Arrange the following compounds in decreasing order of acidity:
(A) o-cresol
(B) Phenol
(C) m-nitrophenol
(D) o-nitrophenol
(E) Ethanol
Choose the correct answer from the options given below:

D>C>B>A>E

The correct answer is Option 4. D>C>B>A>E

Let us analyze the acidity of each compound in detail, focusing on the factors that influence the acidity such as the inductive effect, resonance, and the electron-withdrawing or electron-donating nature of the substituents.

The acidity of a compound is primarily determined by how easily it can lose a proton \((H^+)\) and how stable the resulting conjugate base (the anion) is after deprotonation.

Electron-withdrawing groups (EWGs): These groups pull electron density away from the molecule, stabilizing the negative charge on the conjugate base and thus increasing acidity.

Electron-donating groups (EDGs): These groups push electron density towards the molecule, destabilizing the negative charge on the conjugate base and thus decreasing acidity.

Let us break down each compound:

E. Ethanol 

Ethanol is an alcohol. Its conjugate base (ethoxide ion, \(\text{CH}_3\text{CH}_2\text{O}^-\)) is not stabilized by resonance or any significant electron-withdrawing effects.

Acidity: Very low compared to phenols because the negative charge on the oxygen in the ethoxide ion is localized and not stabilized by resonance.

Conclusion: Least acidic among the given compounds.

B. Phenol 

A hydroxyl group \((-OH)\) attached to a benzene ring.

Phenol is more acidic than ethanol because the conjugate base (phenoxide ion, \(\text{C}_6\text{H}_5\text{O}^-\)) is stabilized by resonance. The negative charge on the oxygen can delocalize over the aromatic ring.

Acidity: Moderate, due to resonance stabilization of the phenoxide ion.

D. o-Nitrophenol 

Phenol with a nitro group at the ortho position. The nitro group is a strong electron-withdrawing group through both inductive (-I) and resonance (-R) effects. It stabilizes the negative charge on the phenoxide ion significantly.

The ortho position allows the nitro group to have a greater effect because of its proximity to the hydroxyl group. It withdraws electron density both inductively and through resonance.

Acidity: Very high, as the nitro group significantly stabilizes the conjugate base, making o-nitrophenol the most acidic in this group.

C. m-Nitrophenol

Phenol with a nitro group  at the meta position. While the nitro group is still electron-withdrawing, its resonance interaction with the phenoxide ion is weaker at the meta position. The inductive effect is still present but not as strong as in the ortho position.

Acidity: Higher than phenol but lower than o-nitrophenol due to the reduced resonance stabilization at the meta position.

A. o-Cresol

Phenol with a methyl group  at the ortho position. The methyl group is an electron-donating group (+I effect). It donates electron density to the benzene ring, destabilizing the conjugate base (phenoxide ion) by increasing the electron density on the oxygen atom.

Acidity: Lower than phenol because the methyl group destabilizes the negative charge on the phenoxide ion, making it less likely to lose a proton.

Final Order of Acidity:

D. o-Nitrophenol : The nitro group at the ortho position strongly increases acidity due to both inductive and resonance effects.

C. m-Nitrophenol : The nitro group at the meta position increases acidity, but not as much as at the ortho position.

B. Phenol : Moderately acidic due to resonance stabilization of the phenoxide ion.

A. o-Cresol : The methyl group reduces acidity compared to phenol due to its electron-donating effect.

E. Ethanol : Least acidic, as the conjugate base is not resonance-stabilized.

The decreasing order of acidity is: D > C > B > A > E