Organic Compounds and How We Represent Them — Introduction to Organic Chemistry | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 3% of your exam marks.

stable

Rare

Stable3%

Homologous series, IUPAC naming and functional groups; often a short opening question.

What organic chemistry is

Organic chemistry is the chemistry of compounds containing the element carbon
By long-standing convention, a handful of carbon-containing substances are treated as inorganic and not studied as part of organic chemistry: carbon monoxide (CO), carbon dioxide (CO₂) and the metal carbonates (e.g. CaCO₃)
Carbon is uniquely able to form four covalent bonds and to chain with itself, which is why organic compounds span tens of millions of known molecules — from methane to DNA

Hydrocarbons

A hydrocarbon is an organic compound that contains only hydrogen and carbon atoms — no other element
Methane (CH₄), ethane (C₂H₆), petrol (a mixture of C₅–C₁₀ alkanes) and natural rubber are all hydrocarbons
An alcohol such as ethanol (C₂H₅OH) is an organic compound but not a hydrocarbon because it contains an oxygen atom

Exam tip

Define a hydrocarbon

What comes up: "State what is meant by a hydrocarbon" or "Explain why compound X is/is not a hydrocarbon."

Write (two marks): (1) contains only carbon and hydrogen atoms; (2) no other element present. Saying it contains carbon and hydrogen alone scores M1; adding "only" (or "no other elements") secures M2.

Watch out: Writing "molecules" instead of "atoms" is rejected. For the two-mark version, you must explicitly say "only" — the mark scheme awards a dependent mark for that word.

Five ways to write the same molecule

The same organic molecule can be drawn in several different forms; each form throws away or keeps different amounts of detail. Using ethanol as the running example:

Type of formula	What it shows	Ethanol
Empirical formula	Simplest whole-number ratio of atoms	C₂H₆O
Molecular formula	Actual number of each kind of atom in one molecule	C₂H₆O
General formula

Exam tip

Features of a homologous series

Stating features of a homologous series comes up (2–3 marks), so you need to know them: same general formula, same functional group, consecutive members differ by CH₂, similar chemical properties, and a trend in physical properties (e.g. boiling point). Say "trend in physical properties", not "same physical properties" (which is rejected).

Functional groups

A functional group is a particular arrangement of atoms within a molecule that gives it its characteristic chemistry
Two molecules in the same homologous series react in the same way precisely because they share the same functional group

Isomers

Isomers are different compounds that share the same but whose atoms are joined together differently — they therefore have different structural (and often different displayed) formulae
Isomers can have similar physical and chemical properties if they are in the same homologous series, or quite different properties if their atoms are arranged into different functional groups
A clean example: the molecular formula C₃H₆ describes two unrelated compounds
- Propene: a straight-chain alkene with a C=C double bond — CH₂=CH−CH₃
- Cyclopropane: a closed three-carbon ring of single bonds — no C=C double bond, no functional group
Propene reacts with bromine water and decolourises it (because of its C=C); cyclopropane does not. Same molecular formula, very different chemistry

Exam tip

Define isomers

Defining isomers comes up, so you need to know they have the same molecular formula but different structural formulae (atoms arranged differently). "Same empirical formula" or "same general formula" is rejected, and isomers are compounds, not elements.