Organic Chemistry: IGCSE Chemistry 0620

Organic chemistry is the longest topic in 0620 and the biggest single divider between Core and Extended grades: a typical Paper 4 carries 8-12 organic marks, and the Supplement material (esters, condensation polymers, isomerism) is where A* candidates pull away. Examiner reports name the recurring failures precisely: “clear” written instead of “colourless” for the bromine water test, combustion equations that refuse to balance, and displayed formulae missing hydrogen atoms. None of it is hard chemistry; all of it is precision.

Homologous series and naming

A homologous series is a family of compounds with the same general formula, the same functional group, similar chemical properties, and physical properties that change gradually (boiling point rises as the chain lengthens). Each member differs from the next by CH2. The four series and their general formulae:

Series	General formula	Functional group	First member you name
Alkanes	CnH2n+2	None (single bonds only)	Methane, CH4
Alkenes	CnH2n	C=C double bond	Ethene, C2H4
Alcohols	CnH2n+1OH	−OH	Methanol, CH3OH
Carboxylic acids	CnH2n+1COOH	−COOH	Methanoic acid, HCOOH

Naming follows the carbon count: meth-, eth-, prop-, but- for one to four carbons. You must draw displayed formulae showing every atom and every bond. A missing hydrogen is a lost mark, every time.

Fuels

Petroleum is a mixture of hydrocarbons separated by fractional distillation: the column is hot at the bottom, cool at the top, and fractions condense where the temperature matches their boiling point. Small molecules have low boiling points and come off at the top. Learn the fraction-to-use pairs: refinery gas (bottled gas/cooking), gasoline (petrol for cars), naphtha (chemical feedstock), kerosene (jet fuel), diesel, fuel oil (ships and power stations), lubricating fraction, bitumen (roads). Coal, natural gas (mainly methane) and petroleum are the fossil fuels. The pollution consequences of burning them (CO, CO2, particulates, SO2) sit in chemistry of the environment, and the energy changes of combustion link to chemical energetics.

Alkanes

Alkanes are saturated hydrocarbons: every carbon-carbon bond is single. They are unreactive apart from two reactions:

Combustion. Complete combustion gives carbon dioxide and water: CH4 + 2O2 → CO2 + 2H2O. In limited oxygen, incomplete combustion gives carbon monoxide (toxic) or carbon (soot) plus water.

Substitution with chlorine (S). In ultraviolet light, chlorine substitutes for hydrogen one atom at a time: CH4 + Cl2 → CH3Cl + HCl. The three scoring details: UV light is essential, the reaction is a substitution, and HCl is the other product. (S) Cracking breaks long alkanes into shorter alkanes plus alkenes (and sometimes hydrogen), using a high temperature and a catalyst. It balances supply against demand for the lighter fractions and is the industrial source of alkenes.

Alkenes

Alkenes are unsaturated (they contain a C=C double bond), and the double bond makes them reactive. The test for unsaturation is the most-asked test in the syllabus: add aqueous bromine, and an alkene turns it from orange to colourless. An alkane leaves it orange.

Addition reactions (S for the full set) all open the double bond:

• Bromine: C2H4 + Br2 → C2H4Br2 (the chemistry behind the bromine water test)
• Hydrogen: with a nickel catalyst, ethene + H2 → ethane (hydrogenation)
• Steam: with a phosphoric acid catalyst at 300°C and 60 atm, ethene + H2O → ethanol

One reaction, one product: that is what distinguishes addition from substitution, and stating it earns a mark in compare questions.

Alcohols

Ethanol is the exam workhorse. Two manufacturing routes, and Extended candidates must compare them:

• Fermentation: aqueous glucose with yeast at about 25-35°C, anaerobic conditions. Glucose → ethanol + carbon dioxide (C6H12O6 → 2C2H5OH + 2CO2). Renewable feedstock, low energy, but slow, batch-based, and the product needs purifying by fractional distillation.
• Catalytic addition of steam to ethene (S): fast, continuous, pure product, but the ethene comes from non-renewable petroleum and the conditions cost energy.

Ethanol burns cleanly (a fuel and a solvent, its two stated uses) and is oxidised to ethanoic acid, either by bacterial oxidation (how vinegar forms) or by heating with acidified aqueous potassium manganate(VII).

Carboxylic acids

Ethanoic acid, CH3COOH, is a weak acid, only partially dissociated in solution (the strong/weak distinction is Supplement and links back to acids, bases and salts). It does everything a dilute acid does, just more slowly: reacts with metals to give hydrogen, with carbonates to give carbon dioxide, with bases to give a salt and water. Its salts are ethanoates.

