ChemistryMediumClass 12

Acidity & Basicity in Organic — Factors affecting

GOC (General Organic Chemistry)

JEE Qs

Hard

min

Always analyze the stability of the conjugate acid (for basicity) or conjugate base (for acidity) using all relevant electronic effects (resonance, inductive, hybridization, steric) to determine relative strengths.

🧮 Key Formulas

pKa = -log(Ka)

pKb = -log(Kb)

Ka * Kb = Kw (for conjugate acid-base pair in aqueous solution, Kw = 10^-14 at 25°C)

Stronger Acid <=> Smaller pKa (Larger Ka)

Stronger Base <=> Smaller pKb (Larger Kb)

✅ Key Points for JEE

1Acidity of an organic compound is directly proportional to the stability of its conjugate base (anion). Factors that stabilize the conjugate base (e.g., electron-withdrawing groups, delocalization of charge) increase acidity.
2Basicity of an organic compound is directly proportional to the availability of the lone pair of electrons on the basic atom for protonation. Factors that increase electron density or localize the lone pair increase basicity (e.g., electron-donating groups).
3The primary factors affecting acidity and basicity are Inductive Effect (I-effect), Resonance/Mesomeric Effect (M-effect), Hybridization (s-character), Aromaticity/Antiaromaticity, and Steric Effects. Resonance is generally more dominant than inductive effect, but hybridization can be very strong, and the 'ortho-effect' is an important exception for substituted benzoic acids/anilines.
4For comparing basicity of amines in aqueous solution, solvation effects (stability of protonated amine by hydrogen bonding with water) must be considered along with inductive effects, leading to the order: secondary > primary > tertiary (for alkyl amines, though specific order varies slightly depending on alkyl groups).
5Acidity comparison: Carboxylic Acids > Phenols > Alcohols. Basicity comparison: Alkyl amines > Ammonia > Aromatic amines > Amides.

⚠️ Common Mistakes

✕Confusing pKa/pKb values with strength; a lower pKa indicates a stronger acid, and a lower pKb indicates a stronger base.
✕Failing to consider the stability of the *conjugate* species (conjugate base for acidity, conjugate acid for basicity) and instead focusing solely on the original molecule.
✕Incorrectly prioritizing the various electronic effects (inductive, resonance, hyperconjugation, hybridization) or neglecting the 'ortho-effect' or solvation effects in specific scenarios.
✕Not recognizing when a lone pair is involved in resonance or aromaticity, which significantly reduces its availability for basicity.

NCERT Chapters

Class 11 Chemistry Part II Ch 12: Organic Chemistry – Some Basic Principles and Techniques
Class 12 Chemistry Part II Ch 11: Alcohols, Phenols and Ethers
Class 12 Chemistry Part II Ch 12: Aldehydes, Ketones and Carboxylic Acids
Class 12 Chemistry Part II Ch 13: Amines