Organic chemistry

Chemistry 1110 · CCEA GCSE · 10 min read · Updated: 31 Aug 2026

Welcome to Organic Chemistry!

Organic chemistry is the study of carbon-based compounds. Carbon is a truly incredible element—it can bond with itself to form long chains, rings, and complex branched structures. Nearly everything around you, from fuels and plastics to medicines and your own DNA, is built on organic chemistry.
Don't worry if this topic seems like a whole new language at first! By learning a few simple naming rules and patterns, you will easily master this unit.

Did You Know? Carbon atoms are like the ultimate building blocks of the chemical world. Because each carbon atom has 4 outer-shell electrons, it always forms 4 covalent bonds to achieve a full outer shell.

Memory Aid for Naming Carbon Chains:
To name organic molecules, count the number of carbon atoms in the main chain. Use this simple rhyme to remember the first four prefixes:
• Monkeys (1 Carbon) = Meth-
• Eat (2 Carbons) = Eth-
• Peanut (3 Carbons) = Prop-
• Butter (4 Carbons) = But-

1. Key Definitions & Homologous Series

Before diving into specific chemical families, let's understand the core language used in GCSE Organic Chemistry:

• Hydrocarbon: A compound containing hydrogen and carbon atoms only.
• Homologous Series: A "family" of organic compounds that have:
- The same general formula.
- The same functional group.
- Similar chemical properties.
- A gradual trend in physical properties (such as boiling point increasing as chain length increases).
- Each successive member differing by a \( -\text{CH}_2- \) unit.
• Functional Group: An atom or group of atoms responsible for the characteristic chemical reactions of a molecule.
• Saturated: A molecule containing only single carbon-carbon bonds (\( \text{C}-\text{C} \)).
• Unsaturated: A molecule containing at least one carbon-carbon double bond (\( \text{C}=\text{C} \)).

Types of Chemical Formulas:
• Molecular Formula: Shows the actual number of atoms of each element in a molecule (e.g. \( \text{C}_2\text{H}_6 \)).
• Structural Formula: Shows how the atoms are arranged without drawing every bond (e.g. \( \text{CH}_3\text{CH}_3 \)).
• Displayed Formula: Shows all atoms and all bonds in the molecule.

Key Takeaway: Every member of a homologous series reacts in a very similar way because they all share the exact same functional group!

2. The Alkanes

Alkanes are the simplest family of hydrocarbons. They are known as saturated hydrocarbons because all the carbon-carbon bonds are single covalent bonds.

General Formula and Members

The general formula for alkanes is \( \text{C}_n\text{H}_{2n+2} \), where \( n \) is the number of carbon atoms.

• Methane: \( \text{CH}_4 \) (Structural: \( \text{CH}_4 \))
• Ethane: \( \text{C}_2\text{H}_6 \) (Structural: \( \text{CH}_3\text{CH}_3 \))
• Propane: \( \text{C}_3\text{H}_8 \) (Structural: \( \text{CH}_3\text{CH}_2\text{CH}_3 \))
• Butane: \( \text{C}_4\text{H}_{10} \) (Structural: \( \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3 \))

Combustion of Alkanes

Alkanes are widely used as fuels (such as natural gas and petrol) because they burn to release large amounts of heat energy (an exothermic reaction).

1. Complete Combustion (Plenty of Oxygen):
Alkanes burn cleanly to produce only carbon dioxide and water.
Word Equation: Alkane + Oxygen \( \rightarrow \) Carbon Dioxide + Water
Example (Methane):
\( \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} \)

2. Incomplete Combustion (Limited Oxygen):
When oxygen supply is restricted, alkanes burn with a smoky yellow flame, producing toxic carbon monoxide (\( \text{CO} \)) and/or solid carbon particulates (soot, \( \text{C} \)), along with water.
Example (Methane forming carbon monoxide):
\( 2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O} \)
Example (Methane forming soot):
\( \text{CH}_4 + \text{O}_2 \rightarrow \text{C} + 2\text{H}_2\text{O} \)

Danger of Carbon Monoxide:
Carbon monoxide is a colourless, odourless, and tasteless toxic gas. It binds irreversibly to haemoglobin in red blood cells, preventing them from carrying oxygen around the body, which can lead to suffocation and death.

Key Takeaway: Alkanes are saturated hydrocarbons with formula \( \text{C}_n\text{H}_{2n+2} \). Complete combustion gives \( \text{CO}_2 \) and \( \text{H}_2\text{O} \); incomplete combustion produces toxic \( \text{CO} \) or soot.

3. The Alkenes

Alkenes are a homologous series of unsaturated hydrocarbons. Their functional group is the carbon-carbon double bond (\( \text{C}=\text{C} \)). Because of this double bond, alkenes are far more reactive than alkanes.

