HookHow one twisted double bond lets you read this sentence
In the back of your eye sits a molecule called retinal, and it does exactly one job. It holds a single carbon–carbon double bond in a bent, ‘11-cis’ shape until a photon of light strikes it. The energy flips that one double bond straight — into ‘all-trans’ retinal — and the shape change is violent enough to fire the optic nerve. Two molecules, identical atoms, identical bonds, differing only in the geometry around one \(\text{C}=\text{C}\), and that difference is the difference between sight and darkness. It is called stereoisomerism, and it is the dramatic finish to a topic that starts somewhere far more mundane.
Section 3.3.1 is the grammar of the whole of organic chemistry — the half of the A-level that Paper 2 is built on. It asks four things of you and nothing more: name any compound by IUPAC rules; draw it three different ways (displayed, structural and skeletal); track its reactions with curly arrows that show where pairs of electrons move; and recognise when two compounds sharing a molecular formula are actually different substances — structural isomers, or E-Z stereoisomers like the retinal firing in your eye right now. Get the grammar wrong and every organic mark downstream is built on sand, because examiners test naming, arrows and isomerism inside almost every other organic question too.
ModelNaming — the IUPAC machine that never guesses
Organic compounds come in homologous series: families that share a general formula and a functional group, whose members differ by \(\text{CH}_2\) and show a smooth trend in physical properties. The functional group is the reactive part that names the family and decides its reactions. An IUPAC name is assembled from four pieces. The stem counts the longest carbon chain — meth, eth, prop, but, pent, hex for one to six carbons. The suffix announces the main functional group — -ane, -ene, -ol, -al, -one, -oic acid. Prefixes name the substituents hanging off the chain — methyl-, chloro-, hydroxy-. And locants, the numbers, pin down where each feature sits.
Three rules settle almost every disagreement. Number the chain so the main functional group gets the lowest possible locant. List substituents alphabetically (ethyl before methyl), using di-, tri-, tetra- for repeats without disturbing the alphabetical order. And a general formula lets you write any member of a series at a glance — \(\text{C}_n\text{H}_{2n+2}\) for alkanes, \(\text{C}_n\text{H}_{2n}\) for alkenes, \(\text{C}_n\text{H}_{2n+1}\text{OH}\) for alcohols.
Name the alcohol \((\text{CH}_3)_2\text{CHCH}_2\text{CH}_2\text{OH}\). The longest chain containing the \(-\text{OH}\) is four carbons, so the stem is but- and the suffix is -ol. Number so the \(-\text{OH}\) is lowest: it sits on carbon 1, giving butan-1-ol, and a methyl branch sits on carbon 3. There is only one substituent, so no alphabetical ordering is needed. The name is 3-methylbutan-1-ol. Number from the other end instead and the \(-\text{OH}\) would land on carbon 4 and the methyl on carbon 2 — a higher locant for the functional group, so it is wrong. The functional group, not the substituent, always claims the lowest number.
DataThree ways to draw one molecule — and two ways to count its atoms
The same molecule can be drawn at three levels of detail, and AQA expects you to move between them fluently. A displayed formula shows every atom and every bond — unambiguous but slow. A structural formula shows the arrangement with the minimum of detail, grouping atoms as \(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}\) or \((\text{CH}_3)_2\text{CHOH}\). A skeletal formula strips it further: the carbon skeleton is drawn as a zig-zag of lines, a carbon sits at every vertex and every line end, the hydrogens on those carbons are left implied, and only heteroatoms (O, N, halogens) and the hydrogens bonded to them are written in. Skeletal is the working language of organic chemistry because it is fast and stays readable for rings and long chains.
Two more formulae describe composition rather than shape. The empirical formula is the simplest whole-number ratio of atoms; the molecular formula is the actual number of each atom in a molecule. The empirical formula alone can never tell you molecule size — you need the relative molecular mass for that.
A hydrocarbon is 85.7% carbon and 14.3% hydrogen by mass, with \(M_r = 56.0\). Divide each percentage by the atomic mass: carbon \(85.7 \div 12.0 = 7.14\); hydrogen \(14.3 \div 1.0 = 14.3\). The ratio \(7.14 : 14.3\) simplifies to \(1 : 2\), so the empirical formula is \(\text{CH}_2\) with an empirical mass of 14.0. The molecular formula is a whole-number multiple: \(56.0 \div 14.0 = 4\), giving \(\text{C}_4\text{H}_8\). The empirical formula fixed the ratio; only \(M_r\) fixed the true size — miss that final division and you stop at \(\text{CH}_2\), which is not a real molecule at all.
MechanismCurly arrows — the choreography of electron pairs
A curly arrow is not decoration; it is a precise statement about electrons. A double-headed curly arrow shows the movement of a pair of electrons, from where the pair starts — a bond or a lone pair — to where it ends, forming a new bond or a lone pair. A single-headed ‘fishhook’ arrow shows the movement of a single electron, and appears only in free-radical steps. The tail of the arrow must sit on the electrons that move; the head must point at where they go.
