Problem 203 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Aniline reacts with chloroform in presence of alcoholic KOH 0 produce bad smelling compound. The compound produced is: (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NC}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{Cl}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CN}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NHC}_{6} \mathrm{H}_{5}\)

Step-by-Step Solution

Answer

The compound produced is \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NC}\).

Step 1: Understand the Reaction

The reaction described in the problem is known as the carbylamine reaction, an organic reaction that leads to the formation of isocyanides, which are known for their foul smell. It typically involves primary amines reacting with chloroform in the presence of a strong base.

Step 2: Identify the Reactants and Conditions

The primary reactant here is aniline, which is a primary amine. The other reactants mentioned are chloroform and alcoholic potassium hydroxide (KOH), establishing the conditions favorable for the carbylamine reaction.

Step 3: Predict the Product

In the carbylamine reaction, a primary amine (in this case, aniline) will react with chloroform and KOH to form an isocyanide (or isonitrile). The general chemical equation is: \[ ext{RNH}_{2} + ext{CHCl}_{3} + ext{3KOH} ightarrow ext{RNC} + 3 ext{KCl} + 3 ext{H}_{2} ext{O} \] For aniline, this means the reaction forms phenyl isocyanide (\(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NC}\)).

Step 4: Verify the Characteristics of the Product

Phenyl isocyanide is known for its intense and unpleasant odor, which matches the description in the problem of a 'bad smelling compound.' This verification supports the correctness of our predicted product.

Key Concepts

Aniline

Aniline is a fundamental aromatic amine, characterized by its benzene ring bonded to an amino group. Its chemical formula is \( \mathrm{C}_6\mathrm{H}_5\mathrm{NH}_2 \). As an important building block in the field of organic chemistry, aniline serves as a precursor to numerous compounds. This includes dyes, plastics, and pharmaceuticals. Because of its structure, aniline acts as a primary amine, meaning it has one alkyl or aryl group attached to the nitrogen atom. This is crucial in reactions like the carbylamine reaction.

Within reactions, the nitrogen atom in aniline is nucleophilic, meaning it can donate a pair of electrons. This allows aniline to participate in many chemical reactions, especially those involving electrophilic substitution. In the context of the carbylamine reaction, the nucleophilic nature of aniline makes it highly reactive with chloroform in the presence of a base, such as alcoholic KOH. This reactivity is what leads to the formation of isocyanides, products known for their distinctly unpleasant smell.

Isocyanides

Isocyanides, or isonitriles, are unique organic compounds characterized by their extreme and unpleasant odor. Their general structure is \( \mathrm{RNC} \), where \( \mathrm{R} \) represents an alkyl or aryl group. Interestingly, isocyanides are the products of the carbylamine reaction, the focus of many organic synthesis processes.

What makes isocyanides distinct is their linear geometry and the triple bond between carbon and nitrogen. This bonding imparts significant stability, as well as a notable chemical reactivity. The lone pair of electrons on the nitrogen can participate in various chemical processes, making isocyanides useful intermediates in organic chemistry.

• Their formation in the carbylamine reaction involves primary amines like aniline reacting with chloroform under strongly basic conditions.
• This transformation is widely used in synthetic chemistry due to the versatility of isocyanides in creating complex molecular structures.
• However, handling isocyanides requires caution due to their strong, pungent smell and potential toxicity.

Organic Reaction Mechanisms

Organic reaction mechanisms describe the step-by-step sequence of elementary reactions by which an overall chemical change occurs. Understanding these mechanisms is crucial for predicting the behavior and outcome of chemical reactions. The carbylamine reaction provides an excellent example of such a mechanism.

The carbylamine reaction unfolds via several key stages involving aniline, chloroform, and alcoholic KOH. This reaction occurs under basic conditions which promote the deprotonation of the amine. The crucial steps include:

• The formation of a dichlorocarbene intermediate, which then reacts with the primary amine, aniline.
• This intermediate is highly reactive and facilitates the insertion of the carbene into the amino group, forming the isocyanide product.
• Finally, the departure of chloride ions and the stabilization of the isocyanide form the characteristic foul-smelling compound.

By breaking down the reaction into these elementary steps, chemists gain insight into the transformations occurring at the molecular level. This knowledge allows for the development and optimization of new synthetic pathways in organic chemistry.