In simple terms
A friendly intro before the formal notes — no formulas yet.
Sorting Carbon's Millions
There are tens of millions of known organic compounds, far too many to learn one by one. Instead we sort them into families. Every family is defined by a functional group — a small, reactive cluster of atoms — and every member of a family reacts in essentially the same way, so learning the family teaches you the members.
Think of a functional group like the engine of a vehicle and the carbon chain like the chassis it is bolted to. A scooter, a car and a truck have very different chassis, but if they all share the same type of engine they start, run and stall in the same way. Swap the engine — bolt on a different functional group — and the behaviour changes completely, even if the chassis looks almost identical. Chemists predict how a molecule reacts by looking straight past the chain to the group bolted onto it.
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Scan the molecule for anything that is not a plain C–C or C–H single bond: a double or triple bond, or a heteroatom such as O, N or a halogen.
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Identify the exact arrangement around that feature — an O–H, a C=O, a –COOH, an N–H, and so on.
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Match the arrangement to a named functional group, and hence to a homologous series (family).
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Use the family to predict the name pattern, the general formula, and the characteristic reactions.
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Full topic notes
Formal explanation with the rigour you need for the exam.
Homologous series and functional groups
A functional group is the reactive part of an organic molecule — the atom or group of atoms responsible for its characteristic chemical reactions. Everything attached to it, usually a chain of carbon and hydrogen, is comparatively inert and acts as a scaffold. Molecules that carry the same functional group belong to the same homologous series: a family in which every member has the same general formula and successive members differ by a single unit. Because the reactive group is identical across the family, the chemistry is shared; because each step adds only an unreactive , physical properties change gradually. For example, straight-chain alcohols show a steady rise in boiling point as the chain lengthens, because each extra increases the molar mass and surface area, strengthening London (dispersion) forces.
Functional group: the site of chemical reactivity — identify it first, and the chemistry follows.
Homologous series: same functional group, same general formula, successive members differing by .
General formula: an algebraic summary of a series, e.g. for alkanes.
Trends: chemical properties are shared across a series; physical properties (boiling point, density) change smoothly with chain length.
The main classes and their functional groups
For IB Chemistry you must recognise the classes below, know the functional group that defines each, and — for the hydrocarbon and alcohol series — recall the general formula. The naming suffix is itself a clue to the functional group present, so learn the group and the suffix together. The letter R is used to stand for any carbon-containing (alkyl or aryl) group.
Alkane — only C–C and C–H single bonds; general formula ; suffix -ane (e.g. propane).
Alkene — at least one C=C double bond; general formula ; suffix -ene (e.g. propene).
Alkyne — at least one C≡C triple bond; general formula ; suffix -yne (e.g. propyne).
Arene — contains a benzene ring (, delocalised π system); named as a benzene derivative.
Halogenoalkane — one or more halogen atoms (–F, –Cl, –Br, –I) replacing H; prefix fluoro-/chloro-/bromo-/iodo- (e.g. 2-bromopropane).
Alcohol — hydroxyl group ; general formula ; suffix -ol (e.g. propan-1-ol).
Ether — oxygen bridging two carbons, ; named as an alkoxyalkane (e.g. methoxymethane).
Aldehyde — carbonyl C=O at the END of the chain (the C=O carbon carries an H); suffix -al (e.g. propanal).
Ketone — carbonyl C=O WITHIN the chain (the C=O carbon is bonded to two carbons); suffix -one (e.g. propanone).
Carboxylic acid — carboxyl group ; suffix -oic acid (e.g. propanoic acid).
Ester — with the second oxygen joined to a carbon, ; named alkyl alkanoate (e.g. methyl propanoate).
Amine — amino group , or ; suffix -amine or prefix amino- (e.g. propan-1-amine).
Representing organic molecules
The same molecule can be written in several ways, each carrying a different amount of detail. Choosing the right one — and reading it correctly — is a genuine skill that Paper 2 tests directly. Moving up the list adds information about how the atoms are actually joined.
