O-Level Pure Chemistry: Organic Chemistry — Alkanes, Alkenes and Alcohols: Reactions and Identification
Organic chemistry forms a dedicated section of the O-Level Pure Chemistry paper and typically contributes 10–15% of marks across both papers, yet many students enter the exam without a unified picture of how alkanes, alkenes and alcohols connect. This guide maps the key reactions, identification tests and structural patterns for each homologous series, and highlights the exam traps that cost the most marks. A systematic reaction-map approach turns this topic from a memorisation exercise into a pattern-recognition one.
Why Organic Chemistry Rewards a Systematic Approach
The organic chemistry section of O-Level Pure Chemistry is highly predictable. The examiners test the same reactions, the same identification tests and the same structural features year after year. Students who treat it as random facts to memorise struggle; students who build a clear reaction map — linking alkanes, alkenes and alcohols through specific reagents and conditions — find that most questions become pattern-matching exercises.
This guide covers the three homologous series in the syllabus, the reactions that connect them, and the exam-technique points that separate full marks from partial marks.
Homologous Series: The Shared Framework
A homologous series is a family of compounds that share the same general formula and functional group, differ by one –CH2– unit between successive members, and show a gradual change in physical properties. All three series in this guide are hydrocarbons or derived from hydrocarbons, and they are linked by reactions you need to know in both directions.
Part 1: Alkanes
Structure and general formula
Alkanes are saturated hydrocarbons — every carbon atom forms single bonds only. General formula: CnH2n+2. The first four members are methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10). Boiling point increases with chain length due to stronger van der Waals forces.
Reaction 1: Combustion
Complete combustion (excess oxygen) produces carbon dioxide and water only:
CH4 + 2O2 → CO2 + 2H2O
Incomplete combustion (limited oxygen) produces carbon monoxide (toxic) and/or carbon (soot). This distinction is tested in environmental and safety contexts.
Reaction 2: Substitution with chlorine (UV light)
In the presence of ultraviolet light, one hydrogen atom is replaced by a chlorine atom:
CH4 + Cl2 → CH3Cl + HCl (UV light required)
This is a substitution reaction — one atom replaces another, producing two products. The reaction does not occur in the dark. UV light (or sunlight) is needed to break the Cl–Cl bond and start the reaction. Always state the condition when asked.
Why alkanes do NOT decolourise bromine water
Alkanes are saturated and chemically unreactive towards bromine water at room temperature without UV light. Orange-brown bromine water stays orange-brown with an alkane. This contrast with alkenes is the most commonly tested identification point in the organic chemistry section.
Part 2: Alkenes
Structure and general formula
Alkenes are unsaturated hydrocarbons containing at least one carbon-to-carbon double bond (C=C). General formula: CnH2n. The simplest alkene is ethene (C2H4). The C=C double bond is the reactive site for all alkene reactions — it opens up to allow other atoms to add across it.
Reaction 1: Addition of bromine (the bromine water test)
The C=C double bond breaks and bromine atoms add across it:
C2H4 + Br2 → C2H4Br2
The orange-brown colour of bromine water is decolourised (becomes colourless). This is the standard test to distinguish alkenes from alkanes. It is an addition reaction — two reactants combine to form one product.
Reaction 2: Hydrogenation (addition of hydrogen)
Hydrogen adds across the double bond in the presence of a nickel catalyst at approximately 150–200 °C to form an alkane:
C2H4 + H2 → C2H6 (Ni catalyst, ~150 °C)
This is used industrially to harden vegetable oils into margarine by converting C=C double bonds into C–C single bonds.
Reaction 3: Hydration (addition of steam → alcohol)
Steam adds across the C=C bond in the presence of a phosphoric acid (H3PO4) catalyst at high temperature and pressure:
C2H4 + H2O → C2H5OH
This is the industrial route to ethanol. The reverse — dehydration of an alcohol — regenerates the alkene. Both directions and their conditions are examined.
Reaction 4: Addition polymerisation
Many alkene molecules join together to form a long-chain polymer:
n(CH2=CH2) → −(CH2–CH2)n−
In exam questions, you may be asked to draw the repeat unit of a polymer or identify the monomer from a given polymer structure. The repeat unit is drawn with bonds extending from each end inside square brackets with subscript n.
Part 3: Alcohols
Structure and general formula
Alcohols contain the hydroxyl functional group (–OH). General formula: CnH2n+1OH. The two most tested members are methanol (CH3OH — toxic, causes blindness) and ethanol (C2H5OH — used as a fuel, solvent, and in beverages).
Reaction 1: Combustion
Alcohols are flammable and burn to produce carbon dioxide and water:
C2H5OH + 3O2 → 2CO2 + 3H2O
Reaction 2: Oxidation to carboxylic acid
Ethanol is oxidised by acidified potassium dichromate(VI) or by bacteria in the presence of air to form ethanoic acid (CH3COOH) — the acid in vinegar:
C2H5OH + [O] → CH3COOH + H2O
The key point for O-Level: alcohols are oxidised to carboxylic acids. The full balanced equation is not always required, but the direction and the type of reagent (oxidising agent) are.
