An organic compound A having molecular formula C₆H₆O gives a violet colour with neutral FeCl₃ ​ solution. A on treatment with CO₂​ and NaOH at 400 K under pressure gives B which on acidification gives a compound C. The compound C reacts with acetyl chloride to give D which is popular pain killer. Deduce the structure of A, B, C and D.

What is the correct order of acid strength of the following compound? I. Phenol II. p-cresol III. m-Nitrophenol IV. p-nitrophenol

IV > III > I > II

The correct answer is option 2. IV > III > I > II.

Acid Strength in Phenols: Phenols are weak acids due to the ability to release a proton \((H^+)\) from the hydroxyl group \((OH)\) attached to the benzene ring. The ease of releasing this proton determines the strength of the acid.

Impact of Substituents: The presence of substituent groups on the benzene ring can influence the acidity of the phenol. These groups can be electron-withdrawing (EWG) or electron-donating (EDG).

Electron-Withdrawing Groups (EWGs): When EWGs are present on the benzene ring, they withdraw electron density away from the oxygen atom in the hydroxyl group. This makes the oxygen atom more positive, weakening its hold on the proton \((H^+)\) and making it easier to release. As a result, the acidity of the phenol increases.

Electron-Donating Groups (EDGs): Conversely, EDGs donate electron density to the oxygen atom in the hydroxyl group, making it more negative and strengthening its hold on the proton. This decreases the acidity of the phenol.

Now, let us analyze each compound:

I. Phenol: This is the base case with no substituent group on the ring.

II. p-Cresol: This has a methyl group \((CH_3)\) at the para \(p-\) position. Methyl is a weak EDG, so it slightly decreases the acidity compared to phenol.

III. m-Nitrophenol: The nitro group \((NO_2)\) is a strong EWG, regardless of its position (meta or para). However, in the meta \((m-)\)position, its electron-withdrawing effect is slightly less pronounced compared to the para position due to the orientation of the nitro group. Despite that, it still increases the acidity compared to phenol and \(p-\)cresol.

IV. p-Nitrophenol: Similar to m-nitrophenol, the nitro group is a strong EWG. But in the para \((p-)\) position, its electron-withdrawing effect is maximized, leading to the strongest acidity among the four compounds.

Therefore, due to the varying influence of the substituent groups, the order of acidity becomes: p-Nitrophenol (IV) > m-Nitrophenol (III) > Phenol (I) > p-Cresol (II)