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Q8.14(iv):
How will you prepare the following compound from benzene? You may use any inorganic reagent and any organic reagent having not more than one carbon atom (iv) Phenylacetic acid

Solution :

Step 1: Planning the Retrosynthetic Pathway

To synthesize Phenylacetic acid ($\text{C}_6\text{H}_5\text{CH}_2\text{COOH}$) from benzene ($\text{C}_6\text{H}_6$), we must introduce a two-carbon side chain. Since we are restricted to reagents with no more than one carbon atom, we must utilize Friedel-Crafts alkylation or acylation followed by functional group modification. The most efficient route involves the conversion of benzene to toluene, followed by halogenation and nucleophilic substitution with cyanide.

Step 2: Conversion of Benzene to Toluene (Friedel-Crafts Alkylation)

Benzene is reacted with methyl chloride in the presence of an anhydrous Lewis acid catalyst to introduce the methyl group. [Per Friedel-Crafts Alkylation mechanism]

$\text{C}_6\text{H}_6 + \text{CH}_3\text{Cl} \xrightarrow{\text{Anhydrous AlCl}_3} \text{C}_6\text{H}_5\text{CH}_3 + \text{HCl}$

Step 3: Free Radical Halogenation

Toluene is subjected to side-chain chlorination using ultraviolet light (or heat) to yield benzyl chloride. [By Free Radical Substitution mechanism]

$\text{C}_6\text{H}_5\text{CH}_3 + \text{Cl}_2 \xrightarrow{h\nu} \text{C}_6\text{H}_5\text{CH}_2\text{Cl} + \text{HCl}$

Step 4: Nucleophilic Substitution (Cyanide Introduction)

Benzyl chloride reacts with sodium cyanide ($\text{NaCN}$), which provides the second carbon atom required for the acetic acid derivative, forming benzyl cyanide (phenylacetonitrile). [By $S_N2$ mechanism]

$\text{C}_6\text{H}_5\text{CH}_2\text{Cl} + \text{NaCN} \rightarrow \text{C}_6\text{H}_5\text{CH}_2\text{CN} + \text{NaCl}$

Step 5: Acidic Hydrolysis of Nitrile

The nitrile group is hydrolyzed under acidic conditions to produce the final carboxylic acid. [By Nucleophilic Acyl Substitution]

$\text{C}_6\text{H}_5\text{CH}_2\text{CN} + 2\text{H}_2\text{O} + \text{H}^+ \xrightarrow{\Delta} \text{C}_6\text{H}_5\text{CH}_2\text{COOH} + \text{NH}_4^+$

Visual Representation of Reaction Sequence

Benzene Toluene Benzyl Chloride Benzyl Cyanide Phenylacetic Acid
Reagent Step Purpose
$\text{CH}_3\text{Cl} / \text{AlCl}_3$ Alkylation of Benzene
$\text{Cl}_2 / h\nu$ Activation of side chain
$\text{NaCN}$ Carbon chain extension
$\text{H}_3\text{O}^+ / \Delta$ Hydrolysis of Nitrile

Final Solution: The synthesis of Phenylacetic acid proceeds via the Friedel-Crafts alkylation of benzene to toluene, followed by side-chain chlorination, nucleophilic substitution with cyanide, and final acidic hydrolysis to the carboxylic acid.


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