Esterification (S). Heat a carboxylic acid with an alcohol and a few drops of concentrated sulfuric acid (the catalyst) and you get an ester plus water. Ethanoic acid + ethanol → ethyl ethanoate + water. Name the ester correctly: the alcohol gives the first half of the name (ethyl), the acid gives the second (ethanoate). Esters are recognised by their sweet smells and used in flavourings and perfumes.

Polymers

Polymers are long-chain molecules built from repeating monomer units.

Addition polymerisation (Core for poly(ethene)). Alkene monomers join when their double bonds open: n ethene molecules → poly(ethene). Nothing else is produced. You must draw the repeat unit: two carbons, single bond between them, brackets, and n outside.

Condensation polymerisation (S). Two different functional groups react, releasing a small molecule (water) at each link. Polyamides such as nylon form from dicarboxylic acid + diamine monomers (amide linkage); polyesters such as PET form from dicarboxylic acid + diol (ester linkage). The contrast question is reliable: addition uses one monomer with a C=C and produces the polymer only; condensation uses two monomers with two functional groups each and produces water as well. Plastics’ disposal problems (non-biodegradable, toxic combustion products) round off the subtopic.

Isomerism (S)

Structural isomers have the same molecular formula but different structural formulae. Butane and methylpropane are both C4H10; butene has positional isomers too. You are expected to draw and name the isomers of molecules with up to four carbons. The classic trap is drawing the “same” isomer twice: a chain bent on paper is still the same straight chain.

Worked exam question

Compound X has the molecular formula C3H6. Describe a test to show that X is unsaturated, state the observation for X, and name the product formed when X reacts with bromine. [4]

Model answer: Add aqueous bromine / bromine water to X and shake (1). The bromine water turns from orange to colourless (1), because X contains a C=C double bond and bromine adds across it (1). The product is 1,2-dibromopropane / C3H6Br2 (1).

Mark-by-mark: M1 is the named reagent: “bromine water” or “aqueous bromine”, not just “bromine”. M2 is the colour change with both colours stated; “it goes clear” fails because clear is not a colour. M3 is the reason: addition across the double bond. M4 is the product, and on the Core paper “a dibromo compound forms” with the formula would be accepted; naming it precisely is Extended polish.

The mistakes that cost marks

1. “Clear” instead of “colourless”. The bromine water test result is orange to colourless. This single word is flagged in examiner reports series after series.
2. Unbalanced combustion equations. Balance the carbons, then the hydrogens, then count oxygens last, and accept a fraction in front of O2 if needed before doubling through. C2H6 + 3½O2 → 2CO2 + 3H2O is fine.
3. Substitution and addition swapped. Alkanes substitute (UV light, HCl produced); alkenes add (double bond opens, one product only). Stating “alkenes undergo substitution” undoes the whole question.
4. Displayed formulae with missing atoms. Count the hydrogens against the general formula before moving on. A C4 alcohol has nine hydrogens plus the OH hydrogen accounted for.
5. Polymer repeat units drawn with the double bond still in. The C=C opens during polymerisation. A repeat unit containing a double bond scores zero.

How to phrase it for full marks

Student wording	Mark-scheme wording
”Bromine water goes clear"	"Aqueous bromine turns from orange to colourless"
"Alkenes are reactive because of the double bond"	"Alkenes are unsaturated; the C=C double bond opens in addition reactions"
"Yeast turns sugar into alcohol"	"Yeast ferments aqueous glucose anaerobically at 25-35°C, producing ethanol and carbon dioxide"
"Nylon is made by joining monomers"	"A dicarboxylic acid and a diamine join with loss of water at each linkage: condensation polymerisation"
"It’s the same formula but different shape"	"Same molecular formula, different structural formula: structural isomers”

The pattern: name the series, name the reaction type, give both colours in any test. Because so much of this topic is Supplement, check which route you are sitting before you revise it. Our Core vs Extended guide maps exactly which organic content each paper can ask.

The Malaysia note

Organic chemistry is taught last in most Malaysian international schools (Year 11, term one), which leaves a single term before the May/June series to absorb the largest topic in the syllabus. Students aiming at A* routinely tell us esters and condensation polymers “weren’t really covered” because the school ran out of runway. The fix is structured: general formulae first, then one reaction map per series, then the Supplement extras as a separate block. That is a four-to-six-session rebuild for a typical student, and a free trial lesson will show you the reaction-map method on ethanol before you commit to anything.

Every sub-topic in Organic Chemistry

• Homologous Series and Naming
• Fuels
• Alkanes
• Alkenes
• Alcohols
• Carboxylic Acids
• Esters (Supplement)
• Polymers (Addition and Condensation)
• Isomerism (Supplement)