From C7 Organic Chemistry you already know alkanes and alkenes, functional groups and homologous series, and that carbon forms four bonds. A-Level adds the full representation toolkit — especially skeletal formulae — systematic IUPAC naming, the three types of structural isomerism plus E–Z stereoisomerism, and the curly-arrow language that every organic reaction on Paper 2 is written in. This page is the toolkit; the reactions build on it.
The six formulae
A molecule can be written six ways, and A-Level expects you to move between them — and to give the exact one asked for. Each strips away or adds detail:
| Representation | What it shows | Example (butan-1-ol) |
|---|---|---|
| Empirical | simplest whole-number ratio of atoms | C4H10O — already simplest (cf. ethane C2H6 → CH3) |
| Molecular | the actual number of each atom | C4H10O |
| General | the algebraic pattern for the series | CnH2n+1OH |
| Structural | the arrangement, condensed | CH3CH2CH2CH2OH |
| Displayed | every atom and every bond shown | all C, H and O drawn with lines |
| Skeletal | C and C–H implied; only the skeleton & groups | a zig-zag ending in –OH |
One molecule — butan-1-ol, C4H10O — three ways AQA can ask for it
Structural
CH3CH2CH2CH2OH
condensed to one line — no bonds drawn
Displayed
every atom and every bond — including O–H
Skeletal
C at every corner & line-end; C–H implied; OH still drawn
Not skeletal
the carbons are never written in
Not displayed
“OH” hides a bond — displayed must draw the O–H
Bond to the O — never the H
the atom facing the chain must be the O (chain–OH, or HO–chain when the group points left). In the lower drawing the bond lands on the H — that reads C–H–O, a bond to the wrong atom
Carbon atoms are not written: there is a carbon at every corner and at the end of every line. Hydrogens bonded to carbon are left out. But every heteroatom (O, N, halogen…) is drawn, and any hydrogen bonded to it (so –OH, –NH2, –COOH are shown in full).
Interactive — the skeletal builder
Tap a vertex to select it · the buttons grow from the selected atom · tap a bond to switch it single ↔ double
Interactive — displayed ↔ skeletal
- Skeletal is not displayed — the format is worth marks. Examiner reports flag a displayed formula given when a skeletal one was asked for (and vice versa). Read the command word and answer in exactly that form.
- Always draw the –OH. In both skeletal and displayed, the oxygen and its hydrogen are shown; hiding the O–H in a displayed formula loses the mark.
- The bond reaches the O, never the H. The connectivity is C–O–H, so the oxygen faces the chain: chain–OH, or HO–chain when the group points left. A bond drawn to the H reads C–H–O — a different (impossible) molecule.
- Terminal groups are line-ends, not bare corners. A chain ending in CH3 is the end of a line; count the implied hydrogens so the carbon has four bonds.
GCSE groundwork: C7 Organic Chemistry (functional groups, homologous series).
Homologous series & functional groups
Organic chemistry is organised into families. Each family — a homologous series — is built around one functional group, the reactive part that decides how the molecule behaves.
A homologous series is a family of compounds with the same functional group and the same general formula, in which each member differs from the next by CH2. Members have similar chemical properties and a gradual trend in physical properties.
One piece of shorthand before the chart: R stands for “the rest of the molecule” — any alkyl group (CH3–, CH3CH2–…). So R–OH means any alcohol and R–Br any bromoalkane; where two chains can differ they are written R and R′ (an ester is R–COO–R′). X is used the same way for “any halogen”.