A key exam habit: an empirical or molecular formula can never, by itself, tell you which compound you hold, because isomers share it. To identify or name a molecule you must have at least a structural or skeletal representation, which pins down the connectivity — and therefore the functional group.
Empirical formula — the simplest whole-number ratio of atoms, e.g. ethene is . It fixes the ratio only, not the size or shape.
Molecular formula — the actual count of each atom in one molecule, e.g. ethene is . Still says nothing about connectivity, so several isomers can share it.
Full structural formula — every atom and every bond drawn out, e.g. ethanol showing all C–H, C–C, C–O and O–H bonds. Unambiguous but slow to draw.
Condensed structural formula — atoms grouped onto each carbon, e.g. ethanol as . Compact and unambiguous about connectivity.
Skeletal formula — the carbon chain as a zig-zag: each vertex and line-end is a carbon, C–H bonds are implied to complete valency, and heteroatoms (with their own H) are drawn explicitly. Fast to draw and the standard for larger molecules.
IUPAC nomenclature: naming step by step
IUPAC names are built systematically, so a correct name can be read straight off a structure and a structure drawn straight from a name. Follow the same four steps every time and the process is reliable, even for unfamiliar molecules.
1. Find the longest continuous carbon chain — this is the parent chain and sets the stem (meth-, eth-, prop-, but-, pent-...). It need not be drawn in a straight line; turn corners if that gives a longer chain.
2. Number the chain from the end that gives the principal functional group the LOWEST locant. If there is no such group (e.g. an alkane), number to give the set of substituents the lowest locants.
3. Identify substituents (branches such as methyl, ethyl; halogens as chloro-, bromo-) and give each its locant; use di-, tri-, tetra- for repeats.
4. Assemble the name: substituents in alphabetical order (ignoring di-/tri- when alphabetising), then the parent stem, then the suffix with its locant, e.g. 2-methylbutan-1-ol.
Structural isomerism
Structural isomers are compounds with the same molecular formula but different arrangements of atoms — different connectivity. Because connectivity determines the functional group and the shape, isomers can have very different properties even though a molecular formula (or a mass spectrum's molar mass) cannot tell them apart. At SL there are three types to recognise.
Chain isomerism — the carbon skeleton is arranged differently (straight vs branched). Example: butane, , and 2-methylpropane, , both .
Position isomerism — the same functional group is attached at a different point on the chain. Example: propan-1-ol, , and propan-2-ol, , both .
Functional-group isomerism — the atoms are arranged into an entirely different functional group, so the isomers belong to different homologous series. Example: propanal (an aldehyde) and propanone (a ketone), both ; or ethanol (alcohol) and methoxymethane (ether), both .
Linking functional group to reactivity and physical properties
The point of classifying by functional group is prediction. Once you name the group you can anticipate both how the molecule reacts and roughly how it behaves physically, because the group governs bonding, polarity and intermolecular forces. A C=C double bond invites addition; an or introduces polarity and hydrogen bonding; a is acidic. The unreactive alkane framework, by contrast, contributes London forces that scale with chain length.
Reactivity — alkanes are unreactive (combustion, substitution); alkenes undergo addition across the C=C; alcohols undergo oxidation and can be dehydrated; carboxylic acids are weak acids and form esters with alcohols; halogenoalkanes undergo substitution.
Polarity and solubility — polar groups such as , and can hydrogen bond, making small members water-soluble; non-polar hydrocarbon chains are not, so solubility falls as the chain lengthens.
Boiling point — within a series, boiling point rises with chain length (stronger London forces). Between series of similar mass, molecules that hydrogen bond (alcohols, acids) boil far higher than those that cannot (alkanes, ethers).
Common mistakes examiners penalise
Numbering the chain from the wrong end — the principal functional group must get the LOWEST locant; fix that before locating substituents. 2-methylbutan-1-ol, not 3-methylbutan-4-ol.
