Arrange the following in increasing order of their pH values:

(A) p-Nitrophenol
(B) m-Cresol
(C) m-Nitrophenol
(D) Phenol

Choose the correct answer from the options given below:

(A) < (C) < (D) < (B)

The correct answer is Option (2) → (A) < (C) < (D) < (B).

When arranging compounds by pH, we are essentially comparing their acidic strengths. The more acidic a compound is, the lower its pH will be, and the more basic or less acidic it is, the higher its pH will be.

Here, we are considering substituted phenols, and the substituents on the phenol ring (whether they are electron-donating or electron-withdrawing) significantly influence the acidity of the compound.

Let’s break down the reasoning for each compound:

A. p-Nitrophenol :

The nitro group (-NO₂) is an electron-withdrawing group (EWG) and it significantly increases the acidity of the phenol. The para position allows the nitro group to stabilize the negative charge on the oxygen of the conjugate base (phenoxide ion) by delocalizing the electron density through resonance and inductive effects. This increased stabilization makes p-nitrophenol the most acidic of the group, resulting in the lowest pH.

C. m-Nitrophenol :

Like p-nitrophenol, m-nitrophenol also has a nitro group (-NO₂), which is electron-withdrawing. However, the meta position has a weaker effect on the acidity compared to the para position. The electron-withdrawing effect is less effective at stabilizing the negative charge in the phenoxide ion at this position. As a result, m-nitrophenol is less acidic than p-nitrophenol, but still more acidic than phenol.

D. Phenol :

Phenol itself has no strong electron-withdrawing or electron-donating groups directly attached to it. Its acidity is moderate, as the hydroxyl group attached to the aromatic ring can donate some electron density to the ring, but not to the extent of significantly altering the acidity. Phenol is less acidic than the nitrophenols but more acidic than cresols.

B. m-Cresol :

m-Cresol contains a methyl group (-CH₃) at the meta position. The methyl group is an electron-donating group (EDG), which decreases the acidity of the phenol by increasing the electron density on the oxygen. This makes it harder to lose the proton (H⁺), hence lowering the acidity.  Because of this, m-cresol is the least acidic compound in this list and has the highest pH.

Increasing Order of pH (From Most Acidic to Least Acidic):

The most acidic compound will have the lowest pH.

The order based on the electron-withdrawing and electron-donating effects of the substituents is:

p-Nitrophenol (A) is the most acidic.

m-Nitrophenol (C) comes next, as the nitro group is still electron-withdrawing but not as strongly as in the para position.

Phenol (D) has no strong substituents, so its acidity is moderate.

m-Cresol (B) is the least acidic due to the electron-donating methyl group.

Final Order of Increasing pH:

\((A) < (C) < (D) < (B)\)

This means that p-nitrophenol has the lowest pH (most acidic), followed by m-nitrophenol, phenol, and lastly, m-cresol, which has the highest pH (least acidic).