Lesson 1 of 12
Formulae and names
A formula must show which atoms are joined, not just how many there are.
A-Level 8873, revised syllabus (2026-2027)
I can read and name the required organic families and formula representations.Syllabus
Syllabus 8873, 9(a). Read and name the required organic families and formula representations.
I can use organic reaction terminology.Syllabus
Syllabus 8873, 9(b). Use organic reaction terminology.
Read and name an organic structure
A formula must show which atoms are joined, not just how many there are.
An organic structure is a map of which atoms are joined. A functional group is the atom or group of atoms that gives a compound its typical reactions. First find the functional group, then the carbon chain. The same molecular formula can describe compounds with different connections, and so different chemistry.
| Representation | What it tells you |
|---|---|
| Empirical: CH2O | The simplest whole-number atom ratio, C:H:O = 1:2:1. |
| Molecular: C3H6O3 | The actual numbers of atoms in one molecule. |
| Structural: CH3CH(OH)CO2H | The middle carbon bears OH; the final carbon belongs to COOH. Brackets keep the connectivity unambiguous. |
| Displayed | Every atom and bond is shown, including each C-H and O-H bond. |
| Skeletal | Carbon atoms are implicit at line ends and corners; enough attached hydrogen atoms are implied to give each carbon four bonds. Heteroatoms and their attached hydrogens, such as OH, are shown. |
To turn a condensed formula into a displayed formula, draw the carbon chain first. Then draw out each bracketed group where it is attached. Write COOH as C(=O)-O-H and CHO as C(=O)-H. Finally draw every C-H bond and check the bonds: four for each carbon, two for each oxygen and one for each hydrogen. In a skeletal formula, use the same check to find the hidden C-H bonds.
A displayed formula keeps every atom visible
Three carbons form a chain. The first has three hydrogens; the second has one hydrogen and an OH group; the third has a double-bonded oxygen and an OH group. There are three carbons, six hydrogens and three oxygens.
The same molecule in skeletal form
The left endpoint is CH3, the middle corner is CH attached to an explicit OH, and the right corner is the carboxyl carbon with C=O and OH. It represents the same lactic-acid connectivity as the displayed formula.
| Class and general structure | Example and naming cue |
|---|---|
| Alkane: CnH2n+2 for an acyclic saturated hydrocarbon | CH3CH3, ethane; suffix -ane. |
| Alkene: CnH2n for an acyclic hydrocarbon with one C=C | CH2=CHCH3, propene; locate the C=C and use -ene. |
| Benzene: C6H6, a six-carbon aromatic ring | Use a hexagon containing a circle to represent the delocalised ring. A fully displayed benzene ring is not required. |
| Halogenoalkane: R-X; CnH2n+1X for a saturated acyclic monohalogen compound | CH3CH2Br, bromoethane; fluoro-, chloro-, bromo- or iodo- gives the substituent. |
| Alcohol: R-OH; CnH2n+2O for a saturated acyclic monohydric alcohol | CH3CH(OH)CH3, propan-2-ol; number the OH position. |
| Aldehyde: R-CHO; CnH2nO for a saturated acyclic monoaldehyde | CH3CHO, ethanal; the carbonyl carbon has at least one H, and its carbon counts in the chain. |
| Ketone: R-CO-R'; CnH2nO for a saturated acyclic monoketone | CH3COCH3, propanone; the carbonyl carbon is bonded to two carbon groups. |
| Carboxylic acid: R-COOH; CnH2nO2 for a saturated acyclic monocarboxylic acid | CH3COOH, ethanoic acid; the COOH carbon is carbon 1. |
| Ester: R-COO-R'; CnH2nO2 for a saturated acyclic monoester | CH3COOCH2CH3, ethyl ethanoate; name the O-attached alkyl group first, then the acid-derived -oate part. |
| Amine: R-NH2 for a primary amine; N can also carry further carbon groups | CH3CH2NH2, ethylamine (ethanamine). For a saturated acyclic monoamine, CnH2n+3N. |
| Amide: R-CONH2; N-substituted forms include R-CONHR' | CH3CONH2, ethanamide; C=O is directly attached to N. A saturated acyclic monoamide has CnH2n+1NO. |
| Amino acid: contains both NH2 and COOH; an alpha-amino acid is H2N-CH(R)-COOH | H2NCH2COOH, aminoethanoic acid; the amino group is on the carbon next to COOH. For a saturated acyclic compound with one of each group: CnH2n+1NO2. |
Here R and R' stand for groups that contain carbon. They do not have to be the same. In methanal and methanoic acid, the group on the carbonyl carbon is H instead. General formulae only work under their stated conditions. Rings, extra double bonds or extra functional groups change them. A formula alone does not tell you the family: propanal and propanone are both C3H6O.
To name a simple compound, choose the parent chain that contains the main functional group. Number it so that the group or the double bond gets a low number. The stems meth-, eth-, prop-, but-, pent- and hex- mean 1 to 6 carbons. Show the branches and their positions. CH3CH(CH3)CH2OH is 2-methylpropan-1-ol: the carbon joined to OH is carbon 1, so the methyl branch is on carbon 2.
| Alcohol class | Carbon groups attached to the OH-bearing carbon | Example |
|---|---|---|
| Primary | One carbon group; methanol is conventionally treated with primary alcohols. | CH3CH2OH |
| Secondary | Two carbon groups. | CH3CH(OH)CH3 |
| Tertiary | Three carbon groups. | (CH3)3COH |
Check your understanding
What is the name of CH3CH2COOCH3?
Show the answer
Methyl propanoate
The CH3 on the oxygen gives methyl. CH3CH2COO- has three carbons including the carbonyl carbon, so it is propanoate.