1. Six Ways to Write a Formula
Chemists use six types of formula. Each one shows a different amount of detail. Here they are for butan-1-ol.
The simplest whole-number ratio of atoms of each element in a compound.
The actual number of atoms of each element in a molecule.
| Formula type | What it shows | Butan-1-ol |
|---|---|---|
| Empirical | Simplest whole-number ratio of atoms | C₄H₁₀O |
| Molecular | Actual number of atoms of each element | C₄H₁₀O |
| General | A formula for every member of a homologous series | CₙH₂ₙ₊₁OH |
| Structural | The arrangement of atoms, carbon by carbon, with no bonds drawn | CH₃CH₂CH₂CH₂OH |
| Displayed | Every atom and every bond | See the diagram below |
| Skeletal | Carbon chain as a zig-zag line, functional groups drawn in | See the diagram below |
Displayed formula
Every bond is drawn as a line.
Skeletal formula
- Each end of a line and each corner is a carbon atom.
- Hydrogen atoms on carbon are not shown.
- Atoms other than C and H (for example O, Cl) are drawn in, with any H atoms bonded to them.
2. Homologous Series
A family of compounds with the same functional group and the same general formula.
- Each member differs from the next by CH₂.
- Members have similar chemical properties, because they have the same functional group.
- Physical properties change gradually. For example, boiling point rises as the chain gets longer.
Series in Unit 2 and Unit 4
| Series | Functional group | Suffix or prefix | Example |
|---|---|---|---|
| Alkanes | C-C only | -ane | propane |
| Alkenes | C=C | -ene | but-1-ene |
| Halogenoalkanes | C-X | chloro-, bromo-, iodo- | 2-bromopropane |
| Alcohols | -OH | -ol | propan-2-ol |
| Aldehydes | -CHO | -al | propanal |
| Ketones | C=O | -one | propanone |
| Carboxylic acids | -COOH | -oic acid | ethanoic acid |
| Nitriles | -C≡N | -nitrile | propanenitrile |
| Amines | -NH₂ | -amine | ethylamine |
| Esters | -COO- | -yl -oate | methyl ethanoate |
3. IUPAC Naming Rules
You must name chains and rings with up to six carbon atoms. You must also draw a structure from its name.
- Find the longest carbon chain that contains the functional group. Name its stem: meth (1), eth (2), prop (3), but (4), pent (5), hex (6).
- Number the chain from the end that gives the functional group the lowest number. Then give the substituents the lowest numbers.
- Name the substituents (methyl, ethyl, chloro, bromo) in alphabetical order. Use di- or tri- for repeats. Put commas between numbers, and hyphens between numbers and letters.
- Use cyclo- for rings.
Worked examples: structure to name
| Structure | Name | Why |
|---|---|---|
| CH₃CH(CH₃)CH₂CH₂CH₃ | 2-methylpentane | Longest chain is 5 carbons. The methyl group is on C2. |
| C(CH₃)₄ | 2,2-dimethylpropane | Longest chain is 3 carbons. Two methyl groups are on C2. |
| CH₂=CHCHClCH₃ | 3-chlorobut-1-ene | The C=C gets the lowest number (1). Cl is then on C3. |
| CH₂BrCH₂Br | 1,2-dibromoethane | Two Br atoms, so di-. One on each carbon. |
| CH₃COCH₂CH₂CH₃ | pentan-2-one | 5 carbons. The C=O is on C2. |
| (CH₃)₂CHCH₂OH | 2-methylpropan-1-ol | The OH carbon is C1. The methyl group is on C2. |
| CH₃CH=CHCH₂CH₂CH₃ | hex-2-ene | 6 carbons. The C=C starts at C2. |
| C₆H₁₁OH (a six-carbon ring with OH) | cyclohexanol | A ring, so cyclo-. No number is needed for one OH group. |
| (CH₃)₂CHCH₂Br | 1-bromo-2-methylpropane | 3-carbon chain. Bromo before methyl (alphabetical order). |
| CH₃CH₂CH₂COOH | butanoic acid | 4 carbons, including the COOH carbon, which is always C1. |
2-methylbutan-2-ol: a 4-carbon chain. The OH and a methyl group are both on C2. Structure: CH₃C(OH)(CH₃)CH₂CH₃.
3-methylbut-1-ene: a 4-carbon chain. The C=C is between C1 and C2. A methyl group is on C3. Structure: CH₂=CHCH(CH₃)CH₃.
4. Mechanisms: How Chemists Show Electron Movement
A covalent bond can break in two ways.
Homolytic fission
Each atom gets one electron from the bond. This makes two radicals. A radical has an unpaired electron, shown as a dot, for example Cl•.
