Animated Solution for Chemistry - Chemistry in Everyday Life: In the given reaction scheme, $\mathbf{P}$ is a phenyl alkyl ether, $\mathbf{Q}$ is an aromatic compound; $\mathbf{R}$ and $\mathbf{S}$ are the major products. $\mathbf{P}\stackrel{\text{HI}}{\to }\mathbf{Q}\underset{\text{(iii) }{\text{H}}_3{\text{O}}^{+}}{\overset{{\text{(i) NaOH, (ii) CO}}_2}{\to }}\mathbf{R}\underset{\text{(ii) }{\text{H}}_3{\text{O}}^{+}}{\overset{{\text{(i) (CH}}_3{\text{CO)}}_2\text{O}}{\to }}\mathbf{S}$ The correct statement about $\mathbf{S}$ is

The beauty of organic chemistry lies in its ability to transform simple, everyday molecules into complex, life-saving drugs through a logical sequence of reactions. This problem is a perfect example of such a journey, taking us from a basic ether all the way to one of the most famous medicines in human history: Aspirin.

The Cleavage of the Ether

We begin our journey with compound P, which is identified as a phenyl alkyl ether ($\text{Ph-O-R}$). The first step involves treating this ether with hydroiodic acid ($\text{HI}$). When an ether is exposed to a strong acid like $\text{HI}$, it typically undergoes cleavage. However, phenyl alkyl ethers have a unique structural feature: the oxygen atom is directly attached to a benzene ring.

The lone pairs on the oxygen atom are in resonance with the $\pi$-electron system of the benzene ring. This resonance imparts a partial double bond character to the $\text{Ph-O}$ bond, making it exceptionally strong and difficult to break. Consequently, the nucleophilic attack by the iodide ion strictly occurs at the weaker $\text{O-R}$ bond. The alkyl group leaves as an alkyl iodide ($\text{R-I}$), while the oxygen remains attached to the benzene ring, picking up a proton to form Phenol ($\text{Ph-OH}$). Thus, our intermediate aromatic compound Q is phenol.

The Kolbe-Schmitt Reaction

In the next phase, phenol ($\mathbf{Q}$) is subjected to a sequence of reagents: $\text{NaOH}$, followed by ${\text{CO}}_2$, and finally acidic hydrolysis (${\text{H}}_3{\text{O}}^{+}$). This specific sequence should immediately ring a bell—it is the classic Kolbe-Schmitt reaction.

First, sodium hydroxide deprotonates the phenol to form a phenoxide ion. The phenoxide ion is highly reactive towards electrophilic aromatic substitution because the negatively charged oxygen strongly donates electron density into the ring. This allows even a weak electrophile like carbon dioxide (${\text{CO}}_2$) to attack the ring, predominantly at the ortho position due to the proximity of the sodium counter-ion which coordinates with both the phenoxide oxygen and the ${\text{CO}}_2$ oxygen. After acidification, a carboxyl group ($-\text{COOH}$) is generated ortho to the hydroxyl group. The resulting product R is Salicylic Acid.

The Acetylation

Salicylic acid itself has pain-relieving properties, but it is notoriously harsh on the stomach lining. To make it more tolerable, chemists modify it. In our reaction scheme, salicylic acid ($\mathbf{R}$) is treated with acetic anhydride (${\text{(CH}}_3{\text{CO)}}_2\text{O}$) in an acidic medium.

This is an acetylation reaction. The phenolic $-\text{OH}$ group acts as a nucleophile, attacking the carbonyl carbon of the acetic anhydride. The hydrogen of the hydroxyl group is replaced by an acetyl group ($-{\text{COCH}}_3$), forming an ester linkage. The final major product S is Acetylsalicylic Acid, universally known by its trade name, Aspirin.

The Pharmacology of Aspirin

Now that we have identified S as Aspirin, we must evaluate its pharmacological properties to find the correct statement among the options. Aspirin belongs to a class of drugs known as non-narcotic analgesics. Unlike narcotic drugs (such as morphine), which bind to opioid receptors in the brain and can cause addiction, Aspirin acts peripherally.

Its primary mechanism of action is the irreversible inhibition of the cyclooxygenase (COX) enzyme. The COX enzyme is crucial for the biosynthesis of prostaglandins—lipid compounds that act like hormones, triggering inflammation, fever, and pain in response to tissue damage. By inhibiting COX, Aspirin effectively inhibits the synthesis of prostaglandins, thereby reducing pain and inflammation.

Let us review the given options: - (A) Inhibiting noradrenaline degrading enzymes is a mechanism associated with certain antidepressants (like MAOIs), not Aspirin. - (B) It inhibits the synthesis of prostaglandin. This is the exact mechanism of Aspirin. Correct. - (C) It is a narcotic drug. This is false; Aspirin is non-narcotic. - (D) It is ortho-acetylbenzoic acid. This is a classic trap! Because the acetylation occurs on the oxygen atom, an ester is formed. The correct chemical name is ortho-acetoxybenzoic acid. An ortho-acetylbenzoic acid would imply a ketone group directly attached to the ring, which is structurally incorrect.

Therefore, the only correct statement about product S is that it inhibits the synthesis of prostaglandins.