transformations are fundamental for building chemical complexity and enabling the synthesis of pharmaceuticals, polymers, and other valuable substances. They allow chemists to modify the chemical nature, improve functionality, and introduce variability in molecular structures for a wide range of applications.