Introduction to Organic Chemistry
Organic Chemistry · 2 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 3% of your exam marks.
Homologous series, IUPAC naming and functional groups; often a short opening question.
The three reaction types you need to know
- Substitution — one atom (or group) is replaced by another atom or group
- Addition — two reactant molecules merge into one product molecule and nothing else is given off
- Combustion — the organic compound burns in oxygen to give carbon-containing oxides plus water
Substitution reactions
- A characteristic feature: an existing atom or group (often a single C–H or C–X bond) is swapped for a new one, and a small by-product (usually HX) is released
- Example: chlorination of ethane in ultraviolet light
C2H6(g) + Cl2(g) → C2H5Cl(g) + HCl(g)
ethane + chlorine → chloroethane + hydrogen chloride
- One hydrogen on the ethane has been swapped for a chlorine atom; the displaced hydrogen pairs up with the second chlorine atom from Cl2 to make HCl
Addition reactions
- A characteristic feature: two or more molecules combine into one product molecule, with nothing else released
- The molecule that accepts the new atoms typically has a C=C double bond, which "opens up" to single bonds and accommodates the incoming atoms
- Example: hydrogenation of propene (margarine manufacture)
CH3−CH=CH2(g) + H2(g) → CH3−CH2−CH3(g)
propene + hydrogen → propane
- Example: bromination of propene (also the test for an alkene)
CH3−CH=CH2(g) + Br2(l) → CH3−CHBr−CH2Br(l)
propene + bromine → 1,2-dibromopropane
- Notice that both examples have one product, not two
Combustion reactions
- All organic compounds burn in oxygen; whether the products are carbon dioxide or carbon monoxide depends on how much oxygen is available
- (plenty of oxygen): products are carbon dioxide and water only
2 C2H6(g) + 7 O2(g) → 4 CO2(g) + 6 H2O(l) (ethane)
2 C4H10(g) + 13 O2(g) → 8 CO2(g) + 10 H2O(l) (butane)
- (limited oxygen): one or more carbons end up as carbon monoxide (a colourless, odourless poison) or as soot (unburnt carbon particles)
2 C2H6(g) + 5 O2(g) → 4 CO(g) + 6 H2O(l) (ethane — incomplete)
- Incomplete combustion is why gas heaters need good ventilation: a build-up of CO is invisible and lethal
- All combustion reactions are exothermic — they are why fossil fuels are used as energy sources
Why incomplete combustion is dangerous
What comes up: "Explain why incomplete combustion of a hydrocarbon is dangerous" (2 marks) or "State what is produced in incomplete combustion" then "Why is this harmful?" (1 + 1).
Write (two marks): (1) carbon monoxide (CO) is produced; (2) carbon monoxide is toxic/poisonous — it reduces the capacity of blood to transport oxygen around the body. M2 depends on naming carbon monoxide in M1.
Watch out: Simply writing "harmful" or "dangerous" scores nothing for the second mark — the mark scheme explicitly ignores vague terms like "harmful." You must state the mechanism: CO binds to haemoglobin and prevents blood from carrying oxygen. Also note: carbon/soot is a second product of incomplete combustion, but the 2-mark hazard question is about CO specifically.
Quick recognition guide
| Reaction type | Number of reactant molecules | Number of product molecules | Clue |
|---|---|---|---|
| Substitution | 2 | 2 (one bigger, one small by-product) | A small molecule HX (or similar) is given off |
| Addition | 2 | 1 | A single combined product, no by-product |
| Combustion | An organic compound + O2 | CO2 (or CO) + H2O | Heat released; the only products contain only C, H, O |