What you'll learn
Organic chemistry focuses on carbon-based compounds and their reactions. This section covers the structure, naming and properties of homologous series including alkanes, alkenes, alcohols and carboxylic acids. You'll learn to identify functional groups, understand combustion and addition reactions, and apply knowledge of polymerisation and cracking processes essential for the AQA GCSE Chemistry specification.
Key terms and definitions
Hydrocarbon — A compound containing only hydrogen and carbon atoms.
Homologous series — A family of compounds with the same general formula and similar chemical properties, differing by CH₂ in successive members.
Functional group — An atom or group of atoms responsible for the characteristic reactions of a compound.
Saturated hydrocarbon — A hydrocarbon containing only single covalent bonds between carbon atoms (alkanes).
Unsaturated hydrocarbon — A hydrocarbon containing at least one carbon-carbon double bond (alkenes).
Polymerisation — The joining of many small molecules (monomers) to form very large molecules (polymers).
Cracking — The breaking down of long-chain hydrocarbons into shorter, more useful molecules.
Combustion — A chemical reaction with oxygen that releases energy in the form of heat and light.
Core concepts
Crude oil and hydrocarbons
Crude oil is a mixture of many different hydrocarbons formed over millions of years from the remains of ancient marine organisms. It represents a finite resource extracted through drilling.
Fractional distillation separates crude oil into useful fractions:
- Crude oil is heated until most compounds vaporise
- Vapours enter a fractionating column that is hot at the bottom and cooler at the top
- Different fractions condense at different heights depending on their boiling points
- Larger molecules with stronger intermolecular forces condense first (lower down)
- Smaller molecules with weaker intermolecular forces condense higher up
The main fractions in order of increasing boiling point are:
- Gases (used for domestic heating and cooking)
- Petrol (fuel for cars)
- Kerosene (aircraft fuel)
- Diesel oil (fuel for lorries and some cars)
- Fuel oil (fuel for ships and power stations)
- Bitumen (surfacing roads)
Properties of hydrocarbons:
- As chain length increases, boiling point increases (stronger intermolecular forces)
- As chain length increases, viscosity increases (flows less easily)
- As chain length increases, flammability decreases (less easy to ignite)
Alkanes
Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. They form a homologous series containing only single C-C bonds.
First four alkanes:
- Methane: CH₄
- Ethane: C₂H₆
- Propane: C₃H₈
- Butane: C₄H₁₀
Alkanes are relatively unreactive but undergo complete combustion in plentiful oxygen:
alkane + oxygen → carbon dioxide + water
Example: CH₄ + 2O₂ → CO₂ + 2H₂O
Incomplete combustion occurs when oxygen supply is limited, producing carbon monoxide (toxic) or carbon (soot):
2CH₄ + 3O₂ → 2CO + 4H₂O
Alkanes can also undergo substitution reactions with halogens in the presence of UV light, where hydrogen atoms are replaced by halogen atoms.
Alkenes
Alkenes are unsaturated hydrocarbons with the general formula CₙH₂ₙ. They contain at least one C=C double bond, making them more reactive than alkanes.
First four alkenes:
- Ethene: C₂H₄
- Propene: C₃H₆
- Butene: C₄H₈
- Pentene: C₅H₁₀
Testing for alkenes: Alkenes decolourise bromine water (changes from orange to colourless). This is used to distinguish unsaturated compounds from saturated compounds. Alkanes do not decolourise bromine water.
Addition reactions: The C=C double bond allows alkenes to undergo addition reactions where the double bond opens and atoms are added across it.
Hydrogenation: alkene + hydrogen → alkane
C₂H₄ + H₂ → C₂H₆
Requires a nickel catalyst at 150°C. Used in margarine production to convert liquid vegetable oils into solid fats.
Halogenation: alkene + halogen → dihaloalkane
C₂H₄ + Br₂ → C₂H₄Br₂
Hydration: alkene + steam → alcohol
C₂H₄ + H₂O → C₂H₅OH
Requires a phosphoric acid catalyst at 300°C and 60-70 atmospheres pressure. Used to produce ethanol industrially.
