Answer the question on the basis of passage given below:

Strong oxidizing agents, oxidize toluene and its derivatives to benzoic acid. However, it is possible to stop the oxidation at the aldehyde stage with suitable reagents that convert methyl group to an intermediate that is difficult to oxidize further.

Decreasing order of acidic strength will be:

1. Phenol

2. 2-Methyl phenol

3. 3-Methyl phenol

4. 4-Methyl phenol

Choose the correct answer from the options given below:

A > C > D > B

The correct answer is option 2. A > C > D > B.

Let us delve into the detailed reasoning for the decreasing order of acidic strength among phenol and its methyl derivatives:

A. Phenol

Phenol has a hydroxyl group \((-OH)\) directly attached to a benzene ring. Phenol is acidic due to the resonance stabilization of the phenoxide ion (the conjugate base) formed after the loss of a proton. The negative charge on the oxygen atom is delocalized into the aromatic ring, stabilizing the phenoxide ion. Phenol serves as the baseline for comparing the effects of substituents.

B. 2-Methylphenol - Ortho-Methylphenol (also known as o-Cresol)

The methyl group \((-CH_3)\) is attached to the benzene ring at the ortho position relative to the hydroxyl group. The ortho-methyl group has a significant electron-donating +I (inductive) effect, which increases the electron density on the ring. This electron-donating effect destabilizes the negative charge on the phenoxide ion, making the phenoxide ion less stable and thus decreasing the acidity of the phenol. Additionally, there may be steric hindrance affecting the acidity.

C. 3-Methylphenol - Meta-Methylphenol (also known as m-Cresol)

The methyl group is positioned at the meta position relative to the hydroxyl group. The meta-methyl group has a lesser electron-donating effect compared to the ortho-methyl group because it does not directly interact with the phenoxide ion. The electron-donating effect of the meta-methyl group is weaker, resulting in less stabilization of the negative charge on the phenoxide ion. Thus, 3-methylphenol is more acidic than 2-methylphenol but less acidic than phenol.

D. 4-Methylphenol - Para-Methylphenol (also known as p-Cresol)

The methyl group is attached to the benzene ring at the para position relative to the hydroxyl group. The para-methyl group has an electron-donating effect through inductive resonance, but this effect is less impactful compared to the ortho position. The interaction of the para-methyl group with the phenoxide ion is weaker compared to the ortho position. The para-methyl group slightly decreases the acidity of phenol but not as much as the ortho-methyl group. The electron-donating effect is less pronounced here, resulting in less stabilization of the phenoxide ion compared to ortho but more than meta.

Summary of Acidity Order

A. Phenol: Baseline acidity with moderate stabilization of the phenoxide ion.

C. 3-Methylphenol: Meta-methyl group slightly decreases acidity due to weaker electron-donating effects.

D. 4-Methylphenol : Para-methyl group has a less significant impact on acidity compared to the ortho position.

B. 2-Methylphenol : Ortho-methyl group has the strongest electron-donating effect, which greatly reduces the acidity of phenol.

Thus, the correct order of decreasing acidity is: A > C > D > B.