Understanding Acidity Order in Organic Compounds

To determine acidity order, we analyze the stability of the conjugate base. More stable conjugate base = stronger acid.

Step 1: Identify the acidic protons

All compounds have -OH groups that can donate protons, but their acidity differs due to substituent effects.

Step 2: Analyze substituent effects

Electron-withdrawing groups (EWG) stabilize conjugate base (increase acidity). Electron-donating groups (EDG) destabilize conjugate base (decrease acidity).

Step 3: Evaluate each compound

Compound I: Phenol - moderate acidity

Compound II: p-Nitrophenol - strong EWG (-NO$2_{}$) increases acidity

Compound III: p-Cresol - EDG (-CH$3_{}$) decreases acidity

Compound IV: m-Nitrophenol - EWG effect is weaker than para position

Step 4: Compare acidity strength

p-Nitrophenol (II) > m-Nitrophenol (IV) > Phenol (I) > p-Cresol (III)

Final Answer:

II > IV > I > III

Related Concepts

Acidity Constant (K$a$): $Ka=\frac{\left[H^{+}\right]\left[A^{-}\right]}{\left[\mathrm{HA}\right]}$

Hammett Equation: Quantifies substituent effects on acidity: $\log \frac{K_a}{K_a^0}=\mathrm{\rho \sigma }$ where σ is substituent constant, ρ is reaction constant

Key Formulae

pK$a$ = -log${10}_{}$K$a$

Lower pK$a$ = stronger acid