Organic chemistry

Q: What is the precise definition of a hydrocarbon?

A hydrocarbon is a molecule consisting of hydrogen and carbon only .

Q: What is meant by a homologous series and a functional group?

A homologous series is a family of organic compounds sharing the same general formula, similar chemical properties, a gradation in physical properties, and differing by a \(-\text{CH}_2-\) unit. A functional group is the reactive group of atoms within a molecule dictating its characteristic chemical reactions.

Q: What is the functional group of carboxylic acids, and why are they classified as weak acids?

Carboxylic acids contain the carboxyl group (\(-\text{COOH}\)) . They are weak acids because they only partially ionise in aqueous solutions.

Q: What are the differences in products and hazards between complete and incomplete combustion of hydrocarbons?

Complete combustion (plentiful oxygen) produces \(\text{CO}_2\) and \(\text{H}_2\text{O}\). Incomplete combustion (limited oxygen) produces toxic carbon monoxide (\(\text{CO}\)), carbon soot (\(\text{C}\)), and \(\text{H}_2\text{O}\).

Q: What is cracking, and why is it carried out industrially?

Cracking is the thermal decomposition of long-chain alkanes into smaller, more useful short-chain alkanes and alkenes at high temperature (\(500\text{–}700^\circ\text{C}\)) using a catalyst or steam.

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

Add orange bromine water and shake: an alkene causes the solution to turn from orange to colourless via an addition reaction, whereas an alkane leaves the bromine water orange.

Science Double Award 1370 · CCEA GCSE · 9 min read · Updated: 31 Aug 2026

Welcome to Organic Chemistry!

Welcome to one of the most exciting and practical areas of GCSE Chemistry: Organic Chemistry. Organic chemistry is simply the study of carbon-based compounds. Carbon is a truly special element because its atoms can join together in rings and long chains, forming millions of different substances—from the fuels that power our transport, to the plastics we use every day, and even the vinegar in our kitchens!

Don't worry if this topic feels a bit new or overwhelming at first. We will break down every concept step-by-step with clear definitions, helpful memory tricks, and direct exam advice so you can master your CCEA GCSE Double Award Science Unit C2 exam.

1. Fundamental Definitions: The Language of Organic Chemistry

Before looking at specific molecules, there are a few golden definitions that examiners love to test. Learning these exact wordings will guarantee you easy marks in the exam!

1. Hydrocarbon:
A molecule consisting of hydrogen and carbon only.
Examiner Warning: You must include the word "only"! If you write "a molecule made of carbon and hydrogen", you will lose the mark because alcohols and sugars also contain carbon and hydrogen along with oxygen.

2. Homologous Series:
A family of organic compounds that:
• Share the same general formula.
• Show similar chemical properties.
• Show a gradation in physical properties (for example, boiling points increase as chain length increases).
• Have successive members differing by a \(-\text{CH}_2-\) unit.

3. Functional Group:
A reactive group of atoms within a molecule that dictates the characteristic chemical properties and reactions of that homologous series.

4. Saturated vs. Unsaturated:
• Saturated hydrocarbon: Contains single carbon-carbon bonds only (\(-\text{C}-\text{C}-\)).
• Unsaturated hydrocarbon: Contains at least one carbon-carbon double bond (\(-\text{C}=\text{C}-\)).

Memory Trick for Naming Carbon Chains:
How do we know how many carbons are in a molecule? Remember this phrase:
Monkeys Eat Peanut Butter
• Meth- = 1 Carbon atom
• Eth- = 2 Carbon atoms
• Prop- = 3 Carbon atoms
• But- = 4 Carbon atoms

Key Takeaway: Hydrocarbons contain hydrogen and carbon only. Saturated means only single \(-\text{C}-\text{C}-\) bonds, while unsaturated means at least one double \(\text{C}=\text{C}\) bond.

2. The Four Homologous Series

For CCEA Double Award Science, you must know the names, formulas, and physical states at room temperature for the first members of four specific homologous series.

Series 1: Alkanes

Alkanes are the simplest hydrocarbons. They are saturated (single bonds only) and have no functional group.

• General Formula: \(\text{C}_n\text{H}_{2n+2}\)
• First 4 Members (all are gases at room temperature):
1. Methane: \(\text{CH}_4\) (gas)
2. Ethane: \(\text{C}_2\text{H}_6\) (gas)
3. Propane: \(\text{C}_3\text{H}_8\) (gas)
4. Butane: \(\text{C}_4\text{H}_{10}\) (gas)

Series 2: Alkenes

Alkenes are unsaturated hydrocarbons containing a reactive carbon-carbon double covalent bond (\(\text{C}=\text{C}\)) as their functional group. Because you need at least two carbons to make a double bond, there is no "methene"—the series starts at \(n=2\)!