| Series | Functional group | General formula | Named with |
|---|---|---|---|
| Alkane | C–C single bonds | CnH2n+2 | -ane |
| Alkene | C=C | CnH2n | -ene |
| Halogenoalkane | C–X (X = halogen) | CnH2n+1X | chloro-/bromo-/iodo- |
| Alcohol | –OH | CnH2n+1OH | -ol |
| Aldehyde | –CHO (end of chain) | CnH2nO | -al |
| Ketone | C=O (within chain) | CnH2nO | -one |
| Carboxylic acid | –COOH | CnH2n+1COOH | -oic acid |
| Ester | –COO– | CnH2nO2 | -oate |
| Amine | –NH2 | CnH2n+3N | amino- / -amine |
| Nitrile | –C≡N | CnH2n+1CN | -nitrile |
Alkane
CnH2n+2
-ane · butane
Alkene
CnH2n
-ene · but-2-ene
Halogenoalkane
CnH2n+1X
halo- · bromoethane
Alcohol
CnH2n+1OH
-ol · propan-1-ol
Aldehyde
CnH2nO
-al · propanal
Ketone
CnH2nO
-one · propanone
Carboxylic acid
CnH2n+1COOH
-oic acid · propanoic acid
Ester
CnH2nO2
-oate · methyl ethanoate
Amine
CnH2n+1NH2
-amine · ethylamine
Nitrile
CnH2n+1CN
-nitrile · propanenitrile
- General formula is the pattern; functional group is the family. The general formula (CnH2n+2…) generates every member; the functional group decides the reactions.
- Aldehydes and ketones share CnH2nO but are different series — the C=O is at the end (aldehyde) or within the chain (ketone).
🧪 Exam-style questions
Which is the correct general formula for the non-cyclic compounds in the homologous series? Tick (✓) one box.
Which compound has the lowest relative molecular mass? Tick (✓) one box.
Source: AQA A-Level Chemistry past papers.
Naming: the IUPAC rules
Every organic name is built from the same four parts, in this order of decisions:
- Stem — find the longest carbon chain that contains the main functional group: meth (1), eth (2), prop (3), but (4), pent (5), hex (6). A ring adds cyclo-.
- Suffix — the principal functional group (the highest-priority group present — see below) sets the ending: -ane, -ene, -ol, -al, -one, -oic acid…
- Locants — number the chain from the end that gives the functional group (then the substituents) the lowest numbers.
- Prefixes — add substituents (methyl-, chloro-…) with their locants, in alphabetical order, using di/tri for repeats.
What if a molecule carries two functional groups? The higher-priority group wins the suffix (and the lowest locant); everything it outranks is named around it. The priority order for the groups on this page:
carboxylic acid > aldehyde > ketone > alcohol > amine > alkene
So CH2=CHCH2OH is prop-2-en-1-ol: the OH outranks the C=C, so it takes the -ol suffix and carbon 1, while the double bond stays in the name as -en-. Halogens and alkyl branches are always prefixes (2-bromo…, 3-methyl…) — they never take the suffix — and an outranked OH appears as the prefix “hydroxy-”.
Interactive — name ↔ structure
Prefix
Stem — the longest chain
Ending — saturation and principal group
Tap a vertex to select it · the buttons grow from the selected atom · tap a bond to switch it single ↔ double
- Lowest locants win — number from the end that gives the smaller set (pentan-2-ol, not pentan-4-ol).
- Prefixes go alphabetically (ethyl before methyl), each with its own locant; di/tri do not count for alphabetising.
- The name must match the exact structure — examiner reports flag names that are chemically related but not the compound asked for.
🧪 Exam-style questions
What is the IUPAC name for this compound? Tick (✓) one box.
Which is the IUPAC name for this compound? Tick (✓) one box.
Source: AQA A-Level Chemistry past papers.
Structural isomerism
Structural isomers are compounds with the same molecular formula but a different structural formula (a different arrangement of atoms).
