Problem 74 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl} \stackrel{\mathrm{NaCN}}{\longrightarrow} \mathrm{X} \stackrel{\mathrm{Ni} / \mathrm{H}_{2}}{\longrightarrow} \mathrm{Y}\) acetic anhydride \({\longrightarrow} \mathrm{Z}\) \(\mathrm{Z}\) in the above reaction sequence is (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CONHCONH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CONHCH}_{3}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NHCOCH}_{3}\)

Step-by-Step Solution

Answer

Z is (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NHCOCH}_{3}\).

Step 1: Identify the reaction from ethyl chloride to X

The compound \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\) (ethyl chloride) reacts with \(\mathrm{NaCN}\) to undergo a nucleophilic substitution reaction. \(\mathrm{CN}^-\) substitutes the \(\mathrm{Cl}^-\) to form \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CN}\) (propionitrile), which is our compound X.

Step 2: Transform X to Y using Ni/H2

The compound X (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CN}\)) reacts with \(\mathrm{Ni}/\mathrm{H}_{2}\) in a hydrogenation reaction. This converts the nitrile group to an amine group, resulting in \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (propylamine), which is our compound Y.

Step 3: Convert Y to Z using acetic anhydride

The compound Y (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\)) reacts with acetic anhydride in an acylation reaction. The amine group is acetylated to form an amide, resulting in \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NHCOCH}_{3}\), which is our compound Z.

Step 4: Identify the correct option for Z

Compare the derived formula for Z (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NHCOCH}_{3}\)) with the given options. The correct match is option (d).

Key Concepts

Nucleophilic Substitution

Nucleophilic Substitution is an important reaction in organic chemistry. It is a process where a nucleophile replaces a leaving group in a molecule. In the context of our exercise, the reaction starts with ethyl chloride \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\) and sodium cyanide \(\mathrm{NaCN}\). Here, the \(\mathrm{CN}^-\) ion acts as a nucleophile. The nucleophile attacks the carbon atom that is bonded to the chlorine atom and substitutes the chloride ion \(\mathrm{Cl}^-\). This substitution leads to the formation of a new compound known as propionitrile \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CN}\), marked as compound X.

Key points to remember about nucleophilic substitution are:

• It involves a nucleophile, usually a negatively charged ion or a neutral molecule with a lone pair of electrons.
• A leaving group is displaced during the process, in this case the chloride ion.
• The reaction mechanism can be either \(\mathrm{S}_N1\) or \(\mathrm{S}_N2\), depending on the substrate structure and reaction conditions.

Understanding these elements is crucial for predicting the outcome of nucleophilic substitution reactions.

Hydrogenation

Hydrogenation is a chemical reaction that involves the addition of hydrogen \(\mathrm{H}_{2}\) to a compound. This process is often used to convert unsaturated compounds, like alkenes and nitriles, into saturated ones. In the current exercise, the hydrogenation reaction transforms propionitrile \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CN}\) into propylamine \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\), making it compound Y.

This transformation is achieved using a nickel catalyst \(\mathrm{Ni}\), which facilitates the addition of hydrogen across the carbon-nitrogen triple bond in the nitrile group. The nitrile bond \(\mathrm{C}-\mathrm{N}\) becomes an amine bond \(\mathrm{C}-\mathrm{NH}_{2}\).

Key points about hydrogenation include:

• Hydrogenation is typically done in the presence of a catalyst to occur efficiently.
• It reduces or saturates organic compounds by adding hydrogen across multiple bonds.
• Hydrogenation reactions can be selective, meaning they can target specific sites in polyfunctional compounds.

Knowing about hydrogenation is essential for manipulating organic molecules in synthesis and industrial applications.

Acylation

Acylation is a process where an acyl group is introduced into a molecule. This is often achieved using acyl halides or anhydrides. In the step-by-step solution provided, propylamine \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) reacts with acetic anhydride. This interaction results in compound Z, which is identified as \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NHCOCH}_{3}\).

During this process, the amine group in propylamine is transformed into an amide, creating a longer chain with the acyl derivative of acetic acid. This particular reaction is crucial as it alters the functional group, potentially changing the physical and chemical properties of the resulting compound.

Important points about acylation are:

• It's a technique used to modify molecules, enhancing their stability or reactivity.
• Acylation is widely used in pharmaceuticals to create derivatives with improved therapeutic qualities.
• Understanding the mechanism helps in designing targeted synthesis pathways for complex organic compounds.

Mastering acylation reactions expands the toolbox for creating diverse organic materials and compounds.