General Formula and Members

The general formula for alkenes is \( \text{C}_n\text{H}_{2n} \).
Note: There is no "methene" because you need at least two carbon atoms to form a \( \text{C}=\text{C} \) double bond!

• Ethene: \( \text{C}_2\text{H}_4 \) (Structural: \( \text{CH}_2=\text{CH}_2 \))
• Propene: \( \text{C}_3\text{H}_6 \) (Structural: \( \text{CH}_2=\text{CH}-\text{CH}_3 \))
• But-1-ene: \( \text{C}_4\text{H}_8 \) (Double bond between carbon 1 and carbon 2: \( \text{CH}_2=\text{CH}-\text{CH}_2-\text{CH}_3 \))
• But-2-ene: \( \text{C}_4\text{H}_8 \) (Double bond between carbon 2 and carbon 3: \( \text{CH}_3-\text{CH}=\text{CH}-\text{CH}_3 \))

Addition Reactions of Alkenes

Because the \( \text{C}=\text{C} \) double bond contains a pair of shared electrons that can easily open up, alkenes undergo addition reactions. In an addition reaction, two molecules combine to form a single product, turning an unsaturated molecule into a saturated one.

1. Test for Unsaturation (Reaction with Bromine Water):
When an alkene is shaken with orange/brown bromine water (\( \text{Br}_2\text{(aq)} \)), the double bond opens up, adding bromine atoms across the carbons.
• Observation: The solution turns from orange/brown to colourless.
• Alkanes do not react with bromine water under normal conditions (the solution stays orange/brown).
• Equation with ethene: \( \text{C}_2\text{H}_4 + \text{Br}_2 \rightarrow \text{C}_2\text{H}_4\text{Br}_2 \) (1,2-dibromoethane)

2. Hydrogenation (Reaction with Hydrogen):
Adding hydrogen gas converts an alkene into an alkane.
• Conditions: Nickel catalyst (\( \text{Ni} \)) and heat (\( 150^\circ\text{C} \)).
• Equation: \( \text{C}_2\text{H}_4 + \text{H}_2 \rightarrow \text{C}_2\text{H}_6 \)
• Everyday Link: This process is used to harden liquid vegetable oils into solid margarines.

3. Hydration (Reaction with Steam):
Adding steam (\( \text{H}_2\text{O}\text{(g)} \)) converts an alkene into an alcohol.
• Conditions: Phosphoric acid catalyst (\( \text{H}_3\text{PO}_4 \)), high temperature (\( \approx 300^\circ\text{C} \)), and high pressure.
• Equation: \( \text{C}_2\text{H}_4 + \text{H}_2\text{O} \rightarrow \text{C}_2\text{H}_5\text{OH} \) (Ethanol)

Key Takeaway: Alkenes have a \( \text{C}=\text{C} \) double bond, react with bromine water (orange to colourless), and undergo addition reactions to form single-product saturated compounds.

4. Addition Polymers

A polymer is a very large molecule made when hundreds or thousands of small reactive molecules, called monomers, join together in a repeating chain.

Addition Polymerisation

In addition polymerisation, the \( \text{C}=\text{C} \) double bonds in alkene monomers break open and bond to neighbouring molecules to form a long, saturated carbon backbone. No other product is made.

1. Polythene (Poly(ethene)):
• Monomer: Ethene (\( \text{C}_2\text{H}_4 \))
• Equation: \( n(\text{CH}_2=\text{CH}_2) \rightarrow -(\text{CH}_2-\text{CH}_2)_n- \)
• Properties & Uses: Flexible, cheap, waterproof. Used for plastic bags, cling film, and plastic bottles.

2. Poly(vinyl chloride) / PVC (Poly(chloroethene)):
• Monomer: Chloroethene (\( \text{CH}_2=\text{CHCl} \))
• Properties & Uses: Tough, rigid, good electrical insulator. Used for window frames, water pipes, and cable insulation.

Disposal of Addition Polymers

Most synthetic polymers are non-biodegradable because their strong covalent carbon-carbon single bonds are unreactive and cannot be broken down by microorganisms.

• Landfill Sites: Takes up valuable land space; waste remains for hundreds of years without rotting.
• Incineration (Burning): Releases energy to generate electricity, but produces greenhouse gases (\( \text{CO}_2 \)) and toxic gases (e.g. acidic \( \text{HCl} \) gas from burning PVC).
• Recycling: Conserves crude oil reserves and reduces waste, but sorting different types of plastics is expensive and labour-intensive.

Key Takeaway: Alkene monomers join via addition polymerisation to make long-chain plastics. They are durable but create major environmental disposal challenges.

