Organic chemistry is the study of carbon compounds. It underpins the fuels that power vehicles, the plastics in everyday objects and even food chemistry.
This topic is tested heavily in Paper 2 structured questions, so the definitions and trend statements must be learnt word-perfect. The notes run from crude oil and fractional distillation through homologous series and isomerism, alkanes, alkenes and cracking, alcohols and carboxylic acids, to addition and condensation polymers.
What these notes cover
- •Fossil fuels and fractional distillation of crude oil
- •The seven fractions and their trends
- •Fuel or feedstock, and biofuels
- •Homologous series and general formulae
- •Molecular and full structural formulae, and naming
- •Isomerism
- •Alkanes: combustion and substitution
- •Alkenes: cracking and addition reactions
- •The bromine test for unsaturation
- •Alcohols: making and reacting ethanol
- •Carboxylic acids and esterification
- •Addition and condensation polymerisation
Key definitions
- Hydrocarbon
- A compound containing hydrogen and carbon only. Ethanol contains hydrogen and carbon but is not a hydrocarbon because it also contains oxygen.
- Homologous series
- A family of organic compounds in which all members have the same general formula, contain the same functional group, differ from the next member by a CH₂ unit, and have similar chemical properties.
- Isomers
- Compounds with the same molecular formula but different structural formulae, with their atoms joined in a different arrangement.
- Saturated and unsaturated compounds
- A saturated compound contains only carbon–carbon single bonds. An unsaturated compound contains at least one carbon–carbon double bond, C=C.
- Polymer
- A very large molecule (a macromolecule) formed when many small molecules, called monomers, join together in a reaction called polymerisation.
Key formulas
General formula of alkanes
CₙH₂ₙ₊₂
n = number of carbon atoms. Alkanes are saturated, with single C–C bonds only. Using Aᵣ: C = 12, H = 1, Mᵣ = 14n + 2.
General formula of alkenes
CₙH₂ₙ
Alkenes contain a C=C double bond and are unsaturated. The series starts at n = 2 (ethene, C₂H₄); there is no one-carbon alkene.
General formula of alcohols
CₙH₂ₙ₊₁OH
The functional group is the hydroxyl group, –OH, and names end in -ol, e.g. methanol CH₃OH.
Complete combustion of propane
C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l)
With plentiful oxygen, an alkane gives carbon dioxide and water only. Balance C first, then H, then O last.
Substitution of methane with chlorine
CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)
This reaction needs UV light, and stating the condition earns a mark. It continues stepwise to CH₂Cl₂, CHCl₃ and CCl₄.
Esterification of ethanoic acid with ethanol
CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l)
The mixture is warmed with a few drops of concentrated H₂SO₄ as catalyst. The reaction is reversible and the product is ethyl ethanoate.
Worked example
Write the balanced equation, with state symbols, for the complete combustion of butane, C₄H₁₀.
- 1Balance carbon first: 4 carbon atoms on the left give 4CO₂ on the right.
- 2Balance hydrogen next: 10 hydrogen atoms on the left give 5H₂O on the right.
- 3Count oxygen on the right last: 4(2) + 5(1) = 13 atoms, so 13 ÷ 2 molecules of O₂ are needed, giving C₄H₁₀(g) + (13 ÷ 2)O₂(g) → 4CO₂(g) + 5H₂O(l).
- 4A fraction is not acceptable in a final answer, so multiply every coefficient by 2: 2C₄H₁₀(g) + 13O₂(g) → 8CO₂(g) + 10H₂O(l).
Watch out: Do not balance oxygen before carbon and hydrogen. Oxygen goes last, and any half is cleared by doubling every coefficient.
Common mistakes
Leaving out 'only' in the hydrocarbon definition
Wrong: A hydrocarbon contains hydrogen and carbon.
Right: A hydrocarbon contains hydrogen and carbon only. Ethanol contains both elements but is not a hydrocarbon.
Saying physical properties are similar
Wrong: Members of a homologous series have similar physical properties.
Right: Members have similar chemical properties. Physical properties such as melting point, boiling point and viscosity increase steadily down the series.
Describing the bromine test as turning clear
Wrong: The bromine turns clear.
Right: The orange bromine is rapidly decolourised, changing from orange to colourless.
Questions students ask
How do you test for an alkene?
Add aqueous bromine to the compound and shake at room temperature. An alkene rapidly decolourises the bromine from orange to colourless, while an alkane leaves it orange. Write 'rapidly', because alkanes slowly decolourise bromine in UV light.
Why can fractional distillation separate crude oil?
Crude oil is a mixture of hydrocarbons with different boiling points, because the hydrocarbons have different chain lengths. In the fractionating column, which is hot at the bottom and cool at the top, each group of hydrocarbons condenses at a different height and is collected as a separate fraction. A full answer needs both 'mixture' and 'different boiling points'.
Why is cracking needed?
Distillation produces too much of the long-chain fractions and not enough petrol-range alkanes, so cracking converts the surplus into more useful molecules. Cracking is also the main source of alkenes, which are needed for plastics and ethanol and do not occur in crude oil. It uses a high temperature of about 600 °C and a catalyst such as aluminium oxide.
What is the difference between addition and condensation polymerisation?
In addition polymerisation, monomers containing a C=C double bond join to give one product only, the polymer. In condensation polymerisation, monomers with a functional group at each end join with the loss of a small molecule, usually water. An example of condensation is Terylene, made from a dicarboxylic acid and a diol.
In the full PDF
- •Displayed-formula diagrams of methane, ethane, propane, ethene and propene, plus the C₄H₁₀ and C₄H₈ isomer pairs
- •The full table of the seven crude-oil fractions with chain lengths and uses, and model 2-mark answers
- •Further worked examples, including identifying compounds from Mᵣ, cracking hexadecane, naming methyl propanoate and working back from a polymer chain to its monomer
- •The comparison of fermentation and steam hydration routes to ethanol, and the acid reactions of ethanoic acid with equations
- •The remaining common mistakes for every section
- •End-of-section summaries for all six sections