Organic Chemistry: Functional Groups, Isomerism and Reaction Pathways
Functional groups decide everything
Identify the group first: it determines the suffix, the polarity, the intermolecular forces and the reactions.
- Alkane: unreactive except for radical substitution with halogens under UV.
- Alkene and alkyne: addition of H2/Ni, Br2 (decolourises bromine water), HBr, H2O/H+; Markovnikov places H on the carbon with more hydrogens.
- Haloalkane: substitution with OH-, CN- or NH3; elimination with KOH in ethanol.
- Alcohol: oxidation (primary to aldehyde and then acid, secondary to ketone, tertiary resists), esterification with a carboxylic acid and H2SO4 catalyst, dehydration to an alkene.
- Aldehyde and ketone: nucleophilic addition; Tollens' reagent detects the aldehyde.
- Carboxylic acid: acidic hydrogen, esterification, salt formation. Ester: hydrolysis, characteristic smell. Amine: a Brønsted base.
Isomerism
Structural isomers differ in connectivity (chain, position, functional group); C4H10 has butane and 2-methylpropane only. cis-trans needs restricted rotation (C=C or a ring) and two different groups on each carbon. A chiral carbon carries four different groups and gives an enantiomer pair.
Worked example
2-bromopropane, CH3CHBrCH3, heated with KOH dissolved in ethanol, gives propene, CH3CH=CH2, by elimination: ethanol is a poor ionizing solvent, so elimination dominates over substitution. The same substrate with aqueous KOH gives propan-2-ol by substitution. Changing the solvent changes the major product - a classic Bagrut distinction.
Traps
Drawing cis-trans isomers when one carbon carries two identical groups. Putting Br on the terminal carbon when adding HBr to propene. Forgetting that a tertiary alcohol cannot be oxidized. Treating an aldehyde and a ketone of the same formula as identical compounds.