Formulas, functional groups and the naming of organic compounds

· Functional group = atom/group of atoms that dictates the physical and chemical properties of an organic compound.
· In A Level organic chemistry, focus on recognising and naming compounds containing: arene, halogenoarene, phenol, acyl chloride, secondary/tertiary amine, amide, and amino acid groups.
· R = alkyl group; Ar = aromatic group/benzene ring; X = halogen atom.
· Be able to interpret general formula, structural formula, displayed formula, and skeletal formula.
· For compounds containing a benzene ring, a fully displayed formula of the benzene ring is not expected.

This table summarises how functional groups are represented in organic molecules. It is useful for practising quick identification of groups that control chemical behaviour and naming. Source

Arene and halogenoarene

· Arene = aromatic compound containing a benzene ring.
· Benzene is represented as a six-membered ring with a delocalised π system.
· Halogenoarene = benzene ring with a halogen substituent, shown as Ar–X.
· Example: chlorobenzene when X = Cl.
· Naming: use benzene as the parent if there is one benzene ring with simple substituents.

This diagram shows benzene using skeletal formula notation. It represents the aromatic ring used in arenes and aromatic naming questions. Source

Phenol

· Phenol = benzene ring directly bonded to –OH.
· Functional group: phenol group, written as Ar–OH.
· Do not confuse phenol with an alcohol: in phenol, –OH is attached directly to the benzene ring.
· Substituted phenols are named using numbered ring positions, e.g. 2,4,6-tribromophenol.
· Use the lowest possible locants for substituents on the ring.

This image shows the phenol functional group clearly. It highlights the key exam point that the hydroxyl group is bonded directly to an aromatic ring. Source

Acyl chloride

· Acyl chloride functional group = –COCl.
· General structure: RCOCl.
· Example: propanoyl chloride.
· Naming pattern: replace the –oic acid ending of the related carboxylic acid with –oyl chloride.
· Example: propanoic acid → propanoyl chloride.
· The carbonyl C=O and C–Cl bond must both be shown clearly in structural/displayed formulae.

This structure shows the key acyl chloride group, –COCl. It helps distinguish acyl chlorides from carboxylic acids, esters, and amides. Source

Amines and amides

· Secondary amine = nitrogen bonded to two carbon groups and one hydrogen, often shown as R₂NH.
· Tertiary amine = nitrogen bonded to three carbon groups, often shown as R₃N.
· Naming of secondary and tertiary amines is not required in this topic.
· Amide functional group = –CONH₂, –CONHR, or –CONR₂.
· Example: propanamide.
· Naming simple amides: replace –oic acid with –amide.
· Example: propanoic acid → propanamide.
· Amides contain a carbonyl group bonded directly to nitrogen, so always look for C(=O)–N.

This diagram compares the different amide types. It is useful for recognising that all amides contain a carbonyl group attached directly to nitrogen. Source

Amino acids

· Amino acid = molecule containing both an amine group and a carboxyl group.
· General structure: H₂N–CHR–COOH.
· Example: 2-aminoethanoic acid.
· Naming: identify the carboxylic acid as the main functional group and number from the –COOH carbon.
· The amino group is named as an amino substituent, e.g. 2-amino-.
· For amino acids, make sure both –NH₂ and –COOH are shown clearly in displayed or structural formulae.

This image shows the general structure of an amino acid. It clearly identifies the two functional groups required for CIE A-Level recognition and naming. Source

Formula types you must recognise

· General formula = uses R, Ar, or X to represent variable groups.
· Structural formula = shows how atoms/groups are connected, e.g. RCOCl, RCONH₂, Ar–OH.
· Displayed formula = shows all atoms and all bonds.
· Skeletal formula = carbon atoms are shown as line ends/vertices; hydrogen atoms on carbon are usually not shown.
· In skeletal formulae, assume each carbon has enough hydrogens to make four bonds.
· Heteroatoms such as O, N, Cl, Br, and I must be shown.

Systematic nomenclature: aliphatic compounds

· Aliphatic molecules = organic molecules that are not aromatic.
· You must name simple aliphatic molecules with relevant functional groups up to six carbon atoms.
· Cyclic compounds may contain a single ring of up to six carbon atoms.
· For esters and amides, naming may involve six carbons plus six carbons.
· For esters and nitriles, only straight chains are required.
· Naming method: choose the longest appropriate parent chain, number to give the functional group/substituents lowest locants, then add prefixes/suffixes correctly.
· Use commas between numbers and hyphens between numbers and words, e.g. 2,4,6-tribromophenol.

Systematic nomenclature: aromatic compounds

· Aromatic molecules in this topic contain one benzene ring.
· Name simple substituents using position numbers around the benzene ring.
· Use lowest locants for multiple substituents.
· Examples required by the syllabus include 3-nitrobenzoic acid and 2,4,6-tribromophenol.
· Treat groups such as –OH, –COOH, –NO₂, halogens, and alkyl groups as substituents when naming aromatic molecules.
· The benzene ring may be shown using skeletal formula, but a fully displayed benzene ring is not required.

High-yield exam tips

· Always identify the functional group first before naming the molecule.
· Check whether the compound is aliphatic or aromatic.
· For aromatic compounds, number the ring to give the lowest set of locants.
· For acyl chlorides and amides, focus on the carbonyl carbon as part of the main functional group.
· Do not confuse phenol with an alcohol or halogenoarene with halogenoalkane.
· In skeletal formulae, remember that carbon and hydrogen atoms may be implied, but heteroatoms must be shown.