What you'll learn
Organic chemistry is the study of carbon-based compounds. This revision guide covers all WJEC GCSE testable content on hydrocarbons, functional groups, and key reactions. You'll learn to name compounds, balance combustion equations, test for alkenes, and understand the properties of different homologous series.
Key terms and definitions
Organic chemistry — the study of compounds containing carbon atoms, usually bonded to hydrogen and other elements
Homologous series — a family of organic compounds with the same general formula and similar chemical properties, differing by CH₂ units
Hydrocarbon — a compound containing only hydrogen and carbon atoms
Saturated hydrocarbon — a hydrocarbon containing only single carbon-carbon bonds (alkanes)
Unsaturated hydrocarbon — a hydrocarbon containing at least one carbon-carbon double bond (alkenes) or triple bond
Functional group — an atom or group of atoms that determines the characteristic chemical properties of a compound
Cracking — the thermal decomposition of long-chain hydrocarbons into shorter, more useful molecules
Combustion — a chemical reaction where a substance reacts with oxygen, releasing energy as heat and light
Core concepts
Carbon and bonding in organic compounds
Carbon atoms form four covalent bonds. This property allows carbon to:
- Form long chains and branched structures
- Create single, double and triple bonds with other carbon atoms
- Bond with hydrogen, oxygen, nitrogen and other elements
- Produce millions of different organic compounds
Carbon's ability to form stable bonds with itself (catenation) explains the vast diversity of organic molecules.
Alkanes: saturated hydrocarbons
Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. They contain only single C-C and C-H bonds.
The first four alkanes:
- Methane: CH₄
- Ethane: C₂H₆
- Propane: C₃H₈
- Butane: C₄H₁₀
Properties of alkanes:
- Unreactive compared to other organic compounds
- Poor reactivity due to strong C-C and C-H bonds
- Undergo complete combustion in excess oxygen
- Undergo incomplete combustion in limited oxygen
- Viscosity increases with chain length
- Boiling point increases with chain length
- Flammability decreases with chain length
Combustion reactions:
Complete combustion produces carbon dioxide and water:
Methane + oxygen → carbon dioxide + water CH₄ + 2O₂ → CO₂ + 2H₂O
Incomplete combustion occurs with insufficient oxygen, producing carbon monoxide (toxic) or carbon (soot) alongside water:
2CH₄ + 3O₂ → 2CO + 4H₂O
Uses of alkanes:
- Fuels (natural gas, LPG, petrol, diesel)
- Starting materials for making other chemicals
- Heating homes and cooking
Alkenes: unsaturated hydrocarbons
Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C). The general formula is CₙH₂ₙ.
The first four alkenes:
- Ethene: C₂H₄
- Propene: C₃H₆
- Butene: C₄H₈
- Pentene: C₅H₁₀
Properties of alkenes:
- More reactive than alkanes due to the C=C double bond
- Undergo addition reactions
- Used to make polymers
- Can be tested using bromine water
Testing for alkenes:
Add bromine water (orange/brown solution) to the unknown compound. If an alkene is present:
- The bromine water decolourises (turns colourless)
- An addition reaction occurs across the C=C bond
Alkanes do not decolourise bromine water because they lack the reactive double bond.
Addition reactions of alkenes:
The C=C double bond breaks and atoms add across it:
Hydrogenation — adding hydrogen to form an alkane: C₂H₄ + H₂ → C₂H₆
Bromination — adding bromine: C₂H₄ + Br₂ → C₂H₄Br₂
Hydration — adding water (steam) to form an alcohol: C₂H₄ + H₂O → C₂H₅OH
Cracking of hydrocarbons
Cracking is the thermal decomposition of long-chain hydrocarbons into shorter, more useful molecules. Long-chain alkanes from crude oil have limited uses but can be broken down into shorter alkanes and alkenes.
