Organic chemistry
The organic chemistry domain tests functional-group recognition, IUPAC naming, isomer counting, identification of the mechanism type behind a transformation, and the structure and formation of polymers and biomolecules. Answers must follow the exact naming conventions demanded by the exam.
Functional groups and nomenclature
Alkanes, alkenes, alkynes; alcohols (−ol), ethers (alkoxy-), aldehydes (−al), ketones (−one), carboxylic acids (oic acid), esters (alkyl alkanoate), amides, amines and nitriles. Choose the longest chain containing the principal group, number to give that group the lowest locant, and list substituents alphabetically. Priority when choosing the suffix: acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene/alkyne > alkane.
Isomerism
Structural isomers differ in connectivity; stereoisomers share connectivity. cis/trans or E/Z needs restricted rotation, typically a C=C with two different substituents on each carbon. Optical isomerism needs a chiral sp³ carbon bearing four different groups, labelled R/S by Cahn–Ingold–Prelog rules. Watch for functional-group isomerism: alcohols versus ethers, aldehydes versus ketones, acids versus esters.
Reaction types
- Addition: electrophilic addition across C=C; HBr with propene follows Markovnikov; bromine water is decolourised.
- Substitution: free-radical halogenation of alkanes (initiation, propagation, termination); SN1/SN2 in haloalkanes.
- Elimination: KOH in ethanol gives the more substituted alkene (Zaitsev).
- Oxidation: primary alcohols → aldehyde → acid; secondary → ketone; tertiary resistant.
- Esterification: acid + alcohol ⇌ ester + water, catalysed by H₂SO₄.
Polymers and biomolecules
Addition polymers (polyethene, polystyrene, PVC) come from C=C monomers. Condensation polymers (nylon, PET) release water. Biomolecules: proteins use amide (peptide) bonds, starch and cellulose are α- and β-glucose polymers, and nucleic acids use phosphodiester links.
Worked example
For C₄H₁₀O, the alcohols are butan-1-ol, butan-2-ol, 2-methylpropan-1-ol and 2-methylpropan-2-ol. The ethers are methoxypropane, ethoxyethane and 2-methoxypropane. That gives seven structural isomers. Methoxypropane and 2-methoxypropane are distinct because the propyl chain is linear in one case and branched (isopropyl) in the other.
Traps
- Numbering to give a substituent the lowest locant instead of the principal functional group.
- Calling an ester an ether, or an amide an amine.
- Missing functional-group isomers when counting formulas such as C₃H₆O.
- Claiming chirality when two of the four groups on a carbon are identical.