Problem 59 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

Butanenitrile is prepared by heating butyl chloride with KCN (option c).

Step 1: Identify the Starting Material

For synthesizing butanenitrile, recognize that the aim is to introduce a nitrile group (–C≡N) to a four-carbon chain. Butanenitrile has four carbon atoms.

Step 2: Evaluate the Options

Examine the options: (a) propyl alcohol with KCN (b) butyl alcohol with KCN (c) butyl chloride with KCN (d) propyl chloride with KCN. Since we are adding a nitrile to a four-carbon species, we need a starting material having a leaving group attached to a three-carbon (for chain elongation by one carbon) or a four-carbon skeleton. Alcohols typically undergo substitution reactions less readily compared to alkyl halides like chlorides.

Step 3: Consider the Best Reaction Mechanism

The nitrile ion (CN⁻) acts as a nucleophile in nucleophilic substitution reactions. Chlorides are better leaving groups than alcohols, allowing the substitution reaction to proceed more effectively without needing excessive energy or harsh reagents.

Step 4: Select the Correct Option

Since the best nucleophilic substitution to attach –C≡N involves replacing the chloride, and given butanenitrile needs four continuous carbon atoms, the ideal choice is option (c): butyl chloride with KCN. Starting from butyl chloride aligns with the carbon skeleton needed for butanenitrile formation.

Key Concepts

Butanenitrile Synthesis

When synthesizing butanenitrile, the goal is to create a four-carbon compound with a nitrile group, which is represented as \( \text{-C}\equiv\text{N} \). This setup is significant because the formation of nitrile groups adds valuable versatility in organic synthesis, often leading to various derivative compounds. Butanenitrile specifically differs from other nitriles by having a four-carbon linear chain, which is an essential consideration when selecting starting materials and reaction processes.
To effectively synthesize butanenitrile, one ideal method is through nucleophilic substitution. This process involves replacing a suitable leaving group on a carbon skeleton with a nitrile ion \( \text{CN}^- \). Recognizing this, butyl chloride serves well as starting material due to its four-carbon structure, which directly matches the necessary chain length for butanenitrile.

Leaving Group

In chemical reactions, especially nucleophilic substitution reactions, the leaving group plays a notably crucial role. The leaving group is the atom or group of atoms that departs with a pair of electrons during the reaction. Ideally, to have an efficient reaction, the leaving group must be able to stabilize the negative charge after parting.
Common characteristics of good leaving groups include the ability to form stable, weakly basic anions. Halides, such as chlorides, bromides, and iodides, are optimal leaving groups for this type of reaction due to their size and electron-donating behavior.

• Chloride ions are commonly used in organic synthesis as leaving groups because they enable reactions to proceed under milder conditions compared to other groups like hydroxyls.
• In the synthesis of butanenitrile, the use of butyl chloride exploits the chloride's ability to leave efficiently, facilitating the incorporation of the nitrile group.

Nitrile Ion

The nitrile ion \( \text{CN}^- \) is a potent nucleophile in organic chemistry. It is characterized by a carbon atom triple-bonded to a nitrogen atom, which forms a linear structure. Nitriles are incredibly useful in synthesis because they act as an important intermediate in creating a wide array of products, including carboxylic acids, amines, and aldehydes.
As a nucleophile, the nitrile ion is admired for its ability to attack electrophilic centers. During nucleophilic substitution reactions, such as the those used for butanenitrile synthesis, the \( \text{CN}^- \) ion targets an electrophilic carbon that is bonded to a good leaving group, such as a chloride.

• The nitrile ion's high electronegativity and compact structure make it a strong nucleophile, capable of readily displacing weaker nucleophiles.

This aspect underlines its importance in efficiently generating new carbon-nitrogen bonds.

Alkyl Halides

Alkyl halides, also known as haloalkanes, are a class of compounds characterized by one or more halogen atoms bound to an alkyl chain. They are established as crucial substrates in nucleophilic substitution reactions, making them vital components in organic synthesis.
The defining feature of alkyl halides includes their electrophilic carbon, resulting from the polarization of the carbon-halogen bond where the carbon atom bears a slight positive charge. This property makes them compatible with nucleophiles like nitrile ions, which provides a pathway for chain elongation and transmutation into other functional groups.

• Butyl chloride, a specific type of alkyl halide, is chosen for butanenitrile production because of its pre-existing four-carbon chain, which aligns with the structural requirement for butanenitrile.
• Among various halides, chlorides provide a good balance between reactivity and stability as leaving groups, allowing efficient generation of desired nitrile compounds.

Hence, alkyl halides serve as excellent foundations for transforming simple molecules into more complex structures.