This choreography classifies how bonds break and which reagents attack. Heterolytic fission sends both electrons of a bond to one atom, producing oppositely charged ions. Homolytic fission gives one electron to each atom, producing two free radicals — species with an unpaired electron, marked with a dot. Reagents fall into three types: a nucleophile is an electron-pair donor with a lone pair, drawn towards a \(\delta+\) carbon; an electrophile is an electron-pair acceptor, drawn towards electron-rich sites; a free radical carries its unpaired electron into the fray. Every organic mechanism in the whole A-level is one of those three species attacking.
Break the \(\text{C}-\text{Br}\) bond of bromoethane two ways. Heterolytically: a single double-headed arrow from the middle of the \(\text{C}-\text{Br}\) bond to the bromine gives the pair entirely to Br, producing \(\text{CH}_3\text{CH}_2^{+}\) and \(:\text{Br}^{-}\). Homolytically, as when \(\text{Cl}_2\) splits under UV light: two fishhook arrows, one to each atom, split the pair one-and-one to give two radicals, \(2\,\text{Cl}\bullet\). Same bond, same breaking event — but two different arrow conventions and two utterly different products. AQA marks the arrows, not the words, so an arrow starting in the wrong place scores nothing even when the products are right.
ModelStructural isomerism — same formula, different skeleton
Structural isomers share a molecular formula but connect their atoms differently, and AQA names three kinds. Chain isomers differ in how the carbon skeleton branches — butane, a straight chain, versus 2-methylpropane, the branched \(\text{C}_4\text{H}_{10}\). Position isomers keep the same skeleton and the same functional group but move the group along the chain — propan-1-ol versus propan-2-ol. Functional-group isomers rearrange the same atoms into an entirely different functional group — propanal (an aldehyde) and propanone (a ketone) are both \(\text{C}_3\text{H}_6\text{O}\) yet belong to different families with different reactions.
The number of isomers climbs steeply with carbon count, and it matters because isomers are genuinely different substances — different boiling points, different chemistry, sometimes different biological effects. Treating them as ‘the same thing drawn differently’ is the mistake that structural isomerism exists to correct.
How many structural isomers has \(\text{C}_4\text{H}_9\text{Br}\)? Start with the straight four-carbon chain and move the bromine: on an end carbon it is 1-bromobutane, one carbon in it is 2-bromobutane — two position isomers. Now re-shape the skeleton to 2-methylpropane and place the bromine on an outer carbon (1-bromo-2-methylpropane) or the central carbon (2-bromo-2-methylpropane). That is four isomers: two from sliding the halogen, two more from re-branching the chain. Draw them as skeletal formulae and they are plainly four distinct molecules, not four names for one.
CaseE-Z stereoisomerism — when rotation is locked
Stereoisomers connect their atoms in the same order but arrange them differently in space. E-Z isomerism arises at a \(\text{C}=\text{C}\) double bond because the \(\pi\) bond locks rotation — the two ends cannot twist relative to one another the way a freely rotating \(\text{C}-\text{C}\) single bond can. For the isomerism to exist, each carbon of the double bond must carry two different groups; if either carbon holds two identical groups, flipping the molecule just gives the same thing back.
Assign E or Z using Cahn–Ingold–Prelog priority: on each carbon, the group whose first atom has the higher atomic number wins. If the two higher-priority groups lie on the same side of the double bond the isomer is Z (from the German zusammen, together); on opposite sides it is E (entgegen, opposite). The older cis-trans labels only work when there is an obvious matching pair of groups; E-Z generalises them and removes the ambiguity. Retinal’s trick in your eye is precisely a Z-to-E flip driven by a single photon.
Assign the isomers of 1-bromo-2-chloroethene, \(\text{BrCH}=\text{CHCl}\). On the left carbon the choice is bromine (atomic number 35) against hydrogen (1) — bromine wins. On the right carbon it is chlorine (17) against hydrogen (1) — chlorine wins. With Br and Cl on the same side the molecule is Z; on opposite sides it is E. Notice cis-trans cannot describe this cleanly — there is no matching pair to be ‘cis’ to — which is exactly why AQA examines E-Z. Contrast but-2-ene, \(\text{CH}_3\text{CH}=\text{CHCH}_3\), where the two methyls make cis equal Z and trans equal E; that coincidence is what tricks people into thinking Z always means cis.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Naming is an AO1 skill marked to the letter. Examiners award the lowest-locant rule, alphabetical ordering of substituents, correct di-/tri- multipliers, and the punctuation — hyphens between numbers and words, commas between numbers — as separate points, and their reports list careless naming as one of the biggest sources of lost organic marks. When you draw skeletal formulae, remember the heteroatom hydrogens: the ‘H’ of an \(-\text{O}-\text{H}\) must be shown even though carbon hydrogens are implied.
Curly-arrow marks (AO2) are unforgiving about the tail. The arrow must originate at a bond or a lone pair and point to the atom or bond receiving the electrons; an arrow that starts on a positive charge, or on a hydrogen, loses the mark regardless of the products. Draw lone pairs explicitly on nucleophiles so the tail has somewhere legitimate to start.
For E-Z, never simply assert the label — justify it. State the atomic numbers you compared on each carbon, name the higher-priority group, then say which side it sits on. This content is examined on Paper 2 (organic and physical chemistry) and reappears synoptically on Paper 3, where the same naming, drawing and arrow conventions underpin every later mechanism from alkenes to acyl chlorides.