Organic Chemistry: Structure, Reactions and Functional Groups
What this domain tests
Organic items test whether you can name a structure, recognise the functional group, predict the product of a characteristic reaction, and explain it with a mechanism or an electronic argument. Stereochemistry and isomer counting also appear.
Core rules
- Priority for naming: carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene > alkyne > alkane > ether > halide.
- Substitution: tertiary and secondary halides favour SN1 (carbocation intermediate, racemisation); primary halides and strong nucleophiles favour SN2 (backside attack, inversion).
- Electrophilic addition follows Markovnikov's rule (H goes to the carbon with more hydrogens) unless peroxides are present, which reverse the regiochemistry for HBr only.
- Oxidation: primary alcohols → aldehydes → carboxylic acids; secondary → ketones; tertiary alcohols resist oxidation.
- Benzene undergoes electrophilic substitution, not addition, because aromaticity is preserved; activating groups (OH, NH₂, alkyl) direct ortho/para, deactivating groups (NO₂, COOH, SO₃H) direct meta.
- Tests: Tollens' and Fehling's for aldehydes, Brady's reagent for carbonyls, iodoform for CH₃CO– or CH₃CH(OH)–, bromine water for alkenes.
Worked example
Propene reacts with HBr. The proton adds to C-1 to give the more stable secondary carbocation at C-2, so bromide attacks C-2 and the product is 2-bromopropane. With HBr and a peroxide initiator, a bromine radical adds first to the terminal carbon to give the more stable secondary radical, so the product is 1-bromopropane - an anti-Markovnikov result specific to HBr.
Traps that cost marks
- Counting a carbon with two identical groups as a stereocentre (it is not chiral).
- Forgetting that cis/trans isomerism needs restricted rotation - it applies to alkenes and rings, not to open-chain alkanes.
- Assuming HCl and HI also show the peroxide effect; only HBr does.