Problem 181 - A Complete Resource Book in Chemistry for JEE Main

Question

In the following chemical reactions: \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}+\mathrm{CHCl}_{3}+3 \mathrm{KOH} \longrightarrow(\mathrm{A})+(\mathrm{B})+3 \mathrm{H}_{2} \mathrm{O}\) the compounds (A) and (B) are respectively [2007] (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CONH}_{2}\) and \(3 \mathrm{KCl}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NC}\) and \(\mathrm{K}_{2} \mathrm{CO}_{3}\) (c) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{NC}\) and \(3 \mathrm{KCl}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{CN}\) and \(3 \mathrm{KCl}\)

Step-by-Step Solution

Answer

Option (c): \( \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{NC} \) and \( 3 \mathrm{KCl} \).

Step 1: Identify the type of reaction

The given reaction involves the mixing of primary amine (\( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2} \)) with chloroform (\( \mathrm{CHCl}_{3} \)) in presence of a strong base such as \( \mathrm{KOH} \). This setup suggests a Carbylamine reaction, also known as Hofmann Isocyanide Synthesis.

Step 2: Understand the Carbylamine reaction

In the Carbylamine reaction, a primary amine reacts with chloroform and a base to produce an isocyanide (isonitrile) and side products. The reaction can be generalized as: \[\mathrm{RNH}_2 + \mathrm{CHCl}_3 + 3\mathrm{KOH} \rightarrow \mathrm{RNC} + 3\mathrm{KCl} + 3\mathrm{H}_2\mathrm{O}\]

Step 3: Applying the reaction to the given compounds

In the given reaction, the primary amine is \( \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{NH}_{2} \). Following the Carbylamine mechanism, the product formed will be an isocyanide:\( \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{NC} \). The potassium chloride formed comes from the displacement of chlorine atoms in \( \mathrm{CHCl}_{3} \).

Step 4: Confirming the side products

The by-products of the reaction include potassium chloride (3 moles because 3 moles of chlorine are displaced) and water (also 3 moles as per equation balance). Thus, the correct by-products are \( 3{\mathrm KCl} \) and \( 3{\mathrm H_2O} \).

Step 5: Determine the correct answer from options

The major products from the reaction are \( \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{NC} \) and \( 3 \mathrm{KCl} \). These match option (c) \( \mathrm{C}_{2}\mathrm{H}_{5}\mathrm{NC} \) and \( 3 \mathrm{KCl} \).

Key Concepts

Hofmann Isocyanide Synthesis

Hofmann Isocyanide Synthesis, also known as the Carbylamine reaction, is a chemical reaction that produces isocyanides, or isonitriles, from primary amines. This reaction requires specific conditions and reactants. Typically, a primary amine is reacted with chloroform in the presence of a strong base like potassium hydroxide (KOH). The result of this reaction is an isocyanide, along with by-products such as potassium chloride (KCl) and water (H2O). The distinctive aspect of this reaction is the formation of isocyanides, which are compounds with a unique -C≡N bonding different from nitriles.

This mechanism is not only interesting because of the unusual isocyanide product but also for its specific requirements:

• Use of a primary amine: Secondary or tertiary amines do not participate in this reaction.
• Presence of a strong base: This is necessary to facilitate the reaction and neutralize by-products.

The reaction is useful in organic chemistry for synthesizing compounds that are building blocks for more complex synthesis pathways. It's worth noting that isocyanides usually have a characteristic unpleasant odor, which makes them easily recognizable.

Primary amines

Primary amines are a class of organic compounds that are essential to various chemical reactions, such as the Hofmann Isocyanide Synthesis.

In terms of structure, a primary amine has the general formula: \[ ext{R-NH}_2\]where \(\text{R}\) represents an alkyl or aryl group.

These amines are called "primary" because the nitrogen atom is bonded to only one hydrocarbon group. This single bond arrangement imparts specific chemical properties that are favorable in reactions like the Carbylamine reaction. Primary amines:

• Are generally more reactive than secondary or tertiary amines, allowing them to participate in a wider range of chemical reactions.
• Can form hydrogen bonds due to the presence of an -NH group, which gives them higher boiling points compared to similar hydrocarbons.
• Are able to react with acid chlorides, anhydrides, and halides, making them important intermediates in organic synthesis.

These characteristics make primary amines versatile building blocks in organic chemistry, where they are used to synthesize dyes, drugs, and agricultural chemicals.

Reaction mechanisms

Reaction mechanisms describe the step-by-step sequence of elementary reactions by which an overall chemical change occurs. Understanding these mechanisms is crucial to predicting how and when different substances react. For the Hofmann Isocyanide Synthesis, the mechanism begins with mixing the three key reactants: a primary amine, chloroform, and a strong base like potassium hydroxide.

First, the base deprotonates the amine, forming an amide ion. Concurrently, potassium hydroxide reacts with chloroform in a radical elimination process to liberate dichlorocarbene, a highly reactive electron-deficient species:\[ ext{CHCl}_3 + 2 ext{KOH} \rightarrow ext{CCl}_2 + 2 ext{KCl} + ext{H}_2 ext{O}\]The dichlorocarbene then attacks the amide ion, leading to the formation of isocyanide upon release of hydrochloric acid, which is neutralized by the base:

• Carbene generation from chloroform is key to this reaction’s success.
• The formation of isocyanide involves nucleophilic addition followed by rearrangement.
• Potassium chloride and water are by-products, balancing the equation.

Understanding how each molecule and ion interacts during the reaction helps in improving reaction conditions and yields, making mechanism study vital for any chemist involved in synthesis.