• General Formula: \(\text{C}_n\text{H}_{2n}\)
• First 3 Members (all are gases at room temperature):
1. Ethene: \(\text{C}_2\text{H}_4\) (gas)
2. Propene: \(\text{C}_3\text{H}_6\) (gas)
3. But-1-ene / But-2-ene: \(\text{C}_4\text{H}_8\) (gas)

Series 3: Alcohols

Alcohols contain the hydroxyl group (\(-\text{OH}\)) as their functional group. They are not hydrocarbons because they contain oxygen.

• General Formula: \(\text{C}_n\text{H}_{2n+1}\text{OH}\)
• First 4 Members (all are liquids at room temperature):
1. Methanol: \(\text{CH}_3\text{OH}\) (liquid)
2. Ethanol: \(\text{C}_2\text{H}_5\text{OH}\) (liquid)
3. Propan-1-ol: \(\text{C}_3\text{H}_7\text{OH}\) (liquid)
4. Butan-1-ol: \(\text{C}_4\text{H}_9\text{OH}\) (liquid)

Production of Ethanol by Fermentation:
Ethanol can be produced naturally by the fermentation of sugars using yeast.
• Word Equation: \(\text{Glucose} \xrightarrow{\text{yeast}} \text{Ethanol} + \text{Carbon Dioxide}\)
• Symbol Equation: \(\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{yeast}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2\)
• Essential Conditions: Anaerobic conditions (no oxygen present), warm temperature (around \(25\text{–}35^\circ\text{C}\)), and aqueous glucose solution.

Series 4: Carboxylic Acids

Carboxylic acids contain the carboxyl group (\(-\text{COOH}\)) as their functional group. They are weak acids because they only partially ionise in aqueous solutions.

• General Formula: \(\text{C}_n\text{H}_{2n+1}\text{COOH}\) (where \(n=0\) for the first member)
• First 4 Members:
1. Methanoic acid: \(\text{HCOOH}\)
2. Ethanoic acid: \(\text{CH}_3\text{COOH}\) (this is the main active ingredient in vinegar)
3. Propanoic acid: \(\text{C}_2\text{H}_5\text{COOH}\)
4. Butanoic acid: \(\text{C}_3\text{H}_7\text{COOH}\)

Key Takeaway: Alkanes and alkenes are gases at room temperature; alcohols are liquids. Always be careful when counting carbons in carboxylic acids: ethanoic acid has 2 carbons in total (\(\text{CH}_3\text{COOH}\)).

3. Crude Oil & Fractional Distillation

What is Crude Oil?
Crude oil is a finite fossil fuel found underground. It is a complex mixture consisting predominantly of hydrocarbons (mainly alkanes). Because it is a mixture of many different chain lengths, crude oil is not very useful directly; it must be separated into simpler mixtures called fractions.

How Fractional Distillation Works:

Fractional distillation separates crude oil based on the different boiling points of its components:

1. Heating: Crude oil is heated and vaporised before being pumped into the bottom of a tall fractionating column.
2. Temperature Gradient: The column is very hot at the bottom and gradually cooler towards the top.
3. Condensation: The vaporised hydrocarbons rise up the column. As they rise, they cool down and condense into liquids at the levels where the temperature falls below their boiling points.
4. Collection: Fractions are continuously collected at different heights.

Trends Along the Column:

• Small molecules (collected near the cool top, e.g., refinery gases, petrol):
- Low boiling points
- Low viscosity (runny and flow easily)
- High flammability (ignite very easily)

• Large molecules (collected near the hot bottom, e.g., diesel, fuel oil, bitumen):
- High boiling points
- High viscosity (thick and sticky)
- Low flammability (harder to ignite)

Key Takeaway: Fractional distillation separates hydrocarbons by boiling point. Smaller molecules have lower boiling points, are more flammable, and condense at the top of the column.

4. Combustion & Environmental Impacts

Hydrocarbons are primarily used as fuels because their combustion (burning in oxygen) releases large amounts of heat energy.

1. Complete Combustion

This occurs when there is a plentiful supply of oxygen.
• General Equation: \(\text{Hydrocarbon} + \text{Oxygen} \longrightarrow \text{Carbon Dioxide} + \text{Water}\)
• Example (Methane): \(\text{CH}_4 + 2\text{O}_2 \longrightarrow \text{CO}_2 + 2\text{H}_2\text{O}\)
• Observations: A clean, blue flame that releases maximum energy.