Cracking and polymerisation
Cracking breaks down long-chain hydrocarbons into shorter, more useful molecules. This process is essential because crude oil contains a surplus of long-chain hydrocarbons but high demand exists for shorter chains like petrol.
Catalytic cracking:
- Long-chain hydrocarbon vapours passed over hot aluminium oxide catalyst (600-700°C)
- Produces a mixture of shorter alkanes and alkenes
Steam cracking:
- Long-chain hydrocarbons mixed with steam and heated to very high temperatures
- No catalyst required
Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄
Polymerisation joins many small alkene molecules (monomers) to form long-chain molecules (polymers). This is an addition reaction where the C=C double bond breaks and monomers join together.
Common polymers:
- Poly(ethene): made from ethene monomers, used for plastic bags and bottles
- Poly(propene): made from propene monomers, used for crates and ropes
- Poly(chloroethene) or PVC: made from chloroethene monomers, used for window frames and pipes
The polymer name is formed by putting the monomer name in brackets after "poly".
Alcohols
Alcohols form a homologous series with the general formula CₙH₂ₙ₊₁OH. The functional group is -OH (hydroxyl group).
First four alcohols:
- Methanol: CH₃OH
- Ethanol: C₂H₅OH
- Propanol: C₃H₇OH
- Butanol: C₄H₉OH
Properties of alcohols:
- Dissolve in water to form neutral solutions
- React with sodium to produce hydrogen gas
- Burn in air (combustion)
- Used as solvents and fuels
Complete combustion of ethanol: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Production of ethanol:
Method 1 — Fermentation:
- Sugar solution mixed with yeast (contains enzymes)
- Kept at approximately 30°C in anaerobic conditions
- Produces ethanol and carbon dioxide
- Equation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
- Maximum concentration of 15% (enzymes denatured at higher concentrations)
- Used to produce alcoholic drinks
- Renewable process using plant materials
Method 2 — Hydration of ethene (covered in alkenes section):
- Continuous process
- Produces pure ethanol
- Uses non-renewable crude oil feedstock
- Faster reaction rate
Oxidation of alcohols: Alcohols can be oxidised by chemical oxidising agents or by microbial action to produce carboxylic acids.
ethanol + oxygen → ethanoic acid + water
C₂H₅OH + O₂ → CH₃COOH + H₂O
This reaction occurs when wine is left open to air, turning it sour.
Carboxylic acids
Carboxylic acids form a homologous series with the general formula CₙH₂ₙ₊₁COOH. The functional group is -COOH (carboxyl group).
First four carboxylic acids:
- Methanoic acid: HCOOH
- Ethanoic acid: CH₃COOH
- Propanoic acid: C₂H₅COOH
- Butanoic acid: C₃H₇COOH
Properties:
- Weak acids (only partially ionise in water)
- pH values of 3-6 in solution
- React with carbonates to produce carbon dioxide, water and a salt
- React with alcohols in the presence of an acid catalyst to form esters
Reactions:
With metals (reactive): 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂
With metal carbonates: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
Esterification: Carboxylic acids react with alcohols in the presence of a concentrated sulfuric acid catalyst to produce esters and water.
ethanoic acid + ethanol ⇌ ethyl ethanoate + water
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
Esters have distinctive sweet, fruity smells and are used in perfumes and food flavourings. The reaction is reversible.
Worked examples
Example 1: A student performs a cracking experiment by passing hexane vapour over a heated catalyst. The products are propene and another hydrocarbon X.
a) Write a balanced equation for this cracking reaction. [2 marks]
b) Describe a test to show that propene is unsaturated. Include the result. [2 marks]
Solution:
a) C₆H₁₄ → C₃H₆ + C₃H₈ [1 mark for formula of X; 1 mark for balanced equation]
b) Add bromine water to propene [1 mark]. The orange/brown bromine water will turn colourless [1 mark].
