Problem 185 - A Complete Resource Book in Chemistry for JEE Main

Question

Compound \(\left(\right.\) A), \(\mathrm{C}_{\mathrm{s}} \mathrm{H}_{9} \mathrm{Br}\), gives a white precipitate when warmed with alcoholic \(\mathrm{AgNO}_{j}\). Oxidation of (A) gives an acid (B), \(\mathrm{C}_{8} \mathrm{H}_{6} \mathrm{O}_{4}\). (B) easily forms anhydride on heating. Identify the compound (A). [2013]\\}

Step-by-Step Solution

Answer

Compound (A) is 2-bromoethylbenzene.

Step 1: Analyze Information from the Reaction with AgNO3

The fact that compound (A) forms a white precipitate with gNO3 suggests that (A) contains a bromine atom bonded to a benzyl group, which can form silver bromide ( gBr) when reacted with AgNO3 in an alcoholic medium. This is a typical reaction for aryl halides.

Step 2: Determine the Structure of Compound (B)

Compound (B) has the molecular formula c_8h_6o_4, indicating it has a benzene ring, as the C/H ratio suggests aromaticity. It forms an anhydride easily, which is a characteristic of dicarboxylic acids that are ortho-substituted (phthalic acids). Thus, (B) is likely phthalic acid or a similar derivative.

Step 3: Infer the Transformation from (A) to (B)

The transformation from compound (A) to compound (B) indicates the introduction of two additional oxygen atoms through oxidation. Given that (A) must be oxidizable to a dicarboxylic acid, it likely contains an aromatic ring with an alkyl chain that can be oxidized to form (B).

Step 4: Determine the Structure of Compound (A)

Combining all information, compound (A) should be a benzylic bromo-alkane since it's oxidizable to phthalic acid. The simplest and suitable structure aligning with the given formula, c_sH_9br, is 2-bromoethylbenzene. The ethyl group allows for oxidation to introduce carboxylic groups when aligned ortho to each other.

Step 5: Confirm Identification of Compound (A)

2-bromoethylbenzene satisfies the given chemical reactions: it reacts with alcoholic AgNO3 to give a white precipitate, and upon oxidation can form phthalic acid (compound B) due to the existence of two benzylic carbons that can be converted to carboxylic acids.

Key Concepts

Aryl Halides

Aryl halides are a fascinating group of organic compounds that consist of a halogen atom bonded to an aromatic ring, such as a benzene ring. Unlike alkyl halides, where the halogen is attached to an aliphatic carbon, aryl halides have their halogen atom directly connected to an sp² hybridized carbon of an aromatic ring. This structural difference imparts unique reactivity and stability characteristics to aryl halides which are crucial in organic reactions.

**Key Characteristics of Aryl Halides**:

• Stability: Due to the resonance stabilization provided by the aromatic ring, aryl halides are generally more stable than their alkyl counterparts.
• Reactivity: They participate in nucleophilic aromatic substitution reactions, but typically require more extreme conditions due to the delocalization of electrons on the benzene ring.
• Utility: Aryl halides are key intermediates in various synthetic processes, including the formation of new carbon-carbon bonds through coupling reactions.

In our exercise, compound (A) involving a bromine atom forming a white precipitate when reacted with \( ext{AgNO}_3\) suggests it is an aryl halide. This highlights the tendency of the bromine to be replaced by the silver ion, a typical characteristic of halogens in this group when exposed to certain reagents.

Phthalic Acid

Phthalic acid stands out as a significant compound in organic chemistry, recognized for its structure consisting of an aromatic ring coupled with two carboxylic acid groups. The most well-known form is the ortho-substituted variant, which refers to the positioning of carboxyl groups directly adjacent to one another on a benzene ring.

**Properties and Significance of Phthalic Acid**:

• Molecular Structure: The formula \( ext{C}_8 ext{H}_6 ext{O}_4\) reveals a benzene core, essential for the characteristic aromaticity, alongside the two carboxylic groups.
• Formation of Anhydrides: Upon heating, phthalic acid easily loses a water molecule to form phthalic anhydride, a key reagent in the production of dyes, perfumes, and synthetic materials.
• Synthesis: It can be synthesized via oxidative reactions, transforming precursors like naphthalene or \( ext{ortho-xylene}\).

In the context of the solution, compound (B) is likely phthalic acid due to its ability to form an anhydride, aligning perfectly with the reaction possibilities laid out. This conversion is vital in understanding the transformation of (A) through oxidation, showcasing the versatility of aromatic compounds in synthetic chemistry.

Oxidation Reactions

Oxidation reactions play a critical role in organic chemistry as they involve the addition of oxygen or the removal of hydrogen, thereby increasing the oxidation state of a molecule. Such reactions are pivotal in the transformation of various functional groups, particularly in synthesizing acids from alcohols or alkyl groups.

**Characteristics of Oxidation Reactions**:

• Functional Group Interconversion: Oxidation is key in converting simpler molecules into more complex structures, such as turning alkyl chains into carboxylic acids or aldehydes into carboxyl groups.
• Types of Reagents: Common oxidizing agents include potassium permanganate \( ext{KMnO}_4\), chromic acid \( ext{H}_2 ext{CrO}_4\), and oxygen itself under catalytic conditions.
• Industrial Applications: These reactions are foundational in manufacturing processes, including the production of phthalic anhydride from phthalic acid.

For the problem at hand, the oxidation of compound (A) to compound (B) is essential, showcasing how a bromine-containing alkylbenzene can be transformed into phthalic acid. This not only highlights the connection between structure and reactivity but also demonstrates the application of oxidation in creating dicarboxylic acids, important in a variety of applications from polymer production to pharmaceuticals.