IB Chemistry: Functional Groups & Homologous Series FAQ

A homologous series is a family of organic compounds sharing the same functional group and general formula, differing only by a CH2 unit between consecutive members. Each step up adds 14 mass units, and members show a gradual trend in physical properties like boiling point, but near-identical chemical behaviour.

A functional group is the specific atom or group responsible for a molecule's characteristic reactions — like -OH or -COOH. A homologous series is the whole family built around that group. Ethanol and propan-1-ol both carry the -OH functional group, so both belong to the same alcohol homologous series.

Start by scanning the structure for the defining atoms — a carbon double-bonded to oxygen signals a carbonyl, -OH signals an alcohol. Name the functional group, state its homologous series and general formula, then connect that identification to whatever the question actually asks about. Examiners mark the connection, not just the label.

Both levels cover alkanes, alkenes, alcohols, halogenoalkanes, aldehydes, ketones, carboxylic acids and esters. HL adds amines, amides, nitriles and arenes, plus deeper mechanism detail for these groups. According to the IB Chemistry guide (first exams 2025), this content is grouped under Structure 3.2, "Functional groups: classification of organic compounds."

Find the longest carbon chain containing the principal functional group, number it to give that group the lowest possible locant, then add substituent names alphabetically with their position numbers. For example, CH3CH(Cl)CH2CH3 becomes 2-chlorobutane — a four-carbon chain with a chloro substituent at carbon 2, since numbering from that end gives the lower number.

Look for the defining bond pattern rather than memorising whole molecules: C=O with an attached -OH is a carboxylic acid, C=O between two carbon chains is a ketone, C=O at the end of a chain is an aldehyde. Skeletal formulas hide hydrogens, so check every vertex and line-end for what's implied.

The one I see most in mock papers is confusing aldehydes and ketones because students forget where the carbonyl sits in the chain. Others lose marks for naming the homologous series but not the specific functional group, or for giving a molecular formula when the question actually asks for a structural one.

They show up across all three papers — as quick recall items in Paper 1's multiple choice, naming and structural questions in Paper 2's short-answer section, and embedded in longer synthesis or mechanism questions in Paper 2's extended response and Paper 3's data-based questions. HL papers test a wider range of functional groups and deeper mechanism links.

Organic chemistry — Structure 3.2 and its related Reactivity 3 mechanisms — trips up more students than almost any other unit, mostly because it rewards pattern recognition over pure memorisation. It's not conceptually harder than equilibrium or kinetics; it just needs a different revision habit: drawing structures repeatedly rather than re-reading notes.

There's no fixed allocation reserved for functional groups alone — they're woven through organic chemistry questions worth anywhere from 1 mark for naming to 6-8 marks for multi-step synthesis or mechanism questions in Paper 2, plus several multiple-choice marks in Paper 1. HL papers generally weight organic content more heavily than SL.

Yes, noticeably — HL adds four extra functional groups (amines, amides, nitriles, arenes), deeper mechanism work including nucleophilic substitution and addition-elimination, and more multi-step synthesis questions. SL organic chemistry stays largely descriptive; HL expects you to predict products and explain mechanisms with curly arrows, which is where most HL students actually lose marks.

Struggling with nomenclature alone isn't a reason to rule out HL — it's a skill built through repetition, not innate talent, and most students improve within a few weeks of consistent practice. What matters more is whether your child generally enjoys mechanism-based problem solving; if organic chemistry is their only weak spot, targeted practice usually closes the gap before mocks.

Draw structures by hand rather than just reading them — the physical act of drawing forces you to notice functional groups you'd otherwise skim past. Build a personal reference table listing every homologous series with its general formula and one distinguishing reaction, then test yourself against past paper structures until naming becomes automatic.