Question

Select the correct order with respect to acid strength.

• A. \(m\text{-Cresol} \lt Phenol \lt m\text{-Nitrophenol} \lt p\text{-Nitrophenol}\)
• B. \(m\text{-Cresol} \lt Phenol \lt p\text{-Nitrophenol} \lt m\text{-Nitrophenol}\)
• C. \(m\text{-Cresol} \lt p\text{-Nitrophenol} \lt Phenol \lt m\text{-Nitrophenol}\)
• D. \(p\text{-Nitrophenol} \lt m\text{-Cresol} \lt Phenol \lt m\text{-Nitrophenol}\)

Hint:

For substituted phenols: \[ \text{Electron withdrawing group} \Rightarrow \text{Acidity increases} \] \[ \text{Electron donating group} \Rightarrow \text{Acidity decreases} \] The para nitro group increases acidity more effectively than the meta nitro group because resonance stabilization is possible.

Answer 1

The Correct Option is A

Concept: Acid strength depends on the stability of the conjugate base (phenoxide ion).

• Electron-donating groups decrease acidity by destabilizing the phenoxide ion.

• Electron-withdrawing groups increase acidity by stabilizing the phenoxide ion.

• The \(-NO_2\) group exhibits both \(-I\) and \(-R\) effects at para position.

Step 1: Compare \(m\)-cresol and phenol. The methyl group shows \(+I\) effect and increases electron density on oxygen. Therefore, \(m\)-cresol is less acidic than phenol. \[\begin{aligned} m\text{-Cresol} \lt Phenol \end{aligned}\]

Step 2: Compare phenol and nitrophenols. The nitro group is strongly electron withdrawing. Hence nitrophenols are more acidic than phenol. \[\begin{aligned} Phenol \lt m\text{-Nitrophenol} \end{aligned}\]

Step 3: Compare \(m\)-nitrophenol and \(p\)-nitrophenol. At meta position, the nitro group exerts mainly the \(-I\) effect. At para position, both \(-I\) and resonance \((-R)\) effects operate, giving greater stabilization to the phenoxide ion. Therefore, \[\begin{aligned} m\text{-Nitrophenol} \lt p\text{-Nitrophenol} \end{aligned}\]

Step 4: Arrange the compounds in increasing order of acidity. Increasing Acid Strength: \[ m\text{-Cresol} \;\lt \; \text{Phenol} \;\lt \; m\text{-Nitrophenol} \;\lt \; p\text{-Nitrophenol} \] \[\begin{aligned} \boxed{ m\text{-Cresol} \lt Phenol \lt m\text{-Nitrophenol} \lt p\text{-Nitrophenol} } \end{aligned}\] Hence, option \(\mathbf{(A)}\) is correct.