Reaction 3: Dehydration to alkene
Heating an alcohol with excess concentrated sulfuric acid (or passing the vapour over hot aluminium oxide) removes water and reforms the C=C double bond:
C2H5OH → C2H4 + H2O (conc. H2SO4, heat)
This is the reverse of hydration. Knowing both directions — hydration of alkene → alcohol, and dehydration of alcohol → alkene — is essential for multi-step synthesis questions.
Reaction 4: Esterification
An alcohol reacts with a carboxylic acid in the presence of a concentrated sulfuric acid catalyst to form an ester and water. The reaction is reversible:
C2H5OH + CH3COOH ⇌ CH3COOC2H5 + H2O
To name an ester: take the alcohol part (replace –ol with –yl) and the acid part (replace –ic acid with –ate). Ethanol + ethanoic acid → ethyl ethanoate. Propan-1-ol + methanoic acid → propyl methanoate.
The Reaction Map: Connecting All Three Series
Memorise these transformations as a connected diagram rather than isolated facts:
- Alkene + H2O (hydration, H3PO4 catalyst) → Alcohol
- Alcohol – H2O (dehydration, conc. H2SO4) → Alkene
- Alkene + H2 (hydrogenation, Ni, ~150 °C) → Alkane
- Alkane + Cl2/UV light (substitution) → Halogenoalkane + HCl
- Alcohol + [O] (oxidation) → Carboxylic acid
- Alcohol + carboxylic acid (esterification, conc. H2SO4) → Ester + H2O
- Alkene (polymerisation) → Addition polymer
Drawing this map from memory is an excellent revision exercise. Any multi-step conversion question in Paper 2 will be a path through this map — identify your starting material, your target, and the sequence of reagents and conditions needed to get there.
Worked Examples
Example 1 — Identifying Compounds from Test Results
A colourless liquid X decolourises bromine water. When X is heated with concentrated H2SO4, a gas Y is produced that also decolourises bromine water. Identify X and Y, and name the reaction that converts X to Y.
Reasoning: X decolourises bromine water → X contains a C=C double bond, suggesting an alkene; but X is a liquid and reacts with conc. H2SO4 to produce a gas — this is the dehydration of an alcohol. Alcohols with C=C bonds can decolourise bromine water under certain conditions. Y is an alkene (gas, decolourises bromine water).
If X is ethanol: X = ethanol, Y = ethene. Reaction type: dehydration.
Example 2 — Writing and Naming an Ester
State the name of the ester formed and write its molecular formula when methanol reacts with propanoic acid.
Methanol → methyl (–yl part). Propanoic acid → propanoate (–anoate part). Ester name: methyl propanoate.
Methanol = CH3OH. Propanoic acid = C2H5COOH. Ester = CH3OOCC2H5, molecular formula C4H8O2.
Exam Traps to Avoid
Trap 1: Saying alkanes decolourise bromine water
They do not — not at room temperature without UV light. The alkene is always the compound that decolourises bromine water in a standard identification question.
Trap 2: Confusing substitution and addition
Addition: two reactants form one product (alkene + Br2 → one dibromo compound). Substitution: one atom replaces another, producing two products (alkane + Cl2 → halogenoalkane + HCl). Mixing these up costs the "type of reaction" mark.
Trap 3: Omitting conditions
Many marks are reserved for the condition alone. Always state: UV light for substitution; Ni catalyst and heat for hydrogenation; H3PO4 or H2SO4 catalyst for hydration; conc. H2SO4 for dehydration and esterification.
Trap 4: Polymer repeat unit drawn without extending bonds
The repeat unit must show bonds extending from both ends of the bracket, not as a standalone molecule. Missing extending bonds loses the mark for the structural drawing.
Trap 5: Confusing methanol and ethanol toxicity
Methanol is toxic — it causes blindness and death. Ethanol is found in alcoholic drinks (though harmful in excess). This distinction appears in "discuss the uses and hazards" style questions.
How to Practise Organic Chemistry Effectively
- Build a reactions table from memory. Three columns: Series | Reagent & Condition | Product & Reaction Type. Fill it in without notes, then check. Repeat until you complete it in under three minutes.
- Practice drawing structural and displayed formulae. Be able to draw the full structural formula for the first three members of each series, showing all bonds and atoms.
- Work identification questions from past papers. Multi-step deduction questions ("Compound X undergoes reaction Y...") appear consistently. Practice at least five from past O-Level Chemistry papers.
- Memorise conditions as pairs with reactions. Never separate them — always learn the reaction and its condition together as one unit.
Organic chemistry is one of the most predictable sections of O-Level Pure Chemistry. A well-organised reaction map and consistent condition recall will reliably convert this topic into marks in both Paper 1 and Paper 2.