Organic Chemistry and Functional Groups
Organic chemistry and functional groups tests nomenclature, isomerism, reaction mechanisms and functional-group conversions, with emphasis on curly-arrow reasoning.
Core rules
- Identify the functional group and the longest carbon chain; number from the end giving the lowest locants. Alcohols, aldehydes, ketones, carboxylic acids, esters, amines, amides and halogenoalkanes each have characteristic reactions.
- Structural isomers differ in chain, position or functional group. E/Z isomerism requires a C=C with two different groups on each carbon.
- Free-radical substitution: initiation by UV light, propagation steps producing new radicals, termination combining radicals.
- Electrophilic addition to alkenes goes through a carbocation; the major product comes from the more stable carbocation (Markovnikov's rule).
- Halogenoalkanes undergo nucleophilic substitution; primary halogenoalkanes favour SN2, tertiary favour SN1. Elimination competes using hot ethanolic KOH.
- Primary alcohols are oxidised to aldehydes (distillation) or carboxylic acids (reflux); secondary alcohols to ketones; tertiary alcohols resist mild oxidation.
Worked example
Propene + HBr. The H+ electrophile adds to the carbon with more hydrogen (C1), forming a secondary carbocation at C2 rather than a primary carbocation at C1. Br− then attacks the carbocation to give 2-bromopropane as the major product. The reasoning is carbocation stability: secondary is more stable than primary because of greater alkyl induction and hyperconjugation.
Traps
Drawing curly arrows from atoms rather than bonds or lone pairs; applying Markovnikov's rule backwards; oxidising a primary alcohol under reflux and expecting the aldehyde; confusing nucleophile and electrophile; forgetting that E/Z requires different groups on each alkene carbon.