Problem 80 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

An organic compound A, \(\mathrm{C}_{8} \mathrm{H}_{4} \mathrm{O}_{3}\), in dry benzene in the presence of anhydrous \(\mathrm{AlCl}_{3}\) gives compound \(\mathbf{B}\). The compound \(\mathbf{B}\) on treatment with \(\mathrm{PCl}_{5}\), followed by reaction with \(\mathrm{H}_{2} / \mathrm{Pd}\left(\mathrm{BaSO}_{4}\right)\) gives compound \(\mathbf{C}\), which on reaction with hydrazine gives a cyclic compound \(\mathbf{D}\left(\mathrm{C}_{14} \mathrm{H}_{10} \mathrm{~N}_{2}\right)\). Identify A, B, C and D. Explain the formation of \(\mathbf{D}\) from \(\mathbf{C}\).

Step-by-Step Solution

Answer

A is phthalic anhydride, B is phthaloyl chloride, C is o-benzoyl alcohol, and D is a cyclic azine.

Step 1: Identify Compound A

Compound A has the molecular formula \( \mathrm{C}_8 \mathrm{H}_4 \mathrm{O}_3 \). This suggests a structure consisting of a benzene ring with additional oxygen-containing groups. A common compound fitting this description is phthalic anhydride, which fits the empirical formula and can easily form interactions with Lewis acids such as \( \mathrm{AlCl}_3 \).

Step 2: Formation of Compound B

The reaction of compound A with anhydrous \( \mathrm{AlCl}_3 \) in dry benzene suggests an acylation reaction. Phthalic anhydride reacts with anhydrous \( \mathrm{AlCl}_3 \) to form phthaloyl chloride, which will be designated as compound B.

Step 3: Formation of Compound C

In the presence of \( \mathrm{PCl}_5 \), phthaloyl chloride can undergo transformation to give o-benzoyl benzoyl chloride. This compound then undergoes reduction with \( \mathrm{H}_2 / \mathrm{Pd(BaSO_4)} \), a Lindlar catalyst, which selectively reduces the carbonyl group to an alcohol while keeping the aromatic rings intact. This results in the formation of a dihydro compound, which we will consider as compound C.

Step 4: Formation of Compound D

Compound C reacts with hydrazine, \( \mathrm{NH_2NH_2} \), to form a cyclic compound D. This transformation is a typical reaction known as the Wolff–Kishner type reaction, wherein the hydrazine causes the loss of carbonyl group(s) as nitrogen gas and formation of a cyclic structure known as benzalazine (or phenanthrene-like structure), leading to compound D \( \mathrm{C}_{14} \mathrm{H}_{10} \mathrm{N}_2 \).

Step 5: Verification

The empirical formulas and structures identified align with known chemical processes: phthalic anhydride reacts to form chlorides in the presence of \( \mathrm{AlCl}_3 \), undergoes reduction to alcohols, and the product forms cyclic structures with hydrazine. This affirms that A is phthalic anhydride, B is phthaloyl chloride, C is an o-benzoyl alcohol derivative, and D is a azine compound.

Key Concepts

Acylation Reaction

An acylation reaction is an essential transformation in organic chemistry. It involves the introduction of an acyl group, \((RCO−)\), into a molecule. This is typically done by reacting an acyl chloride or an acid anhydride, such as phthalic anhydride, with a Lewis acid catalyst like \(\text{AlCl}_3\).

• Acylation can modify the chemical properties of compounds, making it an important tool in organic synthesis.
• This reaction often makes use of Friedel-Crafts acylation, conducive for aromatic substrates like benzene.

In the context of phthalic anhydride and \(\text{AlCl}_3\), the acylation reaction involves the formation of a complex, leading to the formation of phthaloyl chloride. This step highlights how acyl groups bind to aromatic rings, thus creating pathways for further chemical modifications.

Phthalic Anhydride

Phthalic anhydride is an organic compound with molecular formula \(\text{C}_8\text{H}_4\text{O}_3\). It is a widely used chemical in the synthesis of various organic products and serves as a classic example of an acylating agent.

• It is recognized for its structure, a benzene ring flanked with two acyl regions capable of forming anhydrides.
• When reacted with \(\text{AlCl}_3\), phthalic anhydride acts as a substrate for acyl groups' transfer to aromatic compounds.

In many reactions, phthalic anhydride is employed for converting carboxylic groups to acyl chlorides. This makes it a versatile compound for synthetic organic transformations, providing a gateway to complex molecules.

Wolff–Kishner Reaction

The Wolff–Kishner reaction is a noteworthy organic reaction used to convert carbonyl groups into methylene groups. It involves hydrazine \((\text{NH}_2\text{NH}_2)\), aiding in the reduction of ketones or aldehydes.

• This reaction requires strong bases like hydroxides and high temperatures to proceed effectively.
• It removes oxygen from carbonyls, creating extended hydrocarbons or generating nitrogen gas by-products.

In the exercise, when phthalic-derived ketones react with hydrazine, the Wolff–Kishner reaction promotes the removal of carbonyl oxygen, transforming it into a cyclic azine structure. This is a valuable technique for final modifications in complex organic synthesis.

Lindlar Catalyst

The Lindlar catalyst is a special catalyst conceived for hydrogenation reactions. Its main use is to partially reduce alkynes to alkenes under controlled conditions without further reducing them to alkanes.

• Composed of palladium on calcium carbonate, sometimes modified with lead or sulfur.
• Favors selective reduction, maintaining unsaturation in parts of the molecule, especially important for aromatic compounds.

In our context, the Lindlar catalyst helps reduce phthaloyl-derived chlorides to corresponding alcohols without disturbing aromatic integrity. This specificity allows precise control over molecular transformations, critical in synthetic strategies.

Hydrazine Reaction

The hydrazine reaction typically refers to the interactions of hydrazine \(\text{NH}_2\text{NH}_2)\) with different organic substrates, often to induce transformations such as reductions or formation of cyclic compounds.

• Hydrazine is a strong nucleophile and a powerful reducing agent.
• Interacts with carboxyl and carbonyl groups to induce ring closures or eliminate functional groups.

Its role in organic chemistry is pivotal, especially in reactions like the Wolff–Kishner method, where hydrazine drives the elimination of oxygen atoms from carbonyl compounds. In the example given, it converts prepared ketones into cyclic diaryl azines, enveloping reactive sites and facilitating structural complexities in synthetic pathways.