5. The Alcohols

Alcohols are a homologous series containing the hydroxyl functional group: \( -\text{OH} \).

General Formula and Members

The general formula for simple alcohols is \( \text{C}_n\text{H}_{2n+1}\text{OH} \).

• Methanol: \( \text{CH}_3\text{OH} \)
• Ethanol: \( \text{C}_2\text{H}_5\text{OH} \) (or \( \text{CH}_3\text{CH}_2\text{OH} \))
• Propan-1-ol: \( \text{C}_3\text{H}_7\text{OH} \) (hydroxyl group on carbon 1: \( \text{CH}_3\text{CH}_2\text{CH}_2\text{OH} \))
• Propan-2-ol: \( \text{C}_3\text{H}_7\text{OH} \) (hydroxyl group on carbon 2: \( \text{CH}_3\text{CH(OH)}\text{CH}_3 \))
• Butan-1-ol: \( \text{C}_4\text{H}_9\text{OH} \) (hydroxyl group on carbon 1: \( \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH} \))
• Butan-2-ol: \( \text{C}_4\text{H}_9\text{OH} \) (hydroxyl group on carbon 2: \( \text{CH}_3\text{CH}_2\text{CH(OH)}\text{CH}_3 \))

Production of Ethanol

Ethanol is produced industrially by two main routes:

1. Fermentation of Sugars (Biological Route):
Glucose from crops is broken down by enzymes in yeast without oxygen.
• Equation: \( \text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2 \)
• Conditions:
- Yeast (provides enzymes).
- Temperature between \( 25^\circ\text{C} \) and \( 35^\circ\text{C} \) (too cold = reaction too slow; too hot = enzymes denature).
- Anaerobic conditions (absence of air/oxygen): If oxygen is present, yeast produces ethanoic acid (vinegar) instead of ethanol!

2. Hydration of Ethene (Chemical Route):
Addition reaction of ethene with steam (as covered in the alkenes section).

Reactions of Alcohols

• Combustion: Alcohols burn cleanly with a blue flame to form carbon dioxide and water.
\( \text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O} \)
• Oxidation: When exposed to air or reacting with chemical oxidising agents (such as acidified potassium dichromate), alcohols are oxidised to carboxylic acids.
Example: Ethanol oxidises to form ethanoic acid (vinegar).

Key Takeaway: Alcohols contain the \( -\text{OH} \) group. Ethanol is produced via fermentation of glucose under warm, anaerobic conditions, and can be oxidised to form carboxylic acids.

6. The Carboxylic Acids

Carboxylic acids are an organic family containing the carboxyl functional group: \( -\text{COOH} \).

Members and Formulas

• Methanoic acid: \( \text{HCOOH} \) (Structural: \( \text{HCOOH} \))
• Ethanoic acid: \( \text{CH}_3\text{COOH} \) (Structural: \( \text{CH}_3\text{COOH} \)) — the main acid found in household vinegar.
• Propanoic acid: \( \text{C}_2\text{H}_5\text{COOH} \) (Structural: \( \text{CH}_3\text{CH}_2\text{COOH} \))
• Butanoic acid: \( \text{C}_3\text{H}_7\text{COOH} \) (Structural: \( \text{CH}_3\text{CH}_2\text{CH}_2\text{COOH} \))

Acid Nature and Reactions

Carboxylic acids are weak acids. Unlike strong acids (like hydrochloric acid, \( \text{HCl} \)), carboxylic acids only partially ionise in aqueous solution to release \( \text{H}^+ \) ions:
\( \text{CH}_3\text{COOH}\text{(aq)} \rightleftharpoons \text{CH}_3\text{COO}^-\text{(aq)} + \text{H}^+\text{(aq)} \)
They typically have a pH of around 3 to 5.

Even though they are weak, they still undergo all standard acid reactions to form salts called carboxylates (e.g. ethanoates):

1. Reaction with Metals:
Acid + Metal \( \rightarrow \) Salt + Hydrogen
Example with magnesium and ethanoic acid:
\( 2\text{CH}_3\text{COOH} + \text{Mg} \rightarrow (\text{CH}_3\text{COO})_2\text{Mg} + \text{H}_2 \)
• Observation: Effervescence (bubbles of \( \text{H}_2 \) gas), metal dissolves, test with a lit splint produces a squeaky pop.

2. Reaction with Metal Carbonates:
Acid + Metal Carbonate \( \rightarrow \) Salt + Water + Carbon Dioxide
Example with sodium carbonate and ethanoic acid:
\( 2\text{CH}_3\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2 \)
• Observation: Vigorous bubbling; the gas produced turns limewater cloudy/milky.

