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Learn7405Paper 2 · Paper 2: Organic chemistryIntroduction to organic chemistryOrganic nomenclature, isomerism and mechanisms
Introduction to organic chemistry · Subtopic 1 of 1

Organic Nomenclature, Isomerism and Mechanisms

Organic molecules are shown as displayed, structural or skeletal formulae. IUPAC names use the longest chain, the lowest locants and alphabetical prefixes (e.g. 2-methylbutane, 1-bromo-2-chloropropane). Structural isomers have the same molecular formula but different structures (chain, position or functional group isomers). Stereoisomers have the same structure but different spatial arrangement: E/Z isomers arise from restricted rotation about C=C when each C has two different groups. Bonds break by homolytic fission (radicals) or heterolytic fission (ions). Nucleophiles donate an electron pair; electrophiles accept one. Curly arrows show movement of electron pairs.

Cambridge 9701 Paper 1Cambridge 9701 Paper 2Edexcel YCH11 WCH11AQA 7405 Paper 2≈ 50 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Draw and interpret displayed, structural and skeletal formulae.
  • Name organic compounds using IUPAC rules.
  • Identify structural and E/Z isomers.
  • Classify reagents and describe bond fission and curly arrows.
Pause and think

But-2-ene exists as two different compounds with different boiling points. How, if they have the same atoms bonded in the same order?

Section 2 of 14

Core idea

Start hereA language for molecules

"2-methylbutan-2-ol" tells a chemist exactly which molecule you mean, anywhere in the world. Names and curly arrows are organic chemistry's grammar.

The big idea

IUPAC names describe structure. Isomers share a formula but differ in structure or 3D arrangement. Mechanisms track electron pairs with curly arrows.

1. Types of isomerism

Isomerism family tree

Same molecular formula throughout.

Worked exampleAssigning E/Z

CH3CH=C(CH3)CH2CH3\mathrm{CH_3CH{=}C(CH_3)CH_2CH_3}: decide E or Z if the CH₃ on C2 and CH₂CH₃ on C3 are on the same side.

Goal
E or Z.
Strategy
CIP priorities on each carbon.
  1. C2: CH3>H\text{C2: CH}_3 > \text{H}
    C beats H.
  2. C3: CH2CH3>CH3\text{C3: CH}_2\text{CH}_3 > \text{CH}_3
    At the first point of difference, C beats H.
  3. same side⇒Z\text{same side}\Rightarrow Z
    Higher priorities together.
Answer(Z)-3-methylpent-2-ene.
✓ CheckEach C of C=C has two different groups ✓.
Key takeawayReagent types Nucleophiles (:OH⁻, :CN⁻, :NH₃) attack δ+ carbon. Electrophiles (H⁺, Br^δ+, NO₂⁺) attack electron-rich C=C or rings. Radicals (Cl•) form by homolytic fission.
Pause and think

Why can't rotation occur around a C=C bond?

Summary

  • IUPAC rules.
  • Structural and E/Z isomers.
  • Fission types and curly arrows.
Key words
Nucleophile
Electron pair donor.
Electrophile
Electron pair acceptor.
Stereoisomers
Same structural formula, different spatial arrangement.
HistoryIUPAC naming. The Geneva nomenclature of 1892 was the first international agreement on naming organic compounds; before that, the same compound could have dozens of names.
Fun factVision. Light flips a Z double bond in retinal to E inside your eye; that change in shape triggers the nerve signal for vision.
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