4.4.1Organic Chemistry
Alkenes — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
10 min read 12 parts Lesson 11 of 24
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Part 1 of 7Key ideas
Key ideas
General formula of alkenes
CnH2n.
C2H4 + Br2 → C2H4Br2 (1,2-dibromoethane).
Bromine water
Bromine water orange → colourless with alkenes.
Alkenes are more reactive than alkanes because of the C=C double bond.
Ethene + steam → ethanol (phosphoric acid catalyst, about 300 °C, 60–70 atm).
n CH2=CH2 → –(CH2–CH2)n– poly(ethene).
Alkenes are made by cracking long-chain alkanes.
Part 2 of 7Learning objectives
1
State the general formula of alkenes and explain the term unsaturated.
2
Draw and name alkenes up to four carbon atoms, including isomers.
3
Describe the reaction of alkenes with bromine and use it as a test.
4
Compare the reactivity and reactions of alkanes and alkenes.
Part 3 of 7Essential knowledge
Essential knowledge
What are alkenes?
Alkenes are a homologous series of hydrocarbons containing a carbon–carbon double bond (C=C), which is their functional group.
Alkenes are unsaturated hydrocarbons
Alkenes are unsaturated hydrocarbons the double bond means the molecule does not contain the maximum number of hydrogen atoms, and more atoms can be added across the C=C.
The general formula of an alkene is CnH2n
. Examples: ethene C2H4, propene C3H6, butene C4H8. Names end in -ene with a number showing where the double bond starts, e.g. but-1-ene.
Isomers are possible from butene
Isomