Animated Solution for Chemistry - Organic Chemistry: Sodium phenoxide when heated with $C{O}_2$ under pressure at $125^{\circ }C$ yields a product which on acetylation produces $C$. The major product $C$ would be

The synthesis of Aspirin is one of the most iconic and historically significant reaction sequences in organic chemistry. It beautifully demonstrates how we can manipulate functional groups on a benzene ring to create a life-saving medication. Let's break down this elegant transformation step by step.

Activating the Ring

Our journey begins with sodium phenoxide. Phenol itself is a good nucleophile, but converting it into its phenoxide ion makes the benzene ring exceptionally electron-rich. This heightened electron density is crucial because our electrophile, carbon dioxide ($C{O}_2$), is quite weak.

When sodium phenoxide is heated with $C{O}_2$ at $125^{\circ }C$ under a pressure of $5\text{ atm}$, an electrophilic aromatic substitution occurs. Interestingly, the sodium ion plays a vital coordinating role. It chelates with both the phenoxide oxygen and the oxygen of the incoming $C{O}_2$ molecule, effectively "steering" the electrophile to the ortho position. This highly regioselective process is known as the Kolbe-Schmitt reaction, and it yields sodium salicylate as the intermediate.

Reclaiming the Acid

The product of the Kolbe-Schmitt reaction is a sodium salt. To proceed further, we need the free acid. By simply treating the reaction mixture with a strong acid ($H^{+}$), the phenoxide and carboxylate ions are protonated.

This straightforward acidification step gives us salicylic acid (2-hydroxybenzoic acid). Salicylic acid is a fascinating molecule on its own, historically extracted from willow bark and used for pain relief, though it is notoriously harsh on the stomach lining.

The Birth of Aspirin

To make salicylic acid more tolerable for human consumption, we must modify it. This brings us to the final and most crucial step: acetylation. We treat salicylic acid with acetic anhydride ($A{c}_{2}O$) in the presence of an acid catalyst.

Salicylic acid presents a chemical dilemma: it has two reactive hydroxyl groups—the phenolic $-OH$ and the carboxylic $-OH$. Which one will react?

The phenolic $-OH$ is significantly more nucleophilic than the carboxylic $-OH$. The acid catalyst protonates the acetic anhydride, making it a highly potent electrophile. The nucleophilic phenolic oxygen attacks the carbonyl carbon of the anhydride, leading to the cleavage of the anhydride bond. The acetyl group ($-COC{H}_3$) replaces the hydrogen of the phenolic $-OH$, while the carboxylic acid group remains untouched.

The result of this selective acetylation is acetylsalicylic acid, globally celebrated as Aspirin. By understanding the nuances of nucleophilicity and electrophilic aromatic substitution, we have successfully navigated from a simple phenol derivative to one of the most widely used pharmaceuticals in history!