There are three types, and AQA expects you to classify which one you are looking at:
| Type | What differs | Example pair |
|---|---|---|
| Chain | the carbon skeleton (branching) | butane and 2-methylpropane |
| Position | the position of the functional group | propan-1-ol and propan-2-ol |
| Functional group | the functional group itself | an alkene and a cycloalkane; an aldehyde and a ketone |
Chain
butane · 2-methylpropane
same formula, the skeleton branches differently
Position
propan-1-ol · propan-2-ol
same group, different carbon
Functional group
but-1-ene · cyclobutane
an alkene and a cycloalkane — both C4H8
Functional-group isomerism nearly always comes from one of these same-formula pairings — worth knowing cold:
| Shared molecular formula | Pairing | Example pair |
|---|---|---|
| CnH2n | alkene ↔ cycloalkane | but-1-ene & cyclobutane (C4H8) |
| CnH2n+2O | alcohol ↔ ether | propan-1-ol & methoxyethane (C3H8O) |
| CnH2nO | aldehyde ↔ ketone | propanal & propanone (C3H6O) |
| CnH2nO2 | carboxylic acid ↔ ester | propanoic acid & methyl ethanoate (C3H6O2) |
Interactive — the isomer studio
Pick a molecular formula
Tap a vertex to select it · the buttons grow from the selected atom · tap a bond to switch it single ↔ double
- Check the molecular formula first — two structures are only isomers if the formula is identical.
- The functional-group isomer of an alkene is a cycloalkane (same CnH2n) — examiner reports flag students missing this.
- Name the type when asked: chain, position or functional group.
🧪 Exam-style questions
Which compound is an isomer of ethyl ethanoate? Tick (✓) one box.
Source: AQA A-Level Chemistry past papers.
E–Z isomerism & CIP
Stereoisomers have the same structural formula but a different arrangement of atoms in space.
E–Z isomerism is a form of stereoisomerism that arises from restricted rotation about the planar C=C double bond, when each carbon of the double bond carries two different groups.
Interactive — why E–Z isomers exist at all
Both molecules get the same instruction — rotate the right-hand group. Only one of them can obey.
That is the whole origin of E–Z isomerism: because there is no rotation about a C=C double bond, the groups are locked on their side. To label which isomer is which, use the Cahn–Ingold–Prelog (CIP) priority rules:
- At each end of the double bond, rank the two atoms by atomic number — higher atomic number = higher priority. If the first atoms tie, look at the next atoms out.
- If the two higher-priority groups are on the same side → Z (zusammen, together). On opposite sides → E (entgegen, opposite).
- Cis/trans is an older naming you will still meet, used for simple alkenes — ones where each carbon of the C=C carries at least one identical substituent (usually an H on each, as in but-2-ene). Cis = the matching groups on the same side (here the same as Z); trans = opposite sides (E). Once all four groups differ, cis/trans breaks down — E–Z always works, so give E or Z in the exam.
Z-but-2-ene
higher priorities (CH3) together — zusammen
E-but-2-ene
higher priorities apart — entgegen; the double bond stops rotation
CIP in action — 2-chlorobut-2-ene
left C: Cl (17) beats C (6) → priority ① is down. right C: C beats H → ① is up. Opposites → this drawing is E-2-chlorobut-2-ene.
Interactive — assign E or Z
- Each C of the C=C needs two different groups. A terminal or symmetrical alkene (e.g. CH2=CBr2) has no E–Z isomers.
- Rank by atomic number, not by size or eye. Br > Cl > O > N > C > H; if tied, move to the next atom out.
- Z = higher priorities together, E = apart — don’t assume cis/trans without checking priorities.
🧪 Exam-style questions
Which alkene shows E–Z isomerism? Tick (✓) one box.
Which compound has E–Z isomers? Tick (✓) one box.
Explain the difference between structural isomerism and stereoisomerism. Use compounds with the molecular formula C4H8 to show stereoisomerism and the three types of structural isomerism.
