Alkenes

Edexcel International GCSE Chemistry· 4.23–4.28· 12 min read

1. Structure & Classification of Alkenes★★☆☆☆⏱ 3 min

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📘 Definition

Alkene

>C=C<

Unsaturated hydrocarbon with general formula $C_{n} H_{2 n}$ , containing one carbon-carbon double bond as its functional group.

The C=C double bond is the reactive site of all alkene molecules. Alkenes are classified as unsaturated because their double bond can break open to add extra atoms, meaning they are not fully saturated with hydrogen atoms like alkanes.

📐 Worked Example

Explain why propene ( $C_{3} H_{6}$ ) is classified as an unsaturated hydrocarbon, and confirm it fits the general formula for alkenes.

1
1. Identify the functional group: propene contains a C=C double bond.
2
1. Unsaturated hydrocarbons have at least one C=C double bond capable of undergoing addition reactions, so propene is unsaturated.
3
$3. General formula for alkenes is $C_nH_{2n}$. For $n=3$: $C_3H_{2*3} = C_3H_6$, which matches the formula of propene.$

Exam tip:

Always explicitly mention the C=C double bond when explaining why alkenes are unsaturated; answers that only refer to 'a double bond' without specifying carbon-carbon get zero marks.

2. Drawing & Naming Unbranched Alkenes (up to 4 C atoms)★★★☆☆⏱ 4 min

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Unbranched alkenes use the same prefixes as alkanes (eth-, prop-, but-) followed by the suffix -ene. For alkenes with 3 or more carbon atoms, add a number locant to show the position of the double bond, using the lowest possible number for the first carbon of the double bond. Cis/trans or E/Z notation is not required for this specification.

Number of C atoms	Name	Molecular formula	Displayed formula summary
2	Ethene	$C_{2} H_{4}$	Two C atoms double-bonded, each bonded to 2 H atoms
3	Propene	$C_{3} H_{6}$	Double bond between C1 and C2, C3 bonded to 3 H atoms
4	But-1-ene	$C_{4} H_{8}$	Double bond between C1 and C2, C3 and C4 single-bonded
4	But-2-ene	$C_{4} H_{8}$	Double bond between C2 and C3

📐 Worked Example

Draw the displayed formula of but-2-ene, and write its molecular formula.

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1. The prefix 'but-' means 4 carbon atoms in the unbranched chain.
2
1. The locant '2' means the double bond starts at the 2nd carbon, so it lies between C2 and C3.
3
1. Add hydrogen atoms to each carbon to give every carbon 4 total bonds: C1 has 3 H, C2 has 1 H, C3 has 1 H, C4 has 3 H.
4
1. Count the atoms: 4 C and 8 H, so molecular formula is $C_{4} H_{8}$ .

Exam tip:

Never name an alkene with 3+ carbons without the double bond locant: writing 'butene' instead of but-1-ene or but-2-ene loses marks as it is ambiguous.

3. Addition Reaction of Alkenes with Bromine★★★☆☆⏱ 3 min

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📘 Definition

Addition reaction

Reaction where two reactant molecules combine to form a single product, with the alkene double bond opening to add atoms across both carbons of the original double bond.

Alkenes react with pure bromine or bromine water to form colourless dibromoalkanes, with no by-products formed. For example, ethene reacts with bromine to form 1,2-dibromoethane:

C_2H_4 + Br_2 \rightarrow C_2H_4Br_2

📐 Worked Example

Write the balanced symbol equation for the reaction between propene and bromine, and name the product formed.

1
1. Propene has molecular formula $C_{3} H_{6}$ , bromine exists as $B r_{2}$ .
2
1. The double bond opens, one Br atom adds to each carbon of the original double bond, so the product has formula $C_{3} H_{6} B r_{2}$ .
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$3. Balanced equation: $C_3H_6 + Br_2 \rightarrow C_3H_6Br_2$$
4
1. The product is 1,2-dibromopropane.