Why cracking is important:
- Produces high-demand fuels like petrol (short-chain alkanes)
- Generates alkenes for making polymers and other chemicals
- Converts less useful fractions into more valuable products
Conditions for cracking:
Two main industrial methods exist:
Thermal cracking:
- High temperature (700-1200°C)
- High pressure (up to 70 atmospheres)
Catalytic cracking:
- Lower temperature (450-500°C)
- Atmospheric pressure
- Uses zeolite catalysts (aluminium oxide and silicon dioxide)
Example cracking equation:
A long-chain alkane breaks into a shorter alkane and an alkene:
C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane → octane + ethene)
Or producing multiple products:
C₁₅H₃₂ → C₈H₁₈ + C₄H₈ + C₃H₆
Key points:
- Products always include at least one alkene
- Equations must balance for carbon and hydrogen atoms
- Different conditions produce different products
Alcohols
Alcohols form a homologous series containing the functional group -OH (hydroxyl group). The general formula is CₙH₂ₙ₊₁OH.
The 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
- Undergo combustion
- Used as solvents and fuels
- Boiling points increase with chain length
Complete combustion of alcohols:
Alcohols burn in excess oxygen to produce carbon dioxide and water:
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Production of ethanol:
Method 1: Fermentation
- Glucose (from sugar cane or cereals) is converted by yeast enzymes
- Temperature: 25-35°C
- Anaerobic conditions (no oxygen)
- Produces dilute ethanol solution (up to 15%)
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Advantages: Renewable raw materials, low energy requirements Disadvantages: Slow process, produces dilute ethanol requiring distillation
Method 2: Hydration of ethene
- Ethene from cracked crude oil reacts with steam
- Temperature: 300°C
- Pressure: 60-70 atmospheres
- Uses phosphoric acid catalyst
- Produces pure ethanol
C₂H₄ + H₂O → C₂H₅OH
Advantages: Fast, continuous process producing pure ethanol Disadvantages: Non-renewable, high energy costs
Uses of ethanol:
- Alcoholic drinks
- Solvent in perfumes, medicines and inks
- Fuel (bioethanol) or fuel additive
- Antiseptic
Carboxylic acids
Carboxylic acids contain the functional group -COOH. The general formula is CₙH₂ₙ₊₁COOH.
The first four carboxylic acids:
- Methanoic acid: HCOOH
- Ethanoic acid: CH₃COOH
- Propanoic acid: C₂H₅COOH
- Butanoic acid: C₃H₇COOH
Properties:
- Weak acids (partially ionise in water)
- pH 3-5 in dilute aqueous solution
- Dissolve in water
- React with carbonates to produce carbon dioxide
- React with alcohols to form esters
Reactions of carboxylic acids:
With carbonates: CH₃COOH + Na₂CO₃ → CH₃COONa + H₂O + CO₂
With bases: CH₃COOH + NaOH → CH₃COONa + H₂O
Ethanoic acid is found in vinegar (3-5% solution). Carboxylic acids have characteristic sharp, vinegary smells.
Worked examples
Example 1: Balancing combustion equations
Question: Write a balanced symbol equation for the complete combustion of propane (C₃H₈). [3 marks]
Solution:
Step 1: Write the word equation Propane + oxygen → carbon dioxide + water [1 mark]
Step 2: Write the unbalanced symbol equation C₃H₈ + O₂ → CO₂ + H₂O
Step 3: Balance carbon atoms C₃H₈ + O₂ → 3CO₂ + H₂O
Step 4: Balance hydrogen atoms C₃H₈ + O₂ → 3CO₂ + 4H₂O
Step 5: Balance oxygen atoms C₃H₈ + 5O₂ → 3CO₂ + 4H₂O [2 marks]
Mark scheme notes: Award 1 mark for correct reactants and products, 2 marks for fully balanced equation.
