Problem 151 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

A compound \(X\) has the molecular formula \(\mathrm{C}_{7} \mathrm{H}_{7} \mathrm{NO}\). On treatment with \(\mathrm{Br}_{2}\) and \(\mathrm{KOH}, X\) gives an amine \(Y\), The latter gives earbylamine test. \(Y\) upon diazotisation and coupling with phenol gives an azo dye. Thus, \(X\) is: \(\quad\) [MHCET I999] (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{NO}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{C}(\mathrm{OH})=\mathrm{NH}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CONH}_{2}\) (d) none of these

Step-by-Step Solution

Answer

(c) \(\mathrm{C}_{6}\mathrm{H}_{3}\mathrm{CONH}_{2}\).

Step 1: Analyze the Reaction with Bromine and KOH

The compound \(X\) reacts with \(\mathrm{Br}_{2}\) and \(\mathrm{KOH}\), which indicates a Hoffman Bromamide Degradation reaction. This reaction typically transforms an amide \((\mathrm{RCONH}_2)\) into an amine \((\mathrm{RNH}_2)\) with one less carbon atom. Thus, \(X\) must contain the amide group \(\mathrm{CONH}_2\).

Step 2: Determine the Structure of X

Since the molecular formula is \(\mathrm{C}_{7} \mathrm{H}_{7} \mathrm{NO}\) and given the reaction indicates a possible amide, check the options for compatible amide structures. Option (c) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CONH}_{2}\) contains an amide group, which fits the requirements for the starting compound \(X\).

Step 3: Evaluate the Formation of Amine Y

From the Hoffman Bromamide reaction, the amine \(Y\) formed from \(X\) will have the formula \(\mathrm{C}_6\mathrm{H}_5\mathrm{NH}_2\), which is known as aniline. Aniline is known to give a positive earbylamine test, which involves forming a diazonium salt and an azo dye upon coupling.

Step 4: Confirm the Reaction of Diazonisation and Coupling

Aniline \(\mathrm{C}_6\mathrm{H}_5\mathrm{NH}_2\) upon diazotization forms a diazonium salt that can react with phenol to form an azo dye. This confirms that the amine \(Y\) formed in the reaction is indeed aniline, supporting the structure of \(X\) as \(\mathrm{C}_{6}\mathrm{H}_{3}\mathrm{CONH}_2\).

Step 5: Identify the Correct Structure of X

The compound \(X\) must be the option that contains an amide group and allows formation of aniline upon bromine and KOH treatment. Therefore, \(X\) is \(\mathrm{C}_{6}\mathrm{H}_{3}\mathrm{CONH}_2\) from option (c).

Key Concepts

Amide to Amine Conversion

The transformation of amides into amines through the Hofmann Bromamide Degradation is a key chemical process. This reaction specifically involves an amide group (\( \mathrm{RCONH}_2 \)) converting into an amine (\( \mathrm{RNH}_2 \)) through the use of bromine (\( \mathrm{Br}_2 \)) and potassium hydroxide (\( \mathrm{KOH} \)). During this process, the amine that is produced has one less carbon atom than the original amide.

This essentially means that the carbon atom bonded to the nitrogen in the original amide does not appear in the final amine product. This reaction is often used to synthesize primary amines from primary amides.

Key points of amide to amine conversion:

• The original amide group contains a carbon, oxygen, and nitrogen bond.
• The reaction breaks the carbon-to-nitrogen bond.
• It replaces it with a new nitrogen-hydrogen bond, hence forming an amine.

The Hofmann Bromamide Degradation is useful because it provides a straightforward method for reducing carbon chain lengths in organic synthesis.

Diazotization Reaction

Diazotization is an important reaction particularly for amines, where an amine group reacts with nitrous acid to form a diazonium salt. In this process, a primary aromatic amine such as aniline (\( \mathrm{C}_6\mathrm{H}_5\mathrm{NH}_2 \)) reacts to form a diazonium salt.

Steps involved in the diazotization reaction include:

• The reaction of aniline with sodium nitrite and an acid.
• Formation of nitrous acid \( (\mathrm{HNO}_2) \) in situ, which is essential for converting the amine into a diazonium ion.
• The resulting diazonium ion, represented as \( \mathrm{R-N}_2^+ \), is highly reactive and is often used as an intermediate in further chemical transformations.

This reaction allows the conversion of stable amines into more reactive diazonium salts, which can be used in further reactions to modify the structure of the original compound.

Diazotization is crucial in organic chemistry, particularly in the creation of colorful azo compounds.

Azo Dye Formation

Azo dye formation is a fascinating process that involves the use of diazonium salts as intermediates. These diazonium salts are highly reactive and can couple with other aromatic compounds such as phenols to form brightly colored azo dyes. A typical azo dye formation would involve:

• The reaction between a diazonium salt and a coupling component like phenol or another aromatic amine.
• The result is the formation of a compound containing the characteristic azo group (\( -\mathrm{N=N}- \)).
• The presence of this azo linkage introduces vivid colors to the compound.

These dyes are valued in industries for their application in dyeing textiles due to their vibrant and varied hues.

The key steps in forming azo dyes:

• Prepare a diazonium salt from an aromatic amine through diazotization.
• React this diazonium salt with an aromatic compound that acts as a coupling agent.
• The product is an azo dye, identifiable by its deep color and its use in pigments.

Azo dye formation showcases the power of chemical reactions to create substances with desirable properties like color and stability, highlighting the role of chemistry in the fashion and materials science industries.