CCl₃COOH is a stronger acid than CH₃COOH.
Acidity Comparison:
Trichloroacetic acid (CCl₃COOH):
- pKa ≈ 0.66
- Strong acid
- Highly ionized in water
Acetic acid (CH₃COOH):
- pKa ≈ 4.76
- Weak acid
- Partially ionized in water
Why CCl₃COOH Is Stronger:
1. Inductive Effect:
- Chlorine atoms are highly electronegative
- Three Cl atoms withdraw electron density from the carboxyl group
- This withdrawal is transmitted through sigma bonds (negative inductive effect, -I effect)
2. Conjugate Base Stability:
- CCl₃COO⁻ (trichloroacetate ion) is more stable
- Electron-withdrawing Cl atoms disperse negative charge
- More stable conjugate base = stronger acid
3. O-H Bond Weakening:
- Electron withdrawal weakens O-H bond in -COOH
- Easier H⁺ release
- Increased acidity
Comparison: CH₃COOH < CH₂ClCOOH < CHCl₂COOH < CCl₃COOH (Increasing acidity with more Cl substitution)
Key Takeaways:
- CCl₃COOH is a stronger acid
- pKa difference: ~4 units (means 10,000 times stronger)
- Due to electron-withdrawing inductive effect of Cl
- More Cl atoms = stronger acid