2. Incomplete Combustion

This occurs when the supply of oxygen is limited.
• General Equation: \(\text{Hydrocarbon} + \text{Oxygen} \longrightarrow \text{Carbon Monoxide} + \text{Carbon (soot)} + \text{Water}\)
• Observations: A yellow, smoky flame that releases less energy.

Harmful Pollutants to Know for the Exam:

• Carbon Monoxide (\(\text{CO}\)): A toxic, colourless, and odourless gas. It binds irreversibly to haemoglobin in red blood cells, reducing their ability to transport oxygen around the body.
• Carbon Soot (\(\text{C}\)): Tiny unburnt carbon particles that cause respiratory issues (like asthma) and contribute to global dimming.
• Sulfur Dioxide (\(\text{SO}_2\)): Formed when sulfur impurities present in fossil fuels burn in oxygen. Dissolves in rainwater to form acid rain, which damages buildings and harms aquatic life.
• Oxides of Nitrogen (\(\text{NO}_x\)): Formed at very high temperatures inside car engines when nitrogen and oxygen from the air react together. Contributes to acid rain and photochemical smog.

Key Takeaway: Complete combustion makes \(\text{CO}_2\) and \(\text{H}_2\text{O}\). Incomplete combustion makes toxic \(\text{CO}\), soot (\(\text{C}\)), and \(\text{H}_2\text{O}\).

5. Chemical Reactions of Hydrocarbons

Reaction A: Cracking

What is cracking?
Cracking is the thermal decomposition of long-chain saturated alkanes into smaller, more useful short-chain alkanes and alkenes.

Why is cracking necessary?
1. Supply and Demand: Fractional distillation produces a high supply of long-chain hydrocarbons, but there is low market demand for them.
2. Useful Products: There is a huge demand for short-chain alkanes (which make excellent fuels like petrol) and short-chain alkenes (which are needed as monomers to make plastics).

Reaction Conditions:
• High temperature (approx. \(500\text{–}700^\circ\text{C}\))
• Catalyst (such as aluminium oxide, porous pot, or silica) or steam

Example Equation:
\(\text{C}_{10}\text{H}_{22} \longrightarrow \text{C}_8\text{H}_{18} + \text{C}_2\text{H}_4\)
Notice that the total number of carbons and hydrogens is equal on both sides!

Reaction B: Addition Reactions of Alkenes

Because alkenes have a reactive carbon-carbon double bond (\(\text{C}=\text{C}\)), they can undergo addition reactions where the double bond opens up to form a single bond, allowing new atoms to attach.

1. Test for Unsaturation (Bromine Water Test):
• When orange/brown bromine water is shaken with an alkene, it turns colourless.
• When shaken with an alkane, the bromine water remains orange.
• Equation: \(\text{C}_2\text{H}_4 + \text{Br}_2 \longrightarrow \text{C}_2\text{H}_4\text{Br}_2\) (1,2-dibromoethane)
Examiner Warning: Never write that the solution turns "clear". Clear is not a colour—always state colourless!

2. Addition of Hydrogen (Hydrogenation):
• An alkene reacts with hydrogen gas to form an alkane.
• Equation: \(\text{C}_2\text{H}_4 + \text{H}_2 \longrightarrow \text{C}_2\text{H}_6\)
• Real-world use: Used to harden vegetable oils in the manufacture of margarine.

3. Addition of Steam (Hydration):
• An alkene reacts with steam (\(\text{H}_2\text{O}\)) to produce an alcohol.
• Equation: \(\text{C}_2\text{H}_4 + \text{H}_2\text{O} \longrightarrow \text{C}_2\text{H}_5\text{OH}\)
• Real-world use: Industrial synthesis of ethanol.

Key Takeaway: Cracking turns long alkanes into useful short alkanes + alkenes. Alkenes decolourise bromine water from orange to colourless via addition.

6. Addition Polymerisation

What is a Polymer?
A polymer is a very large molecule made when thousands of small, reactive monomer molecules join together in a long chain.

Addition Polymerisation:
The process where many small unsaturated monomer molecules (containing a \(\text{C}=\text{C}\) double bond) join together to form a single long-chain saturated polymer, with no other products formed.

Key Polymers to Know:

• Polyethene:
- Monomer: Ethene (\(\text{C}_2\text{H}_4\))
- Uses: Plastic bags, clingfilm, plastic bottles.

• Polyvinylchloride (PVC / Polychloroethene):
- Monomer: Chloroethene
- Uses: Drainpipes, window frames, electrical wire insulation.

