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CS-991 · Module E-4

Top Exam Mistakes

Exam Preparation Hub

⚠️ Most Repeated Mistakes in Organic Chemistry Exams

❌ Common Mistake

Confusing empirical formula with molecular formula, writing them as if they're always the same.

✅ Correct Approach

Empirical formula is the simplest ratio; molecular formula is the actual count. They're only the same when n = 1 (check using molar mass first).

❌ Common Mistake

Writing SN1 and SN2 mechanisms without stating which substrate (primary/secondary/tertiary) favours which.

✅ Correct Approach

Always link the mechanism to the substrate type: tertiary → SN1 (stable carbocation), primary → SN2 (less steric hindrance).

❌ Common Mistake

Forgetting that heterolytic fission produces TWO different species (a cation and an anion), not two identical ones.

✅ Correct Approach

State clearly: heterolytic fission gives one carbocation and one carbanion; homolytic fission gives two identical free radicals.

❌ Common Mistake

Applying Aldol condensation logic to aldehydes that have no α-hydrogen (like benzaldehyde).

✅ Correct Approach

Check for an α-hydrogen first. No α-H means Cannizzaro reaction (disproportionation) instead of Aldol condensation.

❌ Common Mistake

Mixing up the acidity order of phenols vs alcohols, or forgetting WHY phenols are more acidic.

✅ Correct Approach

Always justify with resonance: the phenoxide ion is stabilized by delocalization into the aromatic ring, which simple alkoxide ions from alcohols cannot do.

❌ Common Mistake

Assuming aromatic amines (like aniline) are just as basic as aliphatic amines.

✅ Correct Approach

Aromatic amines are LESS basic — the nitrogen lone pair is partly delocalized into the ring by resonance, making it less available to accept a proton.

❌ Common Mistake

Writing the general formula for alkenes/alkynes incorrectly (mixing up CₙH₂ₙ and CₙH₂ₙ₋₂).

✅ Correct Approach

Memorize as a set: Alkanes CₙH₂ₙ₊₂ (single bond), Alkenes CₙH₂ₙ (double bond), Alkynes CₙH₂ₙ₋₂ (triple bond).

❌ Common Mistake

Not showing units or balancing steps clearly in numerical problems (empirical formula, % yield), losing method marks.

✅ Correct Approach

Always show each step explicitly (mass → moles → ratio) even when the final answer seems obvious — board marking schemes award marks per correct step.

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