Alkenes

Alkenes: unsaturated hydrocarbons—cracking, bromine water test, and key addition reactions (hydrogenation, hydration, polymerisation).

SEC G3 Pure Chemistry 2027

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Learning objectives

describe the alkenes as a homologous series of unsaturated hydrocarbons with the general formula CnH2n
draw the structures of branched and unbranched alkenes, C2 to C4, and name the unbranched alkenes ethene to butene
describe the manufacture of alkenes and hydrogen by cracking hydrocarbons and recognise that cracking is essential to match the demand for fractions containing smaller molecules from the refinery process
describe the difference between saturated and unsaturated hydrocarbons from their molecular structures and by using aqueous bromine
describe the reactions of alkenes (exemplified by ethene) in terms of combustion, polymerisation (see also 11.4(b)), and the addition with bromine, steam and hydrogen
state the meaning of polyunsaturated when applied to food products
describe the manufacture of margarine by the addition of hydrogen to unsaturated vegetable oils to form a solid product.

Alkenes are not “just hydrocarbons”. They are tested through: (1) the C = C double bond (unsaturation), (2) bromine water test, (3) cracking as a source, and (4) addition reactions (hydrogenation/hydration/polymerisation).

1. Definition

A. Alkene

An alkene is an unsaturated hydrocarbon containing a carbon-carbon double bond, C = C. Alkenes with one double bond form a homologous series with general formula CₙH₂ₙ.

B. Unsaturated hydrocarbon

An unsaturated hydrocarbon is a hydrocarbon that contains at least one C = C bond.

C. Addition reaction

An addition reaction is a reaction where atoms add across a C = C double bond to form a single product.

2. Key Ideas

General formula (for one C = C): CₙH₂ₙ.
There is no “methene”: you need at least two carbon atoms to have C = C.
Alkenes are more reactive than alkanes because the C = C bond can undergo addition reactions.
Bromine water test: alkene decolourises red-brown bromine water to colourless (alkane does not, under normal conditions).
Cracking breaks long-chain alkanes into shorter alkanes and alkenes (and hydrogen may also be produced).
Complete combustion of ethene forms carbon dioxide and water.
Addition polymerisation makes polymers like poly(ethene) from many alkene monomers.
Polyunsaturated means a molecule contains more than one C = C bond.

Alkanes (contrast with substitution): Alkanes
Fuels + cracking context: Fuels and crude oil
Alcohols (product of hydration): Alcohols

3. Detailed Explanations

Alkenes are unsaturated hydrocarbons with a C = C bond; general formula CₙH₂ₙ (for one double bond).
Bromine water test: red-brown Br₂(aq) → colourless (alkene). Alkanes show no change in this test under normal conditions.
Addition reactions happen across C = C: hydrogenation (H₂), hydration (steam → alcohol), polymerisation (many monomers → polymer).
Cracking: long-chain alkanes → shorter alkanes + alkenes (high temperature + catalyst).
Complete combustion of ethene produces CO₂ and H₂O.
You need at least 2 carbons for C = C, so there is no “methene”.

A. General formula and examples

Alkenes (with one C = C) have general formula CₙH₂ₙ.

Unbranched alkenes you should be able to name:

Name (unbranched)	Molecular formula	One-line structural formula
ethene	C₂H₄	CH₂=CH₂
propene	C₃H₆	CH₂=CH–CH₃

Required C4 alkenes and isomers

Name (unbranched)	Molecular formula	One-line structural formula
but-1-ene	C₄H₈	CH₂=CH–CH₂–CH₃
but-2-ene	C₄H₈	CH₃–CH=CH–CH₃

Branched alkene example (same formula, different structure):

Name (branched)	Molecular formula	One-line structural formula
2-methylpropene (methylpropene)	C₄H₈	CH₂=C(CH₃)₂

There is no “methene” because a C = C bond requires at least two carbon atoms.

B. Manufacture: cracking (thermal decomposition)

Cracking breaks long-chain alkanes into smaller molecules (short-chain alkanes and alkenes).

Essential conditions:

high temperature,
catalyst: aluminium oxide (Al₂O₃) or silicon dioxide (SiO₂).

Example equation: C₁₀H₂₂ → [heat][catalyst] C₈H₁₈ + C₂H₄

Cracking can also produce hydrogen, for example: C₂H₆ → C₂H₄ + H₂

Cracking is useful because refineries often have more large hydrocarbon molecules than customers need. It converts them into smaller fuel molecules and alkenes, which are in greater demand and can be used to make other chemicals.

