Organic Chemistry: IGCSE Chemistry 0620

Organic Chemistry is easiest to revise as a connected reaction map rather than a list of compounds. The main sequence is:

petroleum fractions → alkanes → cracking → alkenes → alcohols / polymers → carboxylic acids → esters

The topic contains both Core and Supplement outcomes. Each detailed subtopic page identifies the relevant tier rather than marking the whole organic cluster as one level.

Petroleum, fuels and fractional distillation (Core)

Petroleum is a mixture of hydrocarbons. It is separated into fractions by fractional distillation.

The fractionating column is hot at the bottom and cooler at the top. Molecules with high boiling points condense lower down; molecules with lower boiling points rise further before condensing.

Required fraction-use relationships include:

Fraction	Typical use
Refinery gas	bottled gas for heating and cooking
Gasoline / petrol	fuel for cars
Naphtha	chemical feedstock
Kerosene / paraffin	jet fuel
Diesel / gas oil	fuel for diesel engines
Fuel oil	fuel for ships and home heating systems
Lubricating fraction	lubricants, waxes and polishes
Bitumen	road surfaces and roofing

As hydrocarbon molecules become larger:

• boiling point increases
• viscosity increases
• volatility decreases
• ease of ignition decreases

The pollution produced by burning fossil fuels belongs to Chemistry of the Environment.

Homologous series (Core)

A homologous series is a family of organic compounds with:

• the same functional group
• the same general formula
• similar chemical properties
• a gradual trend in physical properties
• successive members differing by CH2

Series	General formula	Functional group
Alkanes	CnH2n+2	carbon-carbon single bonds only
Alkenes	CnH2n	C=C
Alcohols	CnH2n+1OH	–OH
Carboxylic acids	CnH2n+1COOH	–COOH

Use meth-, eth-, prop- and but- for one to four carbon atoms.

Alkanes (Core)

Alkanes are saturated hydrocarbons. They contain carbon and hydrogen only and have carbon-carbon single bonds.

Complete and incomplete combustion

Complete combustion produces carbon dioxide and water:

CH4 + 2O2 → CO2 + 2H2O

Limited oxygen can produce carbon monoxide and/or carbon particles as well as water.

Substitution with chlorine

In ultraviolet light, chlorine can replace a hydrogen atom in an alkane:

CH4 + Cl2 → CH3Cl + HCl

The reaction type is substitution. The conditions and products matter.

Cracking (Core)

Cracking breaks larger alkane molecules into smaller hydrocarbons. Products include a smaller alkane and an alkene.

Conditions:

• high temperature
• a catalyst

Example:

C10H22 → C8H18 + C2H4

Cracking is used because the supply of larger fractions does not always match demand, while smaller fuels and alkenes are commercially useful.

The previous version of this page incorrectly labelled cracking as Supplement. It is Core for the current syllabus cycle.

Alkenes (Core with Supplement depth)

Alkenes are unsaturated because they contain a carbon-carbon double bond.

Test for unsaturation

Add aqueous bromine:

• alkene: orange to colourless
• alkane: remains orange under the test conditions

Write colourless, not “clear”.

Addition reactions

The double bond opens and atoms add to the two carbon atoms.

• bromine adds to form a dibromo compound
• hydrogen adds to form an alkane, usually with a nickel catalyst
• steam adds to ethene to form ethanol under industrial conditions

One organic product is formed in a simple addition reaction.

Alcohols (mixed Core and Supplement)

Ethanol is produced by fermentation of glucose:

C6H12O6 → 2C2H5OH + 2CO2

Conditions include yeast, about 25-35°C and absence of oxygen.

Ethanol can also be produced by adding steam to ethene at about 300°C and 60 atm with a phosphoric acid catalyst. Comparing the two manufacturing routes is Supplement depth.

Ethanol is used as a fuel and solvent.

Complete combustion produces carbon dioxide and water:

C2H5OH + 3O2 → 2CO2 + 3H2O

Controlled oxidation produces ethanoic acid, for example using acidified aqueous potassium manganate(VII) or bacterial oxidation in air. Combustion is not a route to ethanoic acid.

See Alcohols for the full Core/Supplement breakdown.

Carboxylic acids (mixed Core and Supplement)

Carboxylic acids contain the –COOH functional group. Ethanoic acid reacts as an acid with:

• metals, producing hydrogen
• carbonates, producing carbon dioxide
• bases, producing a salt and water

Its salts are ethanoates.

Esterification (Supplement)

A carboxylic acid reacts with an alcohol to form an ester and water. Concentrated sulfuric acid is used as a catalyst and the mixture is heated.

ethanoic acid + ethanol ⇌ ethyl ethanoate + water

The alcohol supplies the first part of the ester name; the acid supplies the alkanoate ending.

Polymers (mixed Core and Supplement)

A polymer is a long-chain molecule made from many monomers.

Addition polymerisation

Alkene monomers join when their double bonds open. No small molecule is eliminated.

A repeat unit must show:

• the double bond changed to a single bond
• the correct side groups
• continuation bonds passing through brackets
• n outside the brackets

Condensation polymerisation (Supplement)

Monomers with two suitable functional groups join and eliminate a small molecule at each link.

• dicarboxylic acid + diamine can form a polyamide
• dicarboxylic acid + diol can form a polyester

The key contrast:

• addition polymerisation normally uses an alkene and forms the polymer only
• condensation polymerisation uses bifunctional monomers and forms the polymer plus a small molecule

Structural isomerism (Supplement)

Structural isomers have the same molecular formula but different structural formulae.

Butane and methylpropane are both C4H10. A bent drawing of the same chain is not a new isomer; the atom connectivity must differ.

Worked examination question

Compound X has molecular formula C3H6.

(a) Describe a test for unsaturation and the positive observation. [2]

• add aqueous bromine [1]
• orange changes to colourless [1]

(b) Explain the result. [2]

• X contains a C=C double bond / is unsaturated [1]
• bromine adds across the double bond [1]

(c) State the reaction type. [1]

• addition [1]

Common mistakes

1. Labelling cracking as Supplement. It is Core in the current syllabus.
2. Using the wrong fuel-oil use. Learn the syllabus relationship: ships and home heating systems.
3. Writing “clear” for the bromine-water result. The colour is colourless.
4. Leaving the double bond inside a polymer repeat unit. It opens during addition polymerisation.
5. Confusing addition and substitution. Alkenes undergo addition; the alkane/chlorine reaction is substitution.
6. Treating combustion as oxidation to ethanoic acid. Complete combustion forms carbon dioxide and water.
7. Mixing fermentation and hydration conditions. Keep the routes in separate columns.
8. Drawing the same molecule in a different orientation and calling it an isomer. Connectivity must change.

Exam-ready reaction map

Build one page containing:

• fraction → use
• alkane → combustion products
• alkane + chlorine under UV → substituted product + HCl
• large alkane → smaller alkane + alkene by cracking
• alkene + aqueous bromine → dibromo product
• ethene + steam → ethanol
• ethanol → ethanoic acid by controlled oxidation
• alcohol + carboxylic acid → ester + water
• alkene → addition polymer

Then use the organic Chemistry exam-technique guide for naming, displayed formulae, conditions and multi-step pathways.

Topic route

1. Fuels and fractional distillation
2. Homologous series and naming
3. Alkanes
4. Alkenes
5. Alcohols
6. Carboxylic acids
7. Esters
8. Polymers
9. Isomerism

Every subtopic in Organic Chemistry

Tier labels follow the current page metadata. “Core + Supplement” means the page identifies the boundary section by section.

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