Show answer
Structural isomers have the same molecular formula but a different structural formula (different arrangement of atoms). 1 mark Stereoisomers have the same structural formula but a different arrangement of atoms in space. 1 mark
Chain: but-1-ene and 2-methylpropene (methylpropene). 1 mark
Position: but-1-ene and but-2-ene. 1 mark
Functional group: butene (an alkene) and cyclobutane (a cycloalkane). 1 mark
Stereoisomerism: but-2-ene exists as E-but-2-ene and Z-but-2-ene. 1 mark
Source: AQA A-Level Chemistry past papers.
Reaction mechanisms & curly arrows
A mechanism shows how a reaction happens by tracking the movement of electrons. A-Level uses two notations, and the reactions you meet later are all written in one of them.
- Free-radical mechanisms — an unpaired electron is shown as a dot (e.g. Cl•). You write balanced equations for each step (initiation, propagation, termination); curly arrows are not used for radicals.
- Electron-pair mechanisms — drawn with curly arrows, each showing a pair of electrons moving.
A curly arrow shows a pair of electrons moving. It must start from a bond or a lone pair and point to where the new bond (or lone pair) forms. To break a bond, the arrow starts on that bond.
From a bond
breaking a bond: the arrow starts on that bond
From a lone pair
making a bond: the arrow starts on the pair that will form it
Heterolytic vs homolytic fission
full head = the pair moves · fish-hook = one electron (radicals — AQA wants equations there, not arrows)
The dodgy one
an arrow never starts from empty space — electrons move, and they must come from somewhere real
Interactive — dodgy or valid?
- A curly arrow never starts from empty space. Examiner reports are explicit: it must start from a bond or a lone pair.
- Radicals use a dot, not an arrow. Half-headed arrows and single electrons belong to radical chemistry — but the spec asks for balanced equations there, not arrows.
- The head points where the electrons go — onto the atom or between the two atoms forming the new bond.
Capstone: read the command word
More organic marks are lost to format than to chemistry. The molecule can be perfect, but if AQA asked for a skeletal formula and you drew a displayed one, the mark is gone. Circle the command word first, then answer in exactly that form:
| Command word | What AQA wants |
|---|---|
| Name | the full IUPAC name (locants, prefixes, stem, suffix) |
| Molecular formula | the actual count of each atom, e.g. C4H10O |
| Structural formula | the condensed arrangement, e.g. CH3CH(OH)CH3 |
| Displayed formula | every atom and every bond — including each O–H |
| Skeletal formula | zig-zag with C and C–H implied; heteroatoms & their H shown |
| Mechanism | structures + curly arrows (from a bond or lone pair) |
Same molecule, six possible answers — the exam decides which one scores. Build the habit here and it protects you on every organic question for the rest of the course.
Interactive — the command-word challenge
Tap a vertex to select it · the buttons grow from the selected atom · tap a bond to switch it single ↔ double
- Six formulae: empirical (ratio), molecular (actual counts), general (algebraic pattern), structural (condensed), displayed (every atom & bond), skeletal (C and C–H implied, heteroatoms & their H shown).
- Skeletal rules: carbon at every corner and line-end; no C–H drawn; but –OH, –NH2, –COOH are shown. Skeletal ≠ displayed — the format is marked.
- Homologous series = same functional group, same general formula, differing by CH2, similar chemistry.
- IUPAC naming: longest chain (stem) + principal group (suffix) + lowest locants + substituents (prefixes, alphabetical); rings get “cyclo-”. Two groups? The suffix goes to the higher priority: acid > aldehyde > ketone > alcohol > amine > alkene.
- Structural isomers (same molecular formula, different connectivity): chain, position, functional group — common pairs: alkene ↔ cycloalkane, alcohol ↔ ether, aldehyde ↔ ketone, acid ↔ ester.
- Stereoisomers (same structure, different spatial arrangement): E–Z from restricted rotation about a C=C, when each double-bond carbon carries two different groups. Rank by CIP (atomic number): higher priorities together = Z, apart = E.
- Curly arrows show an electron pair moving, starting from a bond or a lone pair — never from empty space; radicals use a dot (no arrows).