Organic Mechanisms: An Introduction

This guide covers Topic 13, An introduction to AS Level organic chemistry — subtopics 13.1 to 13.4 in full — from Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. This is AS Level content.

Scope note. This resource covers Topic 13 only. Topics 14 to 16 — Hydrocarbons, Halogen compounds and Hydroxy compounds — each work through a specific class of reactions in enough depth to deserve their own resource rather than a shallow summary here: Hydrocarbons: Alkanes and Alkenes (free-radical substitution and electrophilic addition), Halogenoalkanes: Nucleophilic Substitution and Elimination (SN1/SN2), and Alcohols: Reactions and Oxidation. What follows is genuinely self-contained: the vocabulary and notation Topic 13 introduces is exactly what those later topics assume you already have.

Why this topic exists

Every organic topic that follows uses the same handful of terms — homolytic fission, nucleophile, electrophilic addition, curly arrows — without redefining them each time. Topic 13 is where they’re defined once, properly, so that later topics can focus on the chemistry itself rather than re-explaining vocabulary.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — AS Level, Topic 13

13.1 Formulas, functional groups and the naming of organic compounds — defining a hydrocarbon; understanding alkanes as hydrocarbons with no functional group; that a functional group dictates a compound’s physical and chemical properties; interpreting and using general, structural, displayed and skeletal formulas; systematic nomenclature of simple aliphatic molecules up to six carbon atoms (six-plus-six for esters; straight chains only for esters and nitriles); deducing molecular and empirical formula from a given structural, displayed or skeletal formula.

13.2 Characteristic organic reactions — the terminology of organic compounds and reactions: homologous series; saturated and unsaturated; homolytic and heterolytic fission; free radical, initiation, propagation, termination; nucleophile, electrophile, nucleophilic, electrophilic; addition, substitution, elimination, hydrolysis, condensation; oxidation and reduction (using [O] and [H] notation); and the terminology of mechanism types: free-radical substitution, electrophilic addition, nucleophilic substitution, nucleophilic addition — including the use of curly arrows to represent electron-pair movement, starting at a bond or a lone pair.

13.3 Shapes of organic molecules; σ and π bonds — describing molecules as straight-chained, branched or cyclic; the shape of, and bond angles in, molecules with sp, sp² and sp³ hybridised atoms; the arrangement of σ and π bonds in such molecules; the term “planar,” using ethene as an example.

13.4 Isomerism: structural isomerism and stereoisomerism — structural isomerism, divided into chain, positional and functional group isomerism; stereoisomerism, divided into geometrical (cis/trans) and optical isomerism (E/Z nomenclature is acceptable but not required); geometrical isomerism in alkenes, explained by restricted rotation from the π bond; chiral centres and the two optical isomers (enantiomers) they produce; identifying chiral centres and geometrical isomerism from a structural formula, including cyclic compounds; deducing the possible isomers of a molecule from its molecular formula.

Naming and drawing organic molecules

A general formula describes a whole homologous series (CₙH₂ₙ₊₂ for alkanes); a structural formula shows how atoms are grouped without every bond drawn (CH₃CH₂OH); a displayed formula shows every atom and every bond; a skeletal formula shows only the carbon skeleton as a zigzag line, with functional groups marked and hydrogen atoms on carbon omitted entirely. You’re expected to move fluently between all four for the functional groups in the AS syllabus.

The vocabulary of organic reactions

Two separate sets of terms describe organic chemistry, and it’s worth keeping them apart:

What happens to the bonds — addition (two reactants become one product, nothing lost), substitution (one atom or group replaces another), elimination (a small molecule is lost, typically forming a double bond), hydrolysis (breakdown by reaction with water), condensation (two molecules join with loss of a small molecule, often water).

How the bond breaks — homolytic fission splits a covalent bond so each atom keeps one electron, producing two free radicals; heterolytic fission splits a bond so one atom keeps both electrons, producing two oppositely charged ions.

A nucleophile is an electron-pair donor, attracted to a region of positive or partial positive charge; an electrophile is an electron-pair acceptor, attracted to a region of negative or partial negative charge. Combining the two vocabularies names a mechanism precisely: electrophilic addition is addition initiated by an electrophile attacking an electron-rich double bond; nucleophilic substitution is substitution initiated by a nucleophile attacking an electron-poor carbon atom.

Curly arrows

A curly arrow represents the movement of one electron pair, always starting at a bond or a lone pair and pointing to where that pair ends up. A full curly arrow (not a half-arrow, called a “fish-hook,” used for single-electron movement in free-radical mechanisms) is standard notation throughout AS organic mechanisms.

Shape, hybridisation, and planarity

The same VSEPR and hybridisation ideas from Chemical Bonding: Shapes and Intermolecular Forces apply directly to organic molecules: an sp³ carbon is tetrahedral, an sp² carbon is trigonal planar, and an sp carbon is linear. Ethene, C₂H₄, is described as planar because both carbons are sp² hybridised — all six atoms in the molecule lie in a single flat plane, which is exactly the geometry that allows its π bond to form by sideways overlap above and below that plane.

Isomerism

Structural isomers share a molecular formula but differ in how the atoms are connected: chain isomers differ in the carbon skeleton (butane vs methylpropane), positional isomers differ in where a functional group sits on the same skeleton (propan-1-ol vs propan-2-ol), and functional group isomers have entirely different functional groups from the same molecular formula (an alcohol and an ether can share a formula).

Stereoisomers share both molecular formula and the same atom-to-atom connections, differing only in spatial arrangement. Geometrical (cis/trans) isomerism occurs in alkenes because rotation around the C=C double bond is restricted by the π bond — the two substituents on each carbon are locked on one side or the other. Optical isomerism arises at a chiral centre: a carbon atom bonded to four different groups. A chiral centre produces two non-superimposable mirror-image structures, called enantiomers.

Common mistakes

• Confusing “addition” and “elimination” as opposites in the wrong way. Addition combines two reactants into one product; elimination is the reverse idea applied to a single molecule losing a small fragment — they aren’t simply forward and backward versions of the same reaction.
• Drawing a curly arrow starting from an atom instead of a bond or lone pair. The arrow always starts where the electron pair currently is.
• Mislabelling a mechanism by only naming the bond change, not the attacking species. “Substitution” alone is incomplete — nucleophilic and electrophilic substitution are different mechanisms entirely.
• Missing a chiral centre because a molecule “looks symmetric” at a glance. Check systematically: does that carbon have four genuinely different groups attached, not four different-looking bonds drawn.
• Treating skeletal formulas as omitting information. They omit only carbon and hydrogen-on-carbon atoms for compactness — every functional group and every other atom is still shown explicitly.

Quick revision checklist

• General, structural, displayed and skeletal formulas, and converting between them
• Homolytic vs heterolytic fission; free radical, nucleophile, electrophile
• The four mechanism names: free-radical substitution, electrophilic addition, nucleophilic substitution, nucleophilic addition
• Curly arrow notation, and where an arrow must start
• sp/sp²/sp³ hybridisation applied to organic molecules; planarity
• Structural isomerism: chain, positional, functional group
• Stereoisomerism: geometrical (cis/trans) and optical (chiral centres, enantiomers)

Related resources

• Chemical Bonding: Shapes and Intermolecular Forces — the hybridisation and orbital theory this resource applies to organic molecules
• Atomic Structure: Orbitals and Ionisation Energy — orbitals from first principles
• A Level Organic Chemistry: Naming, Mechanisms and Aromatic Shape — the A Level sequel, adding new functional groups and benzene’s shape
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.