Example 2: Ethanol can be produced by two different methods: fermentation of glucose and hydration of ethene.
a) Write a balanced symbol equation for the fermentation of glucose. [2 marks]
b) State one advantage and one disadvantage of producing ethanol by fermentation compared to hydration of ethene. [2 marks]
Solution:
a) C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ [2 marks for correct balanced equation]
b) Advantage: Uses renewable resources / glucose from plants / carbon neutral [1 mark]
Disadvantage: Produces dilute ethanol solution / batch process is slower / requires distillation / lower concentration [1 mark]
Example 3: A student investigates the complete combustion of propanol.
a) Write a balanced symbol equation for the complete combustion of propanol (C₃H₇OH). [3 marks]
b) State why incomplete combustion is dangerous. [2 marks]
Solution:
a) C₃H₇OH + 4½O₂ → 3CO₂ + 4H₂O or 2C₃H₇OH + 9O₂ → 6CO₂ + 8H₂O [1 mark for correct reactants and products; 1 mark for correct balancing; 1 mark for state symbols if required or correct whole number ratios]
b) Incomplete combustion produces carbon monoxide [1 mark], which is toxic/poisonous/binds to haemoglobin and prevents oxygen transport [1 mark].
Common mistakes and how to avoid them
Confusing alkanes and alkenes: Remember alkanes are saturated (CₙH₂ₙ₊₂, single bonds only) while alkenes are unsaturated (CₙH₂ₙ, contain C=C double bonds). The bromine water test only works for alkenes.
Incorrect naming of carboxylic acids: The -COOH group must be included in the name. Ethanoic acid is CH₃COOH, not C₂COOH. The number of carbons includes the carbon in the carboxyl group.
Writing unbalanced combustion equations: Always count atoms on both sides. Remember water formula is H₂O and carbon dioxide is CO₂. For complete combustion, products are always CO₂ and H₂O only.
Mixing up fermentation conditions: Fermentation requires anaerobic conditions (no oxygen), approximately 30°C, yeast/enzymes, and a sugar solution. Don't confuse with aerobic respiration or hydration of ethene.
Not recognising functional groups: Learn to identify -OH (alcohols), -COOH (carboxylic acids), and C=C (alkenes) in structural formulae. The functional group determines the compound's properties and reactions.
Forgetting that cracking produces alkenes: Cracking always produces at least one alkene alongside shorter alkanes. If asked to name products, include both types of molecules.
Exam technique for "Organic Chemistry"
Command words matter: "State" requires a brief answer without explanation. "Explain" requires reasoning. "Describe" requires characteristics or how something happens. For balancing equations, ensure you show your working to gain method marks even if the final answer is incorrect.
Drawing displayed formulae: Show all covalent bonds as lines and all atoms. Don't use condensed formulae when the question asks for displayed or structural formulae. Check you've shown the correct number of bonds for each atom (carbon = 4, hydrogen = 1, oxygen = 2).
Using correct terminology: Use precise scientific terms like "decolourises" rather than "goes clear" for the bromine water test. Write "saturated" not "no double bonds" when describing alkanes. State "functional group" when identifying -OH or -COOH groups.
Naming organic compounds: Follow the stem (meth-, eth-, prop-, but-) based on carbon number, then add the appropriate ending (-ane for alkanes, -ene for alkenes, -anol for alcohols, -anoic acid for carboxylic acids). For polymers, write poly(monomer name) in brackets.
Quick revision summary
Organic chemistry studies carbon compounds. Alkanes (CₙH₂ₙ₊₂) are saturated hydrocarbons undergoing combustion and substitution. Alkenes (CₙH₂ₙ) are unsaturated, containing C=C bonds, and undergo addition reactions including polymerisation. Cracking breaks long chains into useful shorter hydrocarbons. Alcohols (CₙH₂ₙ₊₁OH) are produced by fermentation or hydration of alkenes and oxidise to carboxylic acids (CₙH₂ₙ₊₁COOH). Carboxylic acids react with alcohols to form esters. Know functional groups, test for unsaturation using bromine water, and balance combustion equations carefully.