Problem 77 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify \(\mathrm{X}\) and \(\mathrm{Y}\) in the following sequence. \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Br} \stackrel{\mathrm{X}}{\longrightarrow}\) product \(\stackrel{\mathrm{Y}}{\longrightarrow} \mathrm{C}_{3} \mathrm{H}_{7} \mathrm{NH}_{2}\) (a) \(\mathrm{X}=\mathrm{KCN}, \mathrm{Y}=\mathrm{LiAlH}_{4}\) (b) \(\mathrm{X}=\mathrm{KCN}, \mathrm{Y}=\mathrm{H}_{3} \mathrm{O}^{+}\) (c) \(\mathrm{X}=\mathrm{CH}_{3} \mathrm{Cl}, \mathrm{Y}=\mathrm{AlCl}_{3} / \mathrm{HCl}\) (d) \(\mathrm{X}=\mathrm{CH}_{3} \mathrm{NH}_{2}, \mathrm{Y}=\mathrm{HNO}_{2}\)

Step-by-Step Solution

Answer

The correct answer is (a) \(\mathrm{X=KCN}, \mathrm{Y=LiAlH_{4}}\).

Step 1: Understand the First Reaction

The starting compound is ethyl bromide (\(\mathrm{C_{2}H_{5}Br}\)). Reaction \(\mathrm{X}\) will convert it to a product that eventually becomes \(\mathrm{C_{3}H_{7}NH_{2}\). Look for a reagent \(\mathrm{X}\) that adds to a carbon chain or forms an intermediate that can be further reacted.

Step 2: Identify Potential Reactions with KCN

Using \(\mathrm{KCN}\) in a nucleophilic substitution reaction could result in \(\mathrm{C_{2}H_{5}CN}\) by replacing the bromide \(\mathrm{Br}\) with cyanide \(\mathrm{CN}\). This creates a nitrile that can be further reduced to an amine by \(\mathrm{LiAlH_{4}}\).

Step 3: Examine the Second Reaction for Each Choice

After nucleophilic substitution by \(\mathrm{KCN}\), the intermediate \(\mathrm{C_{2}H_{5}CN}\) needs to be converted to \(\mathrm{C_{3}H_{7}NH_{2}\) by reduction. \(\mathrm{LiAlH_{4}}\) is a strong reducing agent that can perform this reduction.

Step 4: Compare Options

(a) and (b) both begin with \(\mathrm{KCN}\), which fits the nucleophilic attack replacing bromine. However, only (a) uses \(\mathrm{LiAlH_{4}}\), the reducing agent capable of converting the resulting nitrile to the primary amine required in the final product.

Step 5: Evaluate Incorrect Options

(c) and (d) do not start with the correct reagent for converting \(\mathrm{C_{2}H_{5}Br}\) to \(\mathrm{C_{2}H_{5}CN}\). Additionally, their secondary reagents \(\mathrm{AlCl_{3}/HCl}\) and \(\mathrm{HNO_{2}}\) do not aid in the necessary reduction to an amine.

Key Concepts

Nucleophilic Substitution

Nucleophilic substitution is a cornerstone reaction in organic chemistry. It involves the replacement of a leaving group in a molecule with a nucleophile. A nucleophile is a chemical species that donates an electron pair to form a chemical bond. In this reaction type, the nucleophile attacks a carbon atom that is attached to an electronegative atom or group. In the case of the exercise, we begin with ethyl bromide (\(\text{C}_2\text{H}_5\text{Br}\)), where bromide (\(\text{Br}^{-}\)) serves as the leaving group.

• The reagent potassium cyanide (\(\text{KCN}\)) acts as the nucleophile, bringing about the substitution.
• The cyanide ion (\(\text{CN}^{-}\)) replaces the bromide ion to form ethyl cyanide (\(\text{C}_2\text{H}_5\text{CN}\)).

This transformation is efficient due to the strong nucleophilicity of the cyanide ion. The resulting nitrile is a key intermediate for further chemical transformations.

Reduction Reaction

Reduction reactions involve the gain of electrons or the decrease in oxidation state of a molecule. In organic chemistry, reduction usually refers to the addition of hydrogen or the removal of oxygen.In this exercise, the intermediate product ethyl cyanide (\(\text{C}_2\text{H}_5\text{CN}\)) is reduced to form an amine.

• Lithium aluminium hydride (\(\text{LiAlH}_4\)) is a powerful reducing agent used here.
• It adds hydrogen atoms to the carbon-nitrogen multiple bond in the nitrile, culminating in the formation of propylamine (\(\text{C}_3\text{H}_7\text{NH}_2\)).

This reaction is significant due to the ability of lithium aluminum hydride to effectively reduce nitriles, making it invaluable in amine synthesis.

Reagents and Mechanisms

Understanding the role of different reagents and their mechanisms is crucial in organic transformations. Reagents are substances used to cause chemical reactions. In the exercise, two key reagents play pivotal roles.

• **Potassium Cyanide (\(\text{KCN}\)):** Used in the first step, it provides the cyanide ion as a nucleophile in a nucleophilic substitution reaction. This step lengthens the carbon chain by adding a \(-\text{CN}\) group.
• **Lithium Aluminium Hydride (\(\text{LiAlH}_4\)):** Employed in the second step to reduce the nitrile intermediate to an amine.

The mechanisms involve electron movement to form new bonds, exemplifying how reagents transform starting materials into desired products. Knowing these mechanisms aids in predicting reaction outcomes and designing new synthetic routes.

Amine Formation

Amines are organic compounds characterized by the presence of a nitrogen atom bonded to hydrogen atoms, alkyl groups, or aryl groups. Forming amines is a common goal in organic synthesis because they serve as building blocks in pharmaceuticals and materials science.In this exercise, the ultimate product is propylamine (\(\text{C}_3\text{H}_7\text{NH}_2\)), derived from the reduction of the nitrile intermediate.

• The nitrile's carbon-nitrogen triple bond is reduced to a carbon-nitrogen single bond, creating the primary amine.
• This conversion is key in modifying functional groups and achieving versatility in chemical synthesis.

Amine formation via this method highlights the interplay of nucleophilic substitution and reduction, showcasing a clear path from a simple alkyl halide to a functional amine.