Organic Chemistry: Hydrocarbons, Functional Groups and Reaction Pathways
This domain tests nomenclature, isomerism and the reactions that interconvert functional groups. STPM expects reagents, conditions and mechanisms to be stated precisely.
Nomenclature and isomerism
Name the longest carbon chain and identify the principal functional group. Structural isomerism includes chain, position and functional-group isomerism. cis–trans isomerism requires restricted rotation and different groups on each carbon. Optical isomerism arises when a carbon has four different substituents.
Hydrocarbons
Alkanes undergo free-radical substitution with halogens under UV light. Alkenes undergo electrophilic addition; Markovnikov’s rule predicts that H adds to the carbon with more hydrogens. Alkenes also undergo oxidation and addition polymerisation. Benzene has a delocalised π system and undergoes electrophilic substitution rather than addition.
Functional groups
Primary alcohols oxidise to aldehydes under distillation and to carboxylic acids under reflux; secondary alcohols give ketones; tertiary alcohols resist oxidation. Halogenoalkanes undergo nucleophilic substitution and elimination. Aldehydes are oxidised by Tollens’ and Fehling’s reagents; ketones are not. Carboxylic acids form esters with alcohols under acid catalysis; esters hydrolyse under acid or base.
Worked example
Propan-1-ol with acidified K2Cr2O7 under distillation gives propanal. Under reflux, the aldehyde is further oxidised to propanoic acid. Propan-2-ol gives propanone, which resists further oxidation. This is a primary versus secondary alcohol distinction.
Traps that cost marks
Do not use Markovnikov’s rule for benzene substitution. Specify distillation or reflux for alcohol oxidation. Esterification is reversible and needs a catalyst. Do not confuse Tollens’ and Fehling’s tests. Benzene undergoes substitution, not addition.