29.1 Formulas, functional groups and nomenclature
- Syllabus
- 9701–2028–2029
- Topic
- 29.1
- Level
- A2
Learning objectives
29.1.1Compounds in the table on page 47 contain• Understand: the compounds in the table on page 47 contain a functional group which dictates their physical/chemical properties29.1.2Interpret/use the general, structural,• Interpret/use: the general, structural, displayed and skeletal formulas of the classes of compound stated in the table on page 4729.1.3Systematic nomenclature of simple aliphatic• Understand and use systematic nomenclature of simple aliphatic organic molecules (including cyclic compounds containing a single ring of up to six carbon atoms) with functional groups detailed in the table on page 47 , up to six carbon atoms (six plus six for esters and amides, straight chains only for esters and nitriles)29.1.4Systematic nomenclature of simple aromatic• Understand and use systematic nomenclature of simple aromatic molecules with one benzene ring and one or more simple substituents, e.g. 3-nitrobenzoic acid or 2,4,6-tribromophenol
A functional group's connectivity produces characteristic properties
A functional group is the connected atom arrangement that produces a class's characteristic physical and chemical properties. The rest of the carbon skeleton modifies those properties, but does not replace the functional group's characteristic chemistry.
| Page-47 class | Structural feature | Consequence that distinguishes the class |
|---|---|---|
| arene | delocalised benzene π system | characteristic aromatic rather than alkene-like reaction behaviour |
| phenol | OH directly bonded to an arene ring | ring–OH interaction gives behaviour different from an aliphatic alcohol |
| acyl chloride | polar C=O bonded to Cl | reactive carbonyl derivative with characteristic substitution chemistry |
| amine | N lone pair not directly beside C=O | can accept H⁺ and behave as a base |
| amide | N directly bonded to C=O | N lone pair is delocalised toward the carbonyl, so it is much less available than in an amine |
| amino acid | amine and carboxyl groups in one molecule | can show both basic and acidic behaviour |
Within one homologous class, increasing carbon-chain size can alter boiling point and solubility through intermolecular-force balance. To predict a molecule, identify both the functional group and the size or shape of its carbon framework.
One atom is not a functional-group diagnosis: OH on an arene is phenol, but OH on an sp³ carbon is an alcohol; nitrogen beside C=O is an amide, not an amine.
Translate between general, structural, displayed and skeletal formulae without changing connectivity
| Formula style | What it communicates | Example use for an acyl chloride |
|---|---|---|
| general | the family pattern across different members | RCOCl |
| structural | atom order and grouped connectivity | CH₃CH₂COCl |
| displayed | every atom and every covalent bond in the shown non-benzene structure | show CH₃–CH₂–C with C=O and C–Cl, including all C–H bonds |
| skeletal | carbon framework as lines; C atoms and C-bound H atoms are omitted | a two-segment chain ending at C(=O)Cl |
In a skeletal formula, every unlabelled line end or vertex is carbon; add enough implied H atoms to give carbon four bonds. Heteroatoms such as N, O and Cl are written, and hydrogens attached to heteroatoms are shown when present.
A valid translation preserves carbon count, functional-group connectivity, multiple bonds and overall charge. For CH₃CH₂COCl, every style must still show three carbons and Cl directly bonded to the carbonyl carbon.
A general formula describes a class, not one unique molecule. A molecular formula gives atom counts but may hide connectivity and isomerism; neither can replace a structural formula when bond order matters.
Name bounded aliphatic structures with one systematic workflow
| Step | Naming decision |
|---|---|
| 1 | identify the principal functional group and its suffix |
| 2 | choose the longest parent chain containing that group, or the single parent ring |
| 3 | number from the end that gives the principal group the lowest locant, then the lowest locant set |
| 4 | identify substituents, positions and multiplicative prefixes; alphabetise distinct prefixes |
| 5 | assemble locants, prefixes, parent, unsaturation and suffix with correct punctuation |
| Syllabus boundary | Required range |
|---|---|
| ordinary aliphatic molecules | up to 6 carbon atoms |
| cyclic molecules | one ring only, containing up to 6 carbon atoms |
| esters and amides | up to 6 + 6 carbon atoms in the two carbon-containing parts |
| esters and nitriles | straight chains only |
| secondary and tertiary amines | recognise structures; their naming is not required by the page-47 table |
| Structure | Key parent decision | Systematic name |
|---|---|---|
| CH₃CH₂COCl | 3-carbon acyl chain | propanoyl chloride |
| CH₃CH(NH₂)CO₂H | carboxyl carbon is C1; amino on C2 | 2-aminopropanoic acid |
| CH₃CH₂CONHCH₃ | propanamide parent; methyl substituent is on N | N-methylpropanamide |
| CH₃COOCH₂CH₃ | alcohol-derived ethyl first; acid-derived ethanoate second | ethyl ethanoate |
The parent is not simply the visually longest line if it omits the principal group. In nitriles the C≡N carbon belongs to the parent count; in esters the two sides have different naming roles and are not joined into one chain.
Name one-ring aromatic molecules from the parent group and lowest locants
| Step | Aromatic naming move |
|---|---|
| 1 | choose benzene or the retained functional parent, such as phenol or benzoic acid |
| 2 | assign the parent-group ring carbon as C1 when the parent fixes it |
| 3 | number around the ring in the direction giving the lowest complete set of locants |
| 4 | add substituent locants and di-, tri-, etc.; alphabetise different substituent prefixes where needed |
| Structure description | Numbering | Name |
|---|---|---|
| benzoic acid with NO₂ two ring bonds from C1 | CO₂H-bearing carbon is C1; NO₂ at C3 | 3-nitrobenzoic acid |
| phenol with Br on both adjacent carbons and the opposite carbon | OH-bearing carbon is C1; Br at C2, C4 and C6 | 2,4,6-tribromophenol |
Keep the single benzene ring in its conventional ring form and show each substituent bond at the correct vertex. The spatial drawing and locants must encode the same relative positions.
Do not renumber phenol or benzoic acid from an arbitrary substituent, and do not optimise only the first locant while ignoring the full ordered locant set. This objective is limited to one benzene ring with simple substituents.