Organic Chemistry
Organic Chemistry checks nomenclature, functional-group reactivity, stereochemistry and mechanism selection. The exam favours ranking questions (acidity, reactivity, boiling point) and identifying the product of a substitution or addition.
Core rules
- Suffix priority for naming: carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene/alkyne > ether > halide.
- Isomerism: constitutional, geometric (E/Z by CIP priorities), optical (chiral centre; up to 2ⁿ stereoisomers).
- Aromaticity requires a cyclic, planar, fully conjugated ring with 4n + 2 π electrons.
- SN2 is concerted, backside attack, fastest for methyl and primary halides in polar aprotic solvents; SN1 proceeds through a carbocation, favoured for tertiary substrates and protic solvents, giving racemisation. E2 needs a strong base and an anti-periplanar β-H.
- Carbonyl reactivity: aldehyde > ketone; acid chloride > anhydride > ester > amide.
- Electron-withdrawing groups raise acidity; electron-donating groups lower it.
Worked example
Rank acetic, chloroacetic, dichloroacetic and trichloroacetic acid by strength. Each chlorine withdraws electron density inductively, stabilising the carboxylate anion and weakening the O–H bond. More chlorines give more stabilisation: trichloro (pKa ≈ 0.65) > dichloro (≈1.25) > chloro (≈2.86) > acetic (≈4.76).
Traps
- Applying Markovnikov without checking for carbocation rearrangement or peroxide conditions.
- Calling a planar 4n ring aromatic; cyclobutadiene is antiaromatic.
- Assigning R/S while hydrogen points toward you - invert the result.
- Forgetting that amino acids exist as zwitterions: below the pI they are cationic, above it anionic.