A Level Organic Chemistry: Naming, Mechanisms and Aromatic Shape

This guide covers subtopics 29.1, Formulas, functional groups and the naming of organic compounds, 29.2, Characteristic organic reactions, and 29.3, Shapes of aromatic organic molecules; σ and π bonds, from Topic 29, An introduction to A Level organic chemistry, of Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. This is A Level content — the direct sequel to Organic Mechanisms: An Introduction at AS Level, now introducing the functional groups and shape ideas that Topics 30–34 build on.

Before studying this

This resource assumes nomenclature and mechanism vocabulary (nucleophile, electrophile, curly arrows, addition, substitution) from Organic Mechanisms: An Introduction, and orbital hybridisation and σ/π bonding for simple alkenes from Chemical Bonding: Shapes and Intermolecular Forces.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — A Level, Topic 29

29.1 Formulas, functional groups and the naming of organic compounds — understanding that new A Level functional groups (arene, halogenoarene, phenol, acyl chloride, secondary/tertiary amine, amide, amino acid) dictate physical/chemical properties; interpreting and using their general, structural, displayed and skeletal formulas; naming simple aliphatic molecules (up to six carbons, six plus six for esters/amides) with these groups; naming simple aromatic molecules with one benzene ring and simple substituents.

29.2 Characteristic organic reactions — understanding and using the terms electrophilic substitution and addition-elimination.

29.3 Shapes of aromatic organic molecules; σ and π bonds — describing and explaining the shape of benzene and other aromatic molecules, including sp² hybridisation, in terms of σ bonds and a delocalised π system.

New functional groups at A Level

Five functional groups appear for the first time at A Level, each introduced fully in its own topic later, but worth being able to recognise and name immediately:

Functional group	Example	Name
Arene (benzene ring)	C₆H₆	benzene
Halogenoarene	C₆H₅Cl	chlorobenzene
Phenol	C₆H₅OH	phenol
Acyl chloride	CH₃CH₂COCl	propanoyl chloride
Amide	CH₃CH₂CONH₂	propanamide
Amino acid	HOOCCH(NH₂)H	2-aminoethanoic acid

Naming aliphatic compounds follows the same systematic rules met at AS Level, extended up to six carbons in the main chain (six plus six for esters and amides, counting both sides of the linking group; straight chains only for esters and nitriles). Naming aromatic compounds adds numbering around the benzene ring to locate substituents, for example 3-nitrobenzoic acid (a benzoic acid with a nitro group at position 3) or 2,4,6-tribromophenol (a phenol with bromine at positions 2, 4 and 6 — the exact positions Phenol: Reactions and Acidity explains phenol substitutes at directly).

Secondary and tertiary amines can be recognised and drawn, but naming them systematically is not required at this level.

Electrophilic substitution and addition-elimination

Two new mechanism names are introduced here, ahead of seeing them applied in full in Topics 30 and 33:

Electrophilic substitution is a mechanism where an electrophile attacks an electron-rich system, and — unlike electrophilic addition at a C=C double bond — a hydrogen atom (or another group) is displaced rather than a double bond being broken. This is the mechanism benzene undergoes (Arenes and Halogenoarenes: Electrophilic Substitution and Reactivity covers it fully): the delocalised ring system is disrupted only momentarily, then restored, which is exactly why benzene reacts by substitution rather than addition — see below.

Addition-elimination is a two-step mechanism in which a nucleophile first adds to an electrophilic carbon (forming a tetrahedral intermediate, as in nucleophilic addition), and then a leaving group is eliminated to restore a C=O double bond. This is the mechanism acyl chlorides undergo (Carboxylic Acids and Acyl Chlorides covers it fully) — recognisably built from the same two mechanistic steps (nucleophilic addition, then elimination) already met separately at AS Level.

The shape of benzene

Each carbon in benzene is sp² hybridised: one 2s and two 2p orbitals combine to form three equal-energy sp² hybrid orbitals, arranged trigonally (120° apart) in a plane. Two of these on each carbon form σ bonds to neighbouring carbons around the ring, and the third forms a σ bond to a hydrogen atom — giving benzene’s flat, hexagonal, planar skeleton with 120° bond angles.

The unhybridised p orbital left on each carbon (perpendicular to the ring’s plane) overlaps sideways with the p orbitals on both neighbouring carbons, all the way around the ring. This sideways overlap doesn’t produce six separate, localised π bonds alternating around the ring (the old “cyclohexatriene” picture); instead, the six p electrons delocalise into a continuous system of electron density spread above and below the ring as a whole — one delocalised π system rather than three alternating double bonds.

This delocalisation is why benzene’s actual carbon-carbon bond length (uniform all the way around the ring) is intermediate between a typical C–C single bond and a C=C double bond, why the ring is unusually stable (aromatic stabilisation — the delocalised electrons are at lower energy than three separate, alternating π bonds would be), and — as Arenes and Halogenoarenes: Electrophilic Substitution and Reactivity explains — why reactions that would disrupt this delocalised system are strongly disfavoured in comparison to reactions that leave it intact.

Common mistakes

Drawing benzene with alternating single and double bonds and treating it like three separate alkenes. This localised picture doesn’t match benzene’s real, uniform bond lengths or its resistance to the addition reactions typical alkenes undergo — always represent it (or reason about it) using the delocalised ring model.

Confusing addition-elimination with simple nucleophilic addition. Addition-elimination has a second step: after the nucleophile adds, a leaving group is eliminated to restore the C=O double bond — a genuinely different overall outcome (substitution of one group for another) from a carbonyl’s simple addition reaction with HCN, which keeps all atoms and adds across the double bond.

Missing that the new AS-recognisable naming rules extend, rather than replace, what was learned at AS Level. The same IUPAC logic (longest chain, lowest locants, functional-group priority) applies — only the list of recognisable functional groups and ring-numbering for aromatics is new.

Quick revision checklist

• Five new A Level functional groups: arene, halogenoarene, phenol, acyl chloride, amide (plus amino acid, containing two groups at once)
• Electrophilic substitution: electrophile attacks, a group is displaced, the ring system is preserved
• Addition-elimination: nucleophile adds, then a leaving group is eliminated, restoring C=O
• Benzene carbon: sp² hybridised, 120° bond angles, planar
• σ bonds form the hexagonal skeleton; the unhybridised p orbitals overlap into one delocalised π system above and below the ring

Related resources

• Organic Mechanisms: An Introduction — the AS-level mechanism vocabulary this topic extends
• Chemical Bonding: Shapes and Intermolecular Forces — hybridisation and σ/π bonding for simple molecules
• Arenes and Halogenoarenes: Electrophilic Substitution and Reactivity — the electrophilic substitution mechanism applied in full
• Optical Isomerism and Chirality — the remaining subtopic of Topic 29
• 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.