Ch 1 · Formulae, functional groups and the naming of organic compounds · AS & A Level Chemistry 9701

Topic 1 of 4 in An introduction to A Level organic chemistry — AS & A Level Chemistry 9701 — 48 lessons.

Introduction

The AS course fixed the rules for naming and drawing organic compounds; A2 organic chemistry now applies them to the complete range of functional groups in the syllabus, including acyl chlorides, amides, phenols and, for the first time, aromatic rings. Examiners expect the same precision at every level, so a name or formula that is ambiguous, or that ignores the numbering rules, loses the mark even when the underlying chemistry is understood.

Key ideas — Functional groups still dictate behaviour

A functional group is the atom or group of atoms responsible for a molecule's characteristic physical and chemical reactions, regardless of chain length. Every carboxylic acid, for example, reacts with a carbonate to release carbon dioxide because each contains -COOH, whether the rest of the molecule is a two-carbon or a twelve-carbon chain. The A2 syllabus adds several new functional groups to those met at AS: the halogenoarene C_6H_5-X, the phenol -OH attached directly to a benzene ring, the acyl chloride -COCl, the amide -CONH_2, and the aromatic ring itself, C_6H_5- (phenyl), each with its own characteristic reactivity that later topics build on.

🧑‍🏫 Teacher's explanation

Think of a functional group as a molecule's 'personality'. Imagine a crowd of people walking down a street; most people look similar (the carbon chain), but if one person is wearing a bright neon sign (the functional group), that sign is what everyone notices and reacts to. The length of the carbon chain might change how 'heavy' or 'sticky' the molecule is, but the functional group determines how it behaves in a chemical reaction.

📖 Definitions

🔍 Worked example — Easy

Identify the functional group in the molecule ceCH3CH2CH2OH and state its class.

🔍 Worked example — Standard

Draw the structural formula for 2-chlorobutane.

🔍 Worked example — Exam-style

Calculate the molar mass (M_r) of benzoic acid, ceC6H5COOH. (Use C=12, H=1, O=16)

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — General, structural, displayed and skeletal formulae

The four formula types from AS extend to every new class of compound. For phenylethanamide, C_6H_5NHCOCH_3: the structural formula groups atoms without showing every bond; the displayed formula shows every atom and every covalent bond, including the six carbons and delocalised bonding of the ring drawn as a hexagon with an inscribed circle or alternating double bonds; the skeletal formula shows the ring as a plain hexagon (with the circle or alternating bonds to represent delocalisation) and the chain as a zig-zag line, with the -NHCOCH_3 group shown explicitly and all C–H bonds omitted. A general formula, such as mathrmC_nH_2n+1COCl for acyl chlorides, still gives the simplest algebraic formula for a homologous series.

🧑‍🏫 Teacher's explanation

Think of organic formulae like different ways of describing a building. A general formula is like saying 'a three-story house with a front door'; it tells you the basic structure but not the exact layout. A structural formula is like a floor plan showing which rooms connect to which; it shows the connectivity but skips the fine details like the exact position of every light switch.

📖 Definitions

🔍 Worked example — Easy

Draw the displayed formula for ethanol, C_2H_5 extOH.

🔍 Worked example — Standard

A compound has the general formula C_nH_2n+1COCl. If n=3, determine the molecular formula and write its structural formula.

🔍 Worked example — Exam-style

Identify the functional group in the molecule C_6H_5NHCOCH_3 and name the class of compound.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Naming aliphatic compounds, including rings

The AS rules apply unchanged: find the longest chain containing the principal functional group, number it to give that group the lowest locant, and list substituents alphabetically with locants, using di-, tri- and tetra- for repeats. Two extensions matter at A2. First, simple ring compounds are named with the prefix cyclo-, treating the ring as the parent chain, e.g. cyclohexanol (mathrmC_6H_11OH) or 1,2-dichlorocyclopentane. Second, when a compound has more than one functional group, the principal group (the one giving the suffix) is chosen by a fixed seniority order — carboxylic acid > acyl chloride > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene/alkane — with any lower-ranked group named as a prefix instead, e.g. 4-hydroxybutanoic acid has -COOH as the suffix and -OH as the prefix "hydroxy-".

🧑‍🏫 Teacher's explanation

Think of naming organic molecules like giving someone directions to a specific house in a complex city. The 'parent chain' is the main road, and the 'substituents' (the extra bits attached) are the specific house numbers and names. To avoid confusion, everyone must follow the same rules so that one name always corresponds to exactly one structure.

📖 Definitions

🔍 Worked example — Easy

Name the following compound: CH_3CH(Cl)CH_2CH_2OH

🔍 Worked example — Standard

Provide the IUPAC name for a five-membered carbon ring with one hydroxyl group and one bromine atom on adjacent carbons.

🔍 Worked example — Exam-style

Name the molecule: CH_3CH_2COCl

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Naming simple aromatic molecules

Benzene, C_6H_6, is the parent ring for aromatic nomenclature. A single substituent is named directly onto "benzene", e.g. chlorobenzene, nitrobenzene, methylbenzene. Several common derivatives keep retained names that Cambridge accepts in place of the fully systematic form: phenol (C_6H_5OH), phenylamine (C_6H_5NH_2), benzaldehyde (C_6H_5CHO) and benzoic acid (C_6H_5COOH). With two or more substituents, the ring carbons are numbered to give the lowest set of locants, and substituents are listed alphabetically, e.g. 1-chloro-4-nitrobenzene, not 4-chloro-1-nitrobenzene. When benzene is itself a substituent on a longer chain, it is named as the phenyl group, e.g. 2-phenylethanol, C_6H_5CH_2CH_2OH.

🧑‍🏫 Teacher's explanation

Think of benzene as a hexagonal dance floor where six carbon atoms are locked in a circular formation. Unlike regular alkanes where carbons form long chains, benzene is a ring with a special 'cloud' of electrons shared across the whole structure. This makes it very stable, but also very reactive in specific ways.

Ch 1 · Formulae, functional groups and the naming of organic compounds · AS & A Level Chemistry 9701

All chapters in An introduction to A Level organic chemistry