Animated Solution for Chemistry - Organic Chemistry: The number of compound(s) given below which contain(s) — COOH group is ……… . (i) Sulphanilic acid (ii) Picric acid (iii) Aspirin (iv) Ascorbic acid

Welcome, future chemists, to a classic trap set by the examiners! In the fascinating world of organic chemistry, nomenclature can sometimes be a double-edged sword. We are presented with four famous compounds, all bearing the prestigious title of 'acid'. But do they all possess the quintessential carboxylic acid group, the $-\text{COOH}$ group? Let's embark on a structural investigation to uncover the truth.

When we hear the word 'acid' in organic chemistry, our minds immediately jump to the $-\text{COOH}$ group. However, acidity is a measure of a molecule's ability to donate a proton ($H^{+}$), not just the presence of a specific functional group. A proton attached to a highly electronegative atom or a system that strongly stabilizes the resulting conjugate base can also exhibit profound acidity. The examiners know this cognitive bias and have carefully selected compounds to test your structural memory.

Who Hides the Carboxyl Group?

Let's put our four suspects under the structural microscope.

1. Sulphanilic Acid: Despite its name, sulphanilic acid does not contain a carbon-based acid group. Its structure consists of a benzene ring substituted with an amino group ($-{\text{NH}}_2$) and a sulfonic acid group ($-{\text{SO}}_3\text{H}$) at the para position. It often exists as a zwitterion, but crucially, it lacks the $-\text{COOH}$ group.

2. Picric Acid: This is perhaps the most famous imposter! Picric acid is chemically $2,4,6-\text{trinitrophenol}$. It features a central phenolic $-\text{OH}$ group flanked by three strongly electron-withdrawing nitro ($-{\text{NO}}_2$) groups. These nitro groups pull electron density away from the oxygen, making the release of the proton incredibly easy. It is a remarkably strong acid, yet it contains absolutely zero $-\text{COOH}$ groups!

3. Aspirin: Ah, the ubiquitous painkiller. Aspirin's systematic name is acetylsalicylic acid. If we draw its structure, we find a benzene ring adorned with an ester group ($-{\text{OCOCH}}_3$) and, right next to it at the ortho position, a genuine carboxylic acid group ($-\text{COOH}$). We have found our first match!

4. Ascorbic Acid: Widely celebrated as Vitamin C, ascorbic acid is vital for our health. But structurally, it is a five-membered lactone ring (a cyclic ester) decorated with multiple hydroxyl ($-\text{OH}$) groups. Its acidity stems from the enolic hydroxyl groups, which, upon losing a proton, form a highly resonance-stabilized ascorbate anion. Once again, the $-\text{COOH}$ group is nowhere to be found.

Unmasking the True Acid

After a thorough structural interrogation, the truth is laid bare. Out of the four compounds—Sulphanilic acid, Picric acid, Aspirin, and Ascorbic acid—only Aspirin harbors the $-\text{COOH}$ group.

This problem is a beautiful reminder that in chemistry, structure is everything. Memorizing common names is important, but understanding the molecular architecture behind those names is what truly makes you a master of the subject. The final count is exactly $1$.