Example 2: Cracking equations
Question: Hexane (C₆H₁₄) can be cracked to form butane and one other product. Complete the equation: C₆H₁₄ → C₄H₁₀ + __________ [2 marks]
Solution:
Step 1: Calculate carbon atoms needed 6 carbons in hexane - 4 carbons in butane = 2 carbons in other product
Step 2: Calculate hydrogen atoms needed 14 hydrogens in hexane - 10 hydrogens in butane = 4 hydrogens in other product
Step 3: Identify the product C₂H₄ (ethene) [1 mark]
Complete equation: C₆H₁₄ → C₄H₁₀ + C₂H₄ [1 mark]
Mark scheme notes: Award 1 mark for correct molecular formula, 1 mark for balanced equation.
Example 3: Comparing fermentation and hydration
Question: Ethanol for use as a fuel can be produced by fermentation or by hydration of ethene. Compare these two methods. [4 marks]
Model answer:
Fermentation uses renewable resources (sugar crops) whereas hydration uses ethene from non-renewable crude oil [1 mark]. Fermentation operates at low temperatures (25-35°C) while hydration requires 300°C and high pressure [1 mark]. Hydration is a faster, continuous process but fermentation is slower and produces dilute ethanol [1 mark]. Fermentation has lower energy costs but requires distillation to purify the ethanol [1 mark].
Mark scheme notes: Award marks for valid comparisons covering raw materials, conditions, rate/efficiency, or product purity.
Common mistakes and how to avoid them
Confusing alkanes and alkenes: Remember alkanes end in -ane and have single bonds only (CₙH₂ₙ₊₂). Alkenes end in -ene and contain C=C double bonds (CₙH₂ₙ). Check the general formula carefully.
Incorrect balancing of combustion equations: Always balance carbon atoms first, then hydrogen, then oxygen last. Remember complete combustion produces only CO₂ and H₂O, while incomplete combustion produces CO or C as well.
Forgetting that cracking must produce an alkene: Every cracking equation must include at least one unsaturated product (alkene). If your products are all alkanes, the equation is wrong.
Muddling up the bromine water test: Alkenes decolourise bromine water; alkanes do not react. Don't say the bromine water "disappears" or "goes clear" — use the term "decolourises from orange/brown to colourless."
Writing alcohol formulas incorrectly: The -OH group must be shown clearly. Ethanol is C₂H₅OH, not C₂H₆O (which doesn't show the functional group). In structural formulas, ensure -OH is visible.
Reversing fermentation conditions: Fermentation requires anaerobic conditions (no oxygen) at 25-35°C. Don't confuse this with aerobic respiration or other processes requiring oxygen.
Exam technique for "Organic Chemistry"
Command words matter: "State" requires a brief answer with no explanation (1 mark). "Explain" requires reasoning (2-3 marks). "Compare" means give similarities and differences. "Describe" needs a sequence or characteristics without explanation.
Show all working in equation questions: Even if your final answer is wrong, you can gain marks for method. Always write the unbalanced equation first, then balance systematically. Check your final equation has equal atoms on both sides.
Use correct chemical terminology: Write "decolourises" not "goes clear," "thermal decomposition" not "breaking down," "saturated" not "full." Precision scores marks, especially in 3-4 mark questions.
Structure extended answers using the mark allocation: A 4-mark question typically needs four distinct points. Use the marks as a guide to how much detail is required. Link points logically using connectives like "therefore," "because," and "however."
Quick revision summary
Organic chemistry studies carbon compounds. Alkanes (CₙH₂ₙ₊₂) are saturated hydrocarbons with single bonds; alkenes (CₙH₂ₙ) are unsaturated with C=C double bonds that decolourise bromine water. Cracking breaks long-chain hydrocarbons into shorter alkanes and alkenes using heat and catalysts. Alcohols contain the -OH functional group and form via fermentation or ethene hydration. Complete combustion of hydrocarbons and alcohols produces carbon dioxide and water. Carboxylic acids are weak acids containing -COOH groups.