Heterolytic fission
One atom gets both electrons from the bond. This makes two ions, one positive and one negative.
Curly arrow rules
- A curly arrow shows the movement of a pair of electrons. A double-headed arrow means a pair.
- An arrow starts at a lone pair or at a covalent bond.
- It ends at an atom, or between two atoms where a new bond forms.
Radical mechanisms use no curly arrows
For a free radical mechanism, show each radical with a dot. Write a balanced equation for each step: initiation, propagation and termination. For example, for methane and chlorine:
- Initiation: Cl₂ → 2Cl•
- Propagation: Cl• + CH₄ → HCl + •CH₃, then •CH₃ + Cl₂ → CH₃Cl + Cl•
- Termination: •CH₃ + Cl• → CH₃Cl
See the full mechanism on the Alkanes page.
An electron-pair donor. It is attracted to a δ+ carbon atom. Examples: OH⁻, CN⁻, NH₃.
An electron-pair acceptor. It is attracted to regions of high electron density. Examples: H⁺, Br₂ when polarised, NO₂⁺.
5. Structural Isomers
Compounds with the same molecular formula but a different structural formula.
| Type | What changes | Examples |
|---|---|---|
| Chain | The carbon chain is arranged differently (straight or branched) | butane CH₃CH₂CH₂CH₃ and 2-methylpropane CH₃CH(CH₃)CH₃ (both C₄H₁₀) |
| Position | The functional group is in a different position on the same chain | propan-1-ol and propan-2-ol (both C₃H₈O); 1-bromobutane and 2-bromobutane (both C₄H₉Br) |
| Functional group | The functional group is different | propanal CH₃CH₂CHO and propanone CH₃COCH₃ (both C₃H₆O); ethanol CH₃CH₂OH and methoxymethane CH₃OCH₃ (both C₂H₆O) |
6. Stereoisomers and E-Z Isomerism
Compounds with the same structural formula but a different arrangement of the atoms in space.
E-Z isomerism is one type of stereoisomerism. It needs two things:
- Restricted rotation about the C=C double bond.
- Two different groups on each carbon of the C=C.
Cahn-Ingold-Prelog (CIP) priority rules
- Look at each carbon of the C=C on its own. The atom with the higher atomic number has the higher priority.
- If the two atoms are the same, compare the atoms joined to them, at the first point of difference. For example, CH₂CH₃ beats CH₃. Both start with C, but ethyl's next set (C, H, H) beats methyl's (H, H, H).
Z (zusammen, together)
The two higher-priority groups are on the same side of the C=C.
E (entgegen, opposite)
The two higher-priority groups are on opposite sides of the C=C.
Worked example 1: but-2-ene
On each carbon, CH₃ beats H.
Worked example 2: 2-chlorobut-2-ene, CH₃C(Cl)=CHCH₃
On C2, Cl beats CH₃ (atomic number 17 beats 6). On C3, CH₃ beats H. In the Z isomer, Cl and the C3 methyl group are on the same side.
Worked example 3: 3-methylpent-2-ene, CH₃CH=C(CH₃)CH₂CH₃
On C2, CH₃ beats H. On C3, both groups start with C, so use the second-atom rule: ethyl (C, H, H) beats methyl (H, H, H).
But-1-ene, CH₂=CHCH₂CH₃, has no E-Z isomers. C1 carries two H atoms, so it does not have two different groups.
Go deeper
These UK AQA pages cover the same ideas in more detail. They may include content that is not in 9620.
7. Practice Questions
Show Answer & Mark Scheme
- 1-bromobutane, CH₃CH₂CH₂CH₂Br (1 mark)
- 2-bromobutane, CH₃CHBrCH₂CH₃ (1 mark)
- 1-bromo-2-methylpropane, (CH₃)₂CHCH₂Br (1 mark)
- 2-bromo-2-methylpropane, (CH₃)₃CBr (1 mark)
Show Answer & Mark Scheme
- There is restricted rotation about the C=C. (1 mark)
- In pent-2-ene, each carbon of the C=C has two different groups. In pent-1-ene, C1 has two H atoms. (1 mark)
Show Answer & Mark Scheme
- CH₃CH(CH₃)CH₂OH is 2-methylpropan-1-ol. (1 mark)
- CH₃COCH₂CH₃ is butan-2-one. (1 mark)
Show Answer & Mark Scheme
CH₂ (1 mark). Hex-1-ene is C₆H₁₂, and the simplest ratio is 1 : 2.
Show Answer & Mark Scheme
- No. (1 mark)
- One carbon of the C=C has two identical CH₃ groups. (1 mark)