How to Draw Polymer Equations and Repeating Units:

When showing polymerisation in an exam:
1. Draw the monomer with its double bond intact, with a large '\(n\)' in front: \(n(\text{C}_2\text{H}_4)\).
2. Draw the arrow: \(\longrightarrow\).
3. Change the \(\text{C}=\text{C}\) double bond to a single \(-\text{C}-\text{C}-\) bond inside square brackets.
4. Extend the side bonds through the brackets on both sides.
5. Write the subscript '\(n\)' at the bottom right corner outside the bracket.

Environmental Impact & Disposal of Addition Polymers:

• Non-biodegradable: Addition polymers have an unreactive, saturated carbon backbone. Microorganisms cannot break them down easily, so they persist in landfill sites for hundreds of years.
• Incineration: Burning plastic waste releases carbon dioxide (contributing to climate change) and can release toxic gases.

Key Takeaway: Monomers have double bonds (\(\text{C}=\text{C}\)); polymers have single bonds with extending bonds through square brackets and a subscript \(n\).

Quick Summary & Exam Checklist

Before stepping into your Unit C2 exam, ensure you can confidently:
• Define a hydrocarbon using the word "only".
• Recall the general formulas for alkanes (\(\text{C}_n\text{H}_{2n+2}\)), alkenes (\(\text{C}_n\text{H}_{2n}\)), and alcohols (\(\text{C}_n\text{H}_{2n+1}\text{OH}\)).
• State the conditions for cracking (\(500\text{–}700^\circ\text{C}\) and a catalyst) and fermentation (yeast, warm \(25\text{–}35^\circ\text{C}\), anaerobic).
• State that bromine water turns from orange to colourless with an alkene.
• Describe how crude oil is separated in a fractionating column based on boiling points.
• Explain why carbon monoxide is toxic and why plastics do not biodegrade.

Quick check

Can you answer these now?

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

What is the precise definition of a hydrocarbon?

A hydrocarbon is a molecule consisting of hydrogen and carbon only.

What is meant by a homologous series and a functional group?

A homologous series is a family of organic compounds sharing the same general formula, similar chemical properties, a gradation in physical properties, and differing by a \(-\text{CH}_2-\) unit. A functional group is the reactive group of atoms within a molecule dictating its characteristic chemical reactions.

What are the key characteristics and first four members of the alkane series?

Alkanes are saturated hydrocarbons with the general formula \(\text{C}_n\text{H}_{2n+2}\). 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 (\(\text{C}_4\text{H}_{10}\)), all of which are gases at room temperature.

What are the key characteristics and first three members of the alkene series?

Alkenes are unsaturated hydrocarbons containing a \(\text{C}=\text{C}\) double covalent bond with the general formula \(\text{C}_n\text{H}_{2n}\). The first three members are ethene (\(\text{C}_2\text{H}_4\)), propene (\(\text{C}_3\text{H}_6\)), and butene (\(\text{C}_4\text{H}_8\)), all of which are gases at room temperature.

How is ethanol produced by fermentation, and what conditions are required?

Ethanol is produced by fermenting glucose with yeast: \(\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{yeast}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2\). The reaction requires anaerobic conditions, a warm temperature (\(25\text{–}35^\circ\text{C}\)), and an aqueous glucose solution.

What is the functional group of carboxylic acids, and why are they classified as weak acids?

Carboxylic acids contain the carboxyl group (\(-\text{COOH}\)). They are weak acids because they only partially ionise in aqueous solutions.

How does fractional distillation separate crude oil into fractions?

Fractional distillation separates crude oil based on differences in boiling points. Vaporised crude oil enters a column with a temperature gradient (hot at the bottom, cool at the top); hydrocarbons rise, cool, and condense at heights corresponding to their boiling points.

What are the differences in products and hazards between complete and incomplete combustion of hydrocarbons?

Complete combustion (plentiful oxygen) produces \(\text{CO}_2\) and \(\text{H}_2\text{O}\). Incomplete combustion (limited oxygen) produces toxic carbon monoxide (\(\text{CO}\)), carbon soot (\(\text{C}\)), and \(\text{H}_2\text{O}\).

What is cracking, and why is it carried out industrially?

Cracking is the thermal decomposition of long-chain alkanes into smaller, more useful short-chain alkanes and alkenes at high temperature (\(500\text{–}700^\circ\text{C}\)) using a catalyst or steam.

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

Add orange bromine water and shake: an alkene causes the solution to turn from orange to colourless via an addition reaction, whereas an alkane leaves the bromine water orange.

What is addition polymerisation?

Addition polymerisation is a process where many small unsaturated monomer molecules join together by opening their double bonds to form a single long-chain saturated polymer, with no other products formed.

Why do addition polymers present serious environmental disposal problems?

Addition polymers are non-biodegradable because their saturated carbon backbone is chemically unreactive, preventing microorganisms from breaking them down.

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