High-Frequency PYQ Topics

1. Acidity Ordering (most frequent): NEET consistently asks to arrange 4 acids in order of acid strength. Strategy: identify –I groups (Cl, F → increase acidity) and +I groups (alkyl → decrease acidity). Count substituents, assess distance. Formic acid is the baseline for no-substituent comparison.

2. NaBH4 vs LiAlH4 (NEET trap): Appears every 2–3 years. Answer = "no reaction" when NaBH4 is with RCOOH. Full answer when LiAlH4: gives primary alcohol.

3. HVZ Limitations: Every few years NEET asks which acids CANNOT undergo HVZ. Always HCOOH (no alpha-C) and benzoic acid / ArCOOH (sp2 ring, no alpha-H). Also trimethylacetic acid (alpha-carbon is quaternary in some forms — check).

4. Fischer vs Saponification: Comparison questions: Fischer = reversible, H2SO4, gives free acid + alcohol system; Saponification = irreversible, NaOH, gives salt.

5. Named Reaction Products: Kolbe electrolysis: identify the alkane product. Soda lime: identify the (n–1)-carbon alkane. Grignard + CO2: identify the carboxylic acid with one more C than the Grignard alkyl group.

Exam Strategy for Acidity Questions

1. Write out all compounds given.
2. Tag each substituent on the alpha-carbon: Cl/F → –I; alkyl → +I; H → neutral.
3. Count substituents: more –I = stronger; more +I = weaker.
4. Check distance: closer –I = stronger effect.
5. Compare final ranking and select the answer.

Typical PYQ Answer Patterns

• "No reaction" → NaBH4 + RCOOH
• "Cannot undergo HVZ" → HCOOH, benzoic acid
• "Irreversible" → Saponification
• "Anode product" → Kolbe electrolysis alkane
• "One carbon more than Grignard" → Grignard + CO2 product