Structural formula explained for iGCSE Chemistry
A structural formula shows every atom in an organic molecule and the bonds that connect them. For Cambridge iGCSE Chemistry, you need to be able to draw and recognise the structural formulae of the first four members of the alkanes, alkenes, alcohols and carboxylic acids. The structural formula sits between the molecular formula (which only counts atoms) and the displayed formula (which shows every single bond as a line).
This guide covers the three ways organic molecules are represented, how to draw each type, what isomerism looks like on a structural formula, and where students lose marks on the homologous series questions.
Three formula types
Molecular, structural and displayed formulae each give a different level of detail. You need all three for iGCSE.
Show every carbon's neighbours
A structural formula must show what is bonded to each carbon. CH3CH2OH is structural, C2H6O is only molecular.
Distinguishes isomers
Structural formulae let you tell butane apart from methylpropane, even though both have molecular formula C4H10.
The three types of chemical formula
Cambridge iGCSE Chemistry distinguishes three ways to represent an organic molecule. The molecular formula just counts atoms. The structural formula shows the order of atoms and which groups attach to each carbon. The displayed formula draws out every atom and every bond.
Knowing which is which matters because exam questions often specify: "Draw the structural formula" or "Draw the displayed formula". Giving the wrong type loses the mark even if the molecule is correct.
| Formula type | What it shows | Example for ethanol |
|---|---|---|
| Molecular | Total atom count only | C2H6O |
| Structural | Order of atoms and groups attached to each carbon | CH3CH2OH |
| Displayed | Every atom and every bond drawn out as lines | H–C(H)(H)–C(H)(H)–O–H drawn fully |
Read the question carefully Cambridge iGCSE often asks for a specific formula type. "Structural formula" means CH3CH2OH style. "Displayed formula" means draw every bond. "Molecular formula" means C2H6O. Drawing the wrong type wastes time and loses the mark.
How to write a structural formula
A structural formula is written along a single line. Start at one end of the carbon chain and work to the other, writing each carbon and the atoms attached to it. Hydrogens are written immediately after the carbon they are bonded to, usually as subscripts (CH3, CH2). Functional groups go at the end (OH, COOH) or in the middle of the chain.
The key rule is that every carbon must form four bonds, every hydrogen forms one bond, every oxygen forms two bonds. If your structural formula does not add up to the right number of bonds per atom, it is wrong.
Structural formulae of the first four alkanes
Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. They contain only single covalent bonds between carbons. The first four are methane, ethane, propane and butane. You should be able to write the structural formula for each from memory.
| Name | Molecular formula | Structural formula |
|---|---|---|
| Methane | CH4 | CH4 |
| Ethane | C2H6 | CH3CH3 |
| Propane | C3H8 | CH3CH2CH3 |
| Butane | C4H10 | CH3CH2CH2CH3 |
Structural formulae of the first four alkenes
Alkenes contain one carbon-to-carbon double bond and have the general formula CₙH₂ₙ. The double bond is shown in a structural formula by writing CH2=CH or CH=CH, depending on which two carbons share it. The position of the double bond can vary in longer alkenes (this is positional isomerism).
Note that there is no methene, because methene would only have one carbon and you need two carbons to share a double bond.
| Name | Molecular formula | Structural formula |
|---|---|---|
| Ethene | C2H4 | CH2=CH2 |
| Propene | C3H6 | CH3CH=CH2 |
| But-1-ene | C4H8 | CH3CH2CH=CH2 |
| But-2-ene | C4H8 | CH3CH=CHCH3 |
Structural formulae of the first four alcohols
Alcohols contain a hydroxyl group (–OH) attached to a carbon chain. They have the general formula CₙH₂ₙ₊₁OH. The OH group is written at the end of the structural formula, attached to the appropriate carbon. As with alkenes, the OH can sit on different carbons in longer chains, giving positional isomers.
| Name | Molecular formula | Structural formula |
|---|---|---|
| Methanol | CH4O | CH3OH |
| Ethanol | C2H6O | CH3CH2OH |
| Propan-1-ol | C3H8O | CH3CH2CH2OH |
| Butan-1-ol | C4H10O | CH3CH2CH2CH2OH |
Isomerism on a structural formula
Two molecules are structural isomers if they have the same molecular formula but a different structural formula. At iGCSE you mainly meet two types: Chain isomerism (the carbon skeleton is arranged differently) and positional isomerism (the functional group sits on a different carbon).
Butane and methylpropane both have the molecular formula C4H10. Their structural formulae differ. Butane is CH3CH2CH2CH3. Methylpropane is CH3CH(CH3)CH3. The bracket notation shows the methyl group branching off the middle carbon.
Reading bracket notation in structural formulae A bracket like (CH3) indicates a side group attached to the carbon written immediately before it. So CH3CH(CH3)CH3 means: A CH3 group, then a CH carbon with a CH3 side group, then another CH3 group. Brackets save space without losing detail.
Worked example: Drawing the structural formula of propanoic acid
Propanoic acid is a carboxylic acid with three carbons and a –COOH functional group. The molecular formula is C3H6O2.
Step 1: Identify the carbon chain. Three carbons means propan- as the stem.
Step 2: Place the functional group. The –COOH carbon counts as carbon 1 of the chain.
Step 3: Fill in the rest of the chain. The remaining two carbons are CH3 and CH2.
Step 4: Write left to right. The structural formula is CH3CH2COOH. Notice the COOH group is always written at the end of the chain because the acid carbon must be at carbon 1.
Where students lose marks
Cambridge examiner reports flag the same handful of slips on structural formula questions every year. Most are about precision rather than missing knowledge.
Common mistakes that cost easy marks Drawing a displayed formula when the question asked for structural (or vice versa). Forgetting that carbon must always have four bonds. Writing CH3-CH3 with hyphens when the structural formula needs no hyphens (CH3CH3). Putting the OH group in the middle of a chain when it sits at the end (e.g. CH3OHCH2CH3 is wrong, the correct propan-2-ol is CH3CH(OH)CH3). Writing the molecular formula as the structural formula.
Key facts to memorise for the exam
- Molecular formula: Total atom count only (C2H6O)
- Structural formula: Order of atoms and groups along the chain (CH3CH2OH)
- Displayed formula: Every atom and every bond drawn out
- Carbon always forms four bonds, hydrogen one, oxygen two
- Alkanes: General formula CₙH₂ₙ₊₂, all single bonds
- Alkenes: General formula CₙH₂ₙ, one carbon-carbon double bond
- Alcohols: General formula CₙH₂ₙ₊₁OH, hydroxyl functional group
- Structural isomers share a molecular formula but differ in structure
