Problem 26 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Show with equations how the following compounds are prepared (equations need not be balanced):(i) 4 -nitrobenzaldehyde from benzene. (ii) \(p\)-bromonitrobenzene from benzene in two steps. (iii) toluene to \(m\)-nitrobenzoic acid?

Step-by-Step Solution

Answer

Use nitration and Friedel-Crafts reactions for nitrobenzaldehyde. Nitration and bromination for p-bromonitrobenzene. Oxidation and nitration for m-nitrobenzoic acid.

Step 1: Preparing 4-nitrobenzaldehyde from Benzene

Start with benzene \Begin with a nitration reaction. This involves treating benzene with concentrated nitric acid in the presence of concentrated sulfuric acid to form nitrobenzene: \\[ C_6H_6 + HNO_3 \rightarrow C_6H_5NO_2 + H_2O \]\Next, follow this with a Friedel-Crafts acylation to introduce the aldehyde group at the para position. Using formyl chloride \( (HCOCl) \) and an aluminum chloride catalyst \( (AlCl_3) \), form \( 4-\text{nitrobenzaldehyde} \): \\[ C_6H_5NO_2 + HCOCl \xrightarrow{AlCl_3} \text{p-nitrobenzaldehyde} \]

Step 2: Preparing p-bromonitrobenzene from Benzene - Step 1

Start with benzene \Perform a nitration reaction. Treat benzene with a mixture of concentrated sulfuric acid and concentrated nitric acid to form nitrobenzene: \\[ C_6H_6 + HNO_3 \rightarrow C_6H_5NO_2 + H_2O \]

Step 3: Preparing p-bromonitrobenzene from Benzene - Step 2

Use nitrobenzene from Step 2 \Carry out a bromination reaction using bromine in the presence of iron tribromide \( (FeBr_3) \) to selectively brominate at the para position, yielding \( p-\text{bromonitrobenzene} \): \\[ C_6H_5NO_2 + Br_2 \xrightarrow{FeBr_3} \text{p-bromonitrobenzene} \]

Step 4: Converting Toluene to m-nitrobenzoic Acid - Step 1

Start with toluene \First, oxidize toluene to benzoic acid using a strong oxidizing agent like potassium permanganate \((KMnO_4)\): \\[ C_6H_5CH_3 \xrightarrow{KMnO_4} C_6H_5COOH \]

Step 5: Converting Toluene to m-nitrobenzoic Acid - Step 2

Use benzoic acid from Step 4 \Carry out a nitration reaction on benzoic acid using concentrated nitric acid and sulfuric acid to yield \( m-\text{nitrobenzoic acid} \) as the major product due to the meta-directing nature of the carboxylic acid group: \\[ C_6H_5COOH + HNO_3 \xrightarrow{H_2SO_4} \, m-\text{nitrobenzoic acid} \]

Key Concepts

Nitration

Nitration is a fundamental type of aromatic substitution reaction that introduces a nitro group ");