3. Reaction with Metal Hydroxides (Neutralisation):
Acid + Alkali \( \rightarrow \) Salt + Water
Example with sodium hydroxide and ethanoic acid:
\( \text{CH}_3\text{COOH} + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O} \)
• Observation: Exothermic reaction (temperature rise), no gas bubbles produced.

Key Takeaway: Carboxylic acids have the \( -\text{COOH} \) group. They are weak acids that react with metals, bases, and carbonates to produce carboxylate salts.

7. Quick Revision Summary Table

Use this quick guide to remember the four homologous series:

• Alkanes: Functional Group = None (\( \text{C}-\text{C} \) single bonds only) | General Formula = \( \text{C}_n\text{H}_{2n+2} \) | Key Reaction = Combustion
• Alkenes: Functional Group = \( \text{C}=\text{C} \) double bond | General Formula = \( \text{C}_n\text{H}_{2n} \) | Key Reaction = Addition (decolourises bromine water)
• Alcohols: Functional Group = \( -\text{OH} \) | General Formula = \( \text{C}_n\text{H}_{2n+1}\text{OH} \) | Key Reaction = Combustion, Oxidation to carboxylic acids
• Carboxylic Acids: Functional Group = \( -\text{COOH} \) | General Formula = \( \text{C}_n\text{H}_{2n+1}\text{COOH} \) (starting at \( n=0 \)) | Key Reaction = Weak acid reactions with metals, carbonates, and alkalis

Common Mistakes to Avoid:
• Forgetting that the carbon atom in the \( -\text{COOH} \) group counts towards the chain name (e.g. \( \text{CH}_3\text{COOH} \) has 2 carbons in total, so it is ethanoic acid, not methanoic acid!).
• Calling bromine water's colour change "clear" instead of colourless. Always write: orange/brown to colourless.
• Forgetting that in polymer structures, the open single bonds must extend through the brackets.

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is a homologous series?

A homologous series is a family of organic compounds sharing the same general formula and functional group, having similar chemical properties and a gradual trend in physical properties, with each successive member differing by a \( -\text{CH}_2- \) unit.

What is the difference between a saturated and an unsaturated hydrocarbon?

A saturated hydrocarbon contains only single carbon-carbon bonds (\( \text{C}-\text{C} \)), whereas an unsaturated hydrocarbon contains at least one carbon-carbon double bond (\( \text{C}=\text{C} \)).

What is the general formula of the alkanes and what are the names of the first four members?

The general formula is \( \text{C}_n\text{H}_{2n+2} \). The first four members are methane (\( \text{CH}_4 \)), ethane (\( \text{C}_2\text{H}_6 \)), propane (\( \text{C}_3\text{H}_8 \)), and butane (\( \text{C}_4\text{H}_{10} \)).

What are the products of complete versus incomplete combustion of alkanes?

Complete combustion produces carbon dioxide (\( \text{CO}_2 \)) and water (\( \text{H}_2\text{O} \)). Incomplete combustion produces toxic carbon monoxide (\( \text{CO} \)) and/or soot (\( \text{C} \)), alongside water.

What is the chemical test used to distinguish between an alkane and an alkene?

Shake the substance with bromine water; an alkene turns the solution from orange/brown to colourless, while an alkane causes no colour change (remains orange/brown).

What are the required reaction conditions for the hydrogenation and hydration of alkenes?

Hydrogenation requires a nickel catalyst (\( \text{Ni} \)) and \( 150^\circ\text{C} \); hydration requires steam, a phosphoric acid catalyst (\( \text{H}_3\text{PO}_4 \)), \( \approx 300^\circ\text{C} \), and high pressure.

What occurs during addition polymerisation?

Many small unsaturated monomer molecules join together as their \( \text{C}=\text{C} \) double bonds break open, forming a single long-chain polymer with a saturated backbone.

Why do addition polymers cause environmental disposal issues?

They are non-biodegradable because their strong, unreactive \( \text{C}-\text{C} \) bonds cannot be broken down by microorganisms.

What conditions are needed to produce ethanol by the fermentation of sugars?

Fermentation requires yeast (enzymes), warm temperatures between \( 25^\circ\text{C} \) and \( 35^\circ\text{C} \), and anaerobic conditions (absence of oxygen).

What functional group defines carboxylic acids and why are they considered weak acids?

They contain the carboxyl group (\( -\text{COOH} \)) and are weak acids because they only partially ionise in aqueous solution to release \( \text{H}^+ \) ions.

What products are formed when ethanoic acid reacts with magnesium, sodium carbonate, and sodium hydroxide?

With magnesium: magnesium ethanoate and hydrogen; with sodium carbonate: sodium ethanoate, water, and carbon dioxide; with sodium hydroxide: sodium ethanoate and water.

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