Exam tip:

You do not need to draw reaction mechanisms or curly arrows for this reaction; only recall it is an addition reaction that produces a single dibromoalkane product.

4. Bromine Water Test for Alkenes★★☆☆☆⏱ 2 min

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Bromine water is an orange-brown solution of bromine dissolved in water, used as a qualitative test to distinguish alkenes from alkanes when carried out in dark conditions (no UV light):

Alkenes: Orange-brown colour decolourises completely, as bromine reacts with the C=C double bond to form a colourless dibromoalkane.
Alkanes: No reaction occurs, so the orange-brown colour of the solution remains unchanged.

📐 Worked Example

A student has two unlabelled test tubes, one containing hexane (alkane) and one containing hexene (alkene). Describe how they can use bromine water to identify the test tube with hexene.

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1. Add equal volumes of bromine water to both test tubes, shake gently, and keep away from UV light.
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1. The test tube where the orange-brown solution decolourises contains hexene, as it undergoes an addition reaction with bromine.
3
1. The test tube where the orange-brown colour stays the same contains hexane, as no reaction occurs in dark conditions.

Exam tip:

Always specify the test is carried out in the dark: alkanes react with bromine in UV light via substitution, which would also decolourise the solution and give a false positive result.

5. Common Pitfalls

Wrong move:

Saying alkenes are unsaturated because they have fewer hydrogen atoms than alkanes

Why:

Mark schemes require explicit reference to the C=C double bond and its ability to undergo addition reactions; relative hydrogen count alone is not sufficient.

Correct move:

State: 'Alkenes are unsaturated because they contain a C=C double bond that can break to add extra atoms via addition reactions.'

Wrong move:

Naming but-1-ene as 'butene' with no double bond locant

Why:

There are two unbranched isomers of butene (but-1-ene and but-2-ene), so omitting the locant makes the name ambiguous.

Correct move:

Always include the lowest possible locant for the double bond position for alkenes with 3 or more carbon atoms.

Wrong move:

Writing the product of ethene + bromine as $C_{2} H_{4} B r + H B r$

Why:

This is a substitution reaction (characteristic of alkanes); alkenes undergo addition, so only one single product is formed with no by-products.

Correct move:

Write only the dibromoalkane product: $C_{2} H_{4} + B r_{2} \to C_{2} H_{4} B r_{2}$

Wrong move:

Stating alkanes decolourise bromine water under all conditions

Why:

Alkanes only react with bromine in UV light; in dark conditions, no reaction occurs so the orange colour remains.

Correct move:

Specify the test is done in the dark, so only alkenes decolourise bromine water.

Wrong move:

Using the general formula $C_{n} H_{2 n + 2}$ for alkenes

Why:

$C_{n} H_{2 n + 2}$ is the general formula for alkanes; alkenes have one double bond so they have 2 fewer hydrogen atoms per molecule.

Correct move:

Recall the general formula for alkenes is $C_{n} H_{2 n}$ for molecules with one C=C double bond.

6. Quick Reference Cheatsheet

Concept	Key Exam Fact
Functional group	C=C< (carbon-carbon double bond)
General formula	$C_{n} H_{2 n}$ (n ≥ 2)
Classification	Unsaturated hydrocarbon (due to C=C double bond)
Bromine reaction	Addition reaction, forms 1 dibromoalkane product only
Bromine water test (dark)	Alkene: decolourises; Alkane: orange-brown colour remains

What's Next

Now you have mastered core alkene properties, you are ready to progress to related Edexcel IGCSE Chemistry organic chemistry topics. Next, learn about the industrial hydration of ethene to produce ethanol, a key reaction that builds on the reactivity of the C=C double bond. You will also explore addition polymerisation, where alkene monomers join to form long polymer chains such as poly(ethene) and poly(propene) used in everyday materials. Finally, revise cracking of alkanes, the industrial process that produces alkenes as a key feedstock for fuel and chemical manufacturing.