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Q8.14(iii):
How will you prepare the following compounds from benzene? You may use any inorganic reagent and any organic reagent having not more than one carbon atom (iii) p-Nitrobenzoic acid

Solution :

Step 1: Introduction of the Alkyl Group via Friedel-Crafts Alkylation

To synthesize $p$-nitrobenzoic acid from benzene using a reagent with only one carbon atom, we must first convert benzene into a methyl-substituted aromatic ring. This is achieved through Friedel-Crafts alkylation using methyl chloride ($CH_3Cl$) in the presence of an anhydrous Lewis acid catalyst ($AlCl_3$).

Reaction: $C_6H_6 + CH_3Cl \xrightarrow{AlCl_3} C_6H_5CH_3 + HCl$ [Per Friedel-Crafts Alkylation Mechanism]

Step 2: Nitration of Toluene

The methyl group is an ortho/para-directing activator. To obtain the para-isomer, we perform nitration using a mixture of concentrated nitric acid ($HNO_3$) and sulfuric acid ($H_2SO_4$).

Reaction: $C_6H_5CH_3 + HNO_3 \xrightarrow{H_2SO_4, 273-283K} p-O_2NC_6H_4CH_3 + o-O_2NC_6H_4CH_3$

The $p$-nitro-toluene is separated from the ortho-isomer through fractional distillation [Based on differences in boiling points and molecular symmetry].

Step 3: Oxidation of the Methyl Group

The methyl group attached to the aromatic ring is oxidized to a carboxylic acid group using an alkaline potassium permanganate ($KMnO_4$) solution followed by acidic workup ($H^+$).

Reaction: $p-O_2NC_6H_4CH_3 \xrightarrow[2) H^+]{1) KMnO_4, OH^-, \Delta} p-O_2NC_6H_4COOH$ [Per Side-chain Oxidation of Alkylbenzenes]

Visual Representation of the Reaction Sequence

Benzene Toluene p-Nitrotoluene p-Nitrobenzoic acid CH3Cl, AlCl3 HNO3/H2SO4 KMnO4, H+

Summary of Reagents and Conditions

Step Reagent Transformation
1 $CH_3Cl / AlCl_3$ Benzene $\rightarrow$ Toluene
2 $Conc. HNO_3 / Conc. H_2SO_4$ Toluene $\rightarrow$ $p$-Nitrotoluene
3 $KMnO_4, OH^-; H_3O^+$ $p$-Nitrotoluene $\rightarrow$ $p$-Nitrobenzoic acid

Final Solution: The synthesis of p-nitrobenzoic acid from benzene is achieved by: (1) Friedel-Crafts alkylation with methyl chloride to form toluene, (2) electrophilic aromatic nitration to form p-nitrotoluene, and (3) vigorous oxidation of the methyl side chain using alkaline KMnOâ‚„ followed by acidic hydrolysis.


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