CHEM10007 · Fundamentals Of Chemistry
Organic Chemistry II: Alkenes, Alkynes & Aromatics
The final week extends organic structure to unsaturated and cyclic systems. You name cycloalkanes, handle geometrical (cis–trans) isomerism arising from restricted rotation, and study alkenes (sp2, π bonds, E/Z nomenclature) and alkynes (sp, triple bonds). You finish with polyenes (conjugation and colour) and benzene/arenes, with IR spectroscopy appearing lightly as a structure-identification tool.
What this chapter covers
- 01Cycloalkanes: 'cyclo' nomenclature
- 02Geometrical (cis–trans) isomerism in rings and across C=C double bonds
- 03Restricted rotation about a C=C double bond as the cause of cis–trans isomerism
- 04Alkenes CnH2n: sp2 bonding and the π bond in ethene; −ene nomenclature with lowest locants
- 05E/Z isomerism assigned by CIP priorities on each sp2 carbon
- 06Polyenes: conjugation and colour (e.g. carotenoids)
- 07Alkynes: sp bonding and the triple bond in acetylene; −yne nomenclature
- 08Benzene and arenes: delocalised structure and naming of substituted derivatives; characteristic IR absorptions (C=O ≈ 1700–1800 cm−1, O−H/N−H ≈ 3300–3500 cm−1)
E/Z assignment and alkene naming
- 1 mark — parent chain and locantThe longest chain through the double bond has 5 carbons with the C=C between C-2 and C-3, so the parent is pent-2-ene.
- 1 mark — CIP priority on both carbonsOn each sp2 carbon, rank the two attached groups by CIP priority (the carbon chain outranks H on each end).
- 1 mark — correct E/Z assignmentThe two higher-priority groups are on opposite sides of the double bond, so the descriptor is E.
- 1 mark — name and cis/trans limitationFull name: (E)-pent-2-ene. The cis/trans system breaks down when a double-bond carbon carries two different non-hydrogen groups, because 'same/opposite side' is undefined — E/Z (by CIP priority) is unambiguous.
Key terms
- Geometrical (cis–trans) isomerism
- Stereoisomerism caused by restricted rotation about a C=C double bond (or within a ring), giving distinct same-side (cis) and opposite-side (trans) arrangements.
- E/Z isomerism
- An unambiguous descriptor for double-bond geometry: higher-CIP-priority groups on opposite sides = E (entgegen), on the same side = Z (zusammen).
- Alkene
- A hydrocarbon with a C=C double bond, formula CnH2n; the carbons are sp2 hybridised with one σ and one π bond between them.
- Alkyne
- A hydrocarbon with a C≡C triple bond; the carbons are sp hybridised, as in acetylene (ethyne), named with the −yne suffix.
- Conjugation
- Alternating single and double bonds in a polyene that delocalise π electrons, often shifting light absorption into the visible region and producing colour.
Organic Chemistry II: Alkenes, Alkynes & Aromatics FAQ
When does cis–trans isomerism occur?
It occurs when there is restricted rotation — across a C=C double bond or within a ring — and each of the relevant carbons carries two different groups. Free rotation about single bonds (as in alkanes) prevents it.
Why is E/Z preferred over cis/trans?
Cis/trans works only when it is clear which groups are 'the same'. When a double-bond carbon bears two different non-hydrogen substituents, E/Z resolves the ambiguity by comparing CIP priorities on each carbon.
Why are conjugated polyenes often coloured?
Extended conjugation delocalises the π electrons and lowers the energy gap between the bonding and antibonding π levels, so the molecule absorbs visible light — as in the carotenoids responsible for orange and red pigments.
Exam move
Practise alkene and alkyne IUPAC naming with lowest locants, and drill E/Z assignment by always comparing CIP priorities on both ends of the double bond. Be ready to explain the cis/trans limitation in words, since it is a favourite short-answer point. Keep IR spectroscopy light but memorise the two flagged absorptions (C=O and O−H/N−H), as they support structure-identification questions that can appear in the mock and practice exams.
