Problem 21 - Chemistry in 30 days

Question

A compound A has a molecular formula \(\mathrm{C}_{7} \mathrm{H}_{7} \mathrm{NO} .\) On treatment with \(\mathrm{Br}_{2}\) and \(\mathrm{KOH}\), A gives an amine \(B\) which gives carbylamine test. B upon diazotisation and coupling with phenol gives an azo dye. \(\mathrm{A}\) can be (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CONHCOCH}_{3}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CONH}_{2}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NO}_{2}\) (d) \(0, \mathrm{~m}\) - or \(\mathrm{p}-\mathrm{C}_{6} \mathrm{H}_{4}\left(\mathrm{NH}_{2}\right) \mathrm{CHO}\)

Step-by-Step Solution

Answer

The compound A is (d) o, m, or p-C6H4(NH2)CHO.

Step 1: Analyze the Reaction with Br2 and KOH

The compound A, when treated with Br2 and KOH, produces an amine B. This indicates that A is likely an amide, undergoing the Hofmann degradation to form B, an amine.

Step 2: Identify the Amine, B

Since compound B is an amine that gives a carbylamine reaction, B must be a primary amine. This narrows down our options because a primary amine should correspond to one of the initial compounds after the Hofmann degradation.

Step 3: Examine Diazotization and Azo Coupling

Compound B undergoes diazotization, forming a diazonium salt, and then couples with phenol to form an azo dye. This suggests B is an aromatic primary amine like aniline.

Step 4: Determine Compound A

Considering the molecular formula C7H7NO and the requirement of compound A to undergo Hofmann degradation to give a primary amine, the structure must be such that an aniline derivative is formed. (d) o, m, or p-C6H4(NH2)CHO fits because the amide (NH2CHO) group is lost during the reaction to form the primary aromatic amine when treated with Br2 and KOH.

Step 5: Match Formula to Option

The chosen structure (d) matches the molecular formula given because one of the positions contains a formyl group, making the structure consistent with C7H7NO after considering the reaction sequence.

Key Concepts

Diazotization

Diazotization is a chemical process where a primary aromatic amine, such as aniline, converts into a diazonium salt. This transformation is a significant step in organic chemistry, especially in the synthesis of azo dyes.
To perform diazotization, you typically use sodium nitrite ( NaNO₂) dissolved in hydrochloric acid ( HCl), and the reaction is kept icy cool to maintain stability. During the reaction, the amine ( NH₂) group loses an atom of hydrogen and forms a positively charged nitrogen ( N₂⁺) group.

• This step is critical because if conditions are too warm, the diazonium ion may decompose, ruining the reaction.
• Diazonium salts are generally unstable, except in cold temperatures, making them challenging to handle without proper cooling.

Diazotization is vital as it opens pathways to create various synthetic dyes and pigments, expanding its importance across industrial and laboratory settings.

Azo Coupling

Azo coupling is a fascinating chemical reaction where the diazonium component combines with another compound, typically an aromatic compound like phenol, to form an azo compound. These reactions are colorful, producing vibrant azo dyes which are often used for coloring textiles and other materials.
The process generally involves an electrophilic substitution where the N₂⁺ group in the diazonium ion acts as an electrophile. In combination with a coupling partner, such as phenol, the result is an azo linkage ( N=N), a distinctive feature of azo compounds.

• The azo linkage absorbs light, giving these compounds their vivid colors.
• The pH level and choice of solvent can significantly influence the reaction efficiency and color outcome.

Understanding azo coupling is crucial in preparing various dyes and materials, where specific colors and properties are tailored for commercial and artistic use.

Carbylamine Reaction

The carbylamine reaction is a distinctive test for identifying primary amines, which tests for the presence of primary amines in a given compound. When compound B, a primary amine, reacts with chloroform ( CHCl₃) and a strong base like potassium hydroxide ( KOH), it produces an isocyanide (or carbylamine) that has a very sharp and characteristic odor.

• This reaction is highly specific to primary amines and does not occur with secondary or tertiary amines.
• The unpleasant aroma of the isocyanides produced often helps in easily confirming the presence of a primary amine.

In the context of compound A transforming into compound B, confirming that B is a primary amine through the carbylamine reaction supports the correct identification of the molecular structure that matches C₇H₇NO.