Cracking breaks a larger alkane into smaller hydrocarbons. Products include an alkene and may include hydrogen.

C. Combustion

Ethene burns completely in a plentiful supply of oxygen to form carbon dioxide and water: C₂H₄(g) + 3O₂(g) → 2CO₂(g) + 2H₂O(l)

D. Addition reactions

Alkenes undergo addition reactions because the C = C bond opens up and new atoms attach.

1) Hydrogenation (add H₂)

Ethene + hydrogen → ethane: C₂H₄(g) + H₂(g) → C₂H₆(g)

Conditions: heat and a nickel catalyst.

Application: hydrogenation of vegetable oils (unsaturated) to make margarine (more saturated).

2) Bromination (test for unsaturation)

Alkenes react with bromine water (aqueous bromine, Br₂(aq)).

Observation: red-brown → colourless.

Example: C₂H₄(g) + Br₂(aq) → C₂H₄Br₂(aq)

Write: “red-brown bromine water decolourises to colourless”. Also write: “alkanes show no change under normal conditions”.

Bromine water is toxic/irritant. In practical questions, “wear gloves/eye protection” is acceptable.

3) Hydration (add steam)

Ethene + steam → ethanol: C₂H₄(g) + H₂O(g) → [high\ temperature,\ high\ pressure][H₃PO₄] C₂H₅OH(l)

Conditions: high temperature and pressure with a phosphoric(V) acid catalyst (H₃PO₄). Exact temperature and pressure values are not required.

4) Addition polymerisation

Many alkene monomers join to form a polymer.

Ethene → poly(ethene): n(CH₂ = CH₂) → [-CH₂-CH₂-]ₙ

Key terms:

monomer: small molecule that repeats,
polymer: long chain made from many monomers,
repeating unit: the repeated section in the polymer.

In addition polymerisation, each ethene double bond opens and the monomers join. No other product forms.

E. Polyunsaturated food products and margarine

An unsaturated oil contains at least one C = C bond. Polyunsaturated means its molecules contain more than one C = C bond.

During margarine manufacture, hydrogen adds across some of these double bonds. This makes the vegetable oil more saturated and produces a solid product.

4. Common Mistakes

Writing “bromine turns clear” (say “red-brown to colourless”).
Writing that alkanes react with bromine water at room temperature (they do not, without UV).
Mixing up cracking with fractional distillation (cracking is chemical change; distillation is physical separation).
Forgetting conditions (temperature/catalyst) for cracking and hydration.
Writing the hydration product as “ethane” (it is ethanol).

5. Exam Tips

Write: “long-chain alkanes”, “high temperature”, “catalyst (Al₂O₃ or SiO₂)”, “short-chain alkanes and alkenes”.

Write: “steam”, “high temperature and pressure”, and “phosphoric(V) acid catalyst (H₃PO₄)”.

6. Worked Examples

Problem

Can

C₅H₁₀ be an alkene containing one

C = C bond? Show the general-formula check.

Study the worked solution

Choose the alkene formula
Method
Use CₙH₂ₙ for an alkene with one double bond.
Reason
The formula relates the hydrogen subscript to the number of carbon atoms.
Working
For C₅H₁₀, n = 5.
Substitute and compare
Method
Calculate the required number of hydrogen atoms.
Reason
2n = 2(5) = 10, matching the given formula.
Working
C₅H₁₀ fits CₙH₂ₙ, so it can be an alkene.

Problem

You have ethane and ethene. Describe a chemical test and give the expected observation for each substance.

Hints

Hint 1: bond difference

Ethene contains

C = C; ethane contains only

C-C.

Hint 2: observation wording

State both the starting and final colour, and give a separate observation for the alkane.

View solution step by step

Add the test reagent
Method
Add red-brown bromine water to separate samples.
Reason
Bromine undergoes addition across an alkene’s C = C bond.
Working
Use equal small samples so their observations can be compared.
Distinguish the samples
Method
Identify ethene by decolourisation and ethane by no change.
Reason
Ethene is unsaturated; ethane has no double bond and does not react under normal conditions.
Working
Ethene: red-brown → colourless. Ethane: remains red-brown.

Learner structure

Ethene forms poly(ethene). A learner draws the repeating unit as

-CH₂ = CH₂-. Correct the bond in the repeating unit and explain the change.