Missing the longest chain — the parent chain can turn a corner. '2-ethylbutane' is really hexane; '3-methylbutane' is really 2-methylbutane. Always trace every path.
Confusing aldehydes and ketones — a carbonyl at the chain END (C=O carbon carries an H) is an aldehyde (-al); one WITHIN the chain (C=O carbon bonded to two carbons) is a ketone (-one).
Confusing ethers with alcohols, or esters with carboxylic acids — an ether has with no O–H; an ester has with no O–H, whereas the acid keeps its O–H in .
Mixing up the primary/secondary/tertiary rule — for alcohols count carbons on the carbon bearing ; for amines count carbons on the NITROGEN itself.
Getting the general formula wrong — alkanes , alkenes , alkynes . A wrong general formula leads to counting the wrong number of isomers.
Drawing 'new' isomers that are duplicates — a structure that is just the same molecule numbered or flipped differently scores nothing; check each candidate is genuinely distinct.
Model answer — marked the way our engine marks it
This is the showcase for an organic-classification topic. In Paper 2 the marks are analytic: each mark is tied to a specific correct piece of work (a method mark, M, for a valid structure; an answer mark, A, for the correct IUPAC name), and error-carried-forward (ECF) means a correct name written on a student's own drawn structure still scores even if that structure was not on the official list. Study how each mark below is earned by one specific isomer, and how equivalent representations are accepted.
Where this leads
Every reaction topic ahead is organised by the classes you have just met. The reactions of the halogenoalkanes, the addition reactions of the alkenes, the oxidation of alcohols to aldehydes, ketones and carboxylic acids, and esterification all start by identifying a functional group and predicting its behaviour. Master the habit built here — spot the group, name the family, read off the general formula and the likely chemistry — and the rest of organic chemistry becomes variations on a classification you already own.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Give the IUPAC name of the molecule with condensed structure CH₃CH(CH₃)CH₂CH₂OH. [2]
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Step 1 — longest chain. Trace the longest continuous carbon path. Starting from the carbon: HO–CH₂–CH₂–CH(CH₃)–CH₃. The longest chain is 4 carbons, so the parent is butane, and with an it becomes a butan-ol. [M1: correct parent chain and class]
Draw the skeletal or structural formulae of the three structural isomers of C₅H₁₂ and name each. State which type of isomerism relates them. [4]
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fits the alkane general formula (), so all isomers are saturated alkanes; they differ only in how the carbon skeleton branches — this is chain isomerism.
A molecule has the condensed structure HOCH₂CH₂COOH. Identify every functional group present and classify the compound. Predict, with a reason, whether it is likely to be soluble in water. [3]
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Step 1 — scan for anything beyond C–C / C–H. Reading the structure there are two oxygen-containing features: an at one end and a at the other.
Draw the structural formulae of the FOUR structural isomers of C₄H₁₀O that are alcohols, and give the IUPAC name of each. [4]
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Model answer — full working.
How it all connects
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Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
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Quick check
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Revision flashcards
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Functional group
A specific atom or group of atoms within a molecule that is responsible for its characteristic chemical reactions. The rest of the molecule (the carbon–hydrogen scaffold) is comparatively unreactive.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
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Functional group: the site of chemical reactivity — identify it first, and the chemistry follows.
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Homologous series: same functional group, same general formula, successive members differing by .
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General formula: an algebraic summary of a series, e.g. for alkanes.
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Trends: chemical properties are shared across a series; physical properties (boiling point, density) change smoothly with chain length.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Get a Paper 2 organic question marked: name a branched molecule, or draw and name the structural isomers of a formula, with full working
Get a Paper 2 organic question marked: name a branched molecule, or draw and name the structural isomers of a formula, with full working
Extra simulations & links
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Frequently asked
Checkpoint
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Reading it isn’t knowing it — prove it.
Before you move on: do Get a Paper 2 organic question marked: name a branched molecule, or draw and name the structural isomers of a formula, with full working on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.