Track the alkene bond

Bond within the repeating unit

C−C single bondC=C double bond

Repeating unit

−CH2−CH2−−CH−CH−

View solution step by step

Open the double bond
Method
Replace the C = C in each ethene monomer with a C-C bond.
Reason
The additional bonding capacity joins each monomer to its neighbours in the growing chain.
Working
CH₂ = CH₂ monomer → single-bonded carbon atoms in the polymer.
Write the repeating unit
Method
Show an outward bond at both ends of -CH₂-CH₂-.
Reason
Those bonds indicate continuation of the polymer chain.
Working
Repeating unit: -CH₂-CH₂-.

Examination question

State the reagent and conditions used to convert ethene to ethanol by hydration. [4 marks]

Give the complete industrial conditions

Write your answer

View solution step by step

State the reagent
1 mark
Method
Use steam, H₂O(g).
Reason
Hydration adds the components of water across C = C.
Working
Reagent: steam.
State temperature
1 mark
Method
Use a high temperature.
Reason
High temperature is the required level of detail; an exact value is not required.
Working
Condition: high temperature.
State pressure
1 mark
Method
Use high pressure.
Reason
The industrial hydration process operates at elevated pressure.
Working
Condition: high pressure.
State the catalyst
1 mark
Method
Use phosphoric(V) acid, H₃PO₄.
Reason
It catalyses the addition of steam to ethene.
Working
Steam at high temperature and pressure with a H₃PO₄ catalyst.

Self-mark with the mark scheme

Compare your response with each mark point. Select a point only when your response contains that evidence.

Self-mark the reagent, essential conditions and catalyst.

Mark points

1 mark: States steam.
1 mark: States high temperature.
1 mark: States high pressure.
1 mark: States phosphoric(V) acid/H3PO4 catalyst.

Equation transfer

Decane is cracked to produce ethene and one alkane. Deduce the alkane and write one balanced cracking equation.

Hints

Hint 1: subtract atoms

Subtract the atoms in

C₂H₄ from those in

C₁₀H₂₂.

Hint 2: series check

Check that the remainder fits

CₙH₂ₙ₊₂.

View solution step by step

Conserve carbon
Method
Subtract the two carbon atoms in ethene from the ten in decane.
Reason
Cracking rearranges atoms; it does not create or destroy them.
Working
10-2 = 8 carbon atoms remain.
Conserve hydrogen
Method
Subtract the four hydrogen atoms in ethene from the 22 in decane.
Reason
The missing product must contain all remaining hydrogen atoms.
Working
22-4 = 18, giving C₈H₁₈.
Check and write the equation
Method
Confirm C₈H₁₈ is an alkane and write the products.
Reason
For n = 8, 2n + 2 = 18 and both sides now have identical atom totals.
Working
C₁₀H₂₂ → C₈H₁₈ + C₂H₄.

Next step

Try Error analysis: bromine water independently.

7. Mind Stretchers

Mind stretcher 1: Error analysis (bromine water)Extension

Question: A student writes: “Alkenes turn bromine water from colourless to red-brown.” Explain the error and correct it.

Show Answer

The direction is reversed.

Correct observation: bromine water is red-brown and turns colourless when it reacts with an alkene.

Final: red-brown → colourless (not colourless → red-brown).

Mind stretcher 2: Cracking vs distillation (trap)Extension

Question: A student says: “Cracking separates crude oil into fractions.” Replace this with a correct sentence for cracking and a correct sentence for fractional distillation.

Show Answer

Cracking: breaks long-chain hydrocarbons into smaller molecules (alkanes and alkenes) using heat and a catalyst (chemical change).

Fractional distillation: separates crude oil into fractions by different boiling points (physical separation).

Final: Cracking changes molecules; distillation separates a mixture.

8. Quiz

Test general formula, bromine-water observations, cracking conditions, and key addition reactions.

Practice next

Lesson quizCheck this lesson quickly with targeted MCQs.Structured practiceSwitch to exam-style written responses after the quiz.

Continue where you stopped

Learning objectives

1. Definition

A. Alkene

B. Unsaturated hydrocarbon

C. Addition reaction

2. Key Ideas

3. Detailed Explanations

A. General formula and examples

Required C4 alkenes and isomers

B. Manufacture: cracking (thermal decomposition)

C. Combustion

D. Addition reactions

1) Hydrogenation (add H₂)

2) Bromination (test for unsaturation)

3) Hydration (add steam)

4) Addition polymerisation

E. Polyunsaturated food products and margarine

4. Common Mistakes

5. Exam Tips

6. Worked Examples

7. Mind Stretchers

Mind stretcher 1: Error analysis (bromine water)Extension

Mind stretcher 2: Cracking vs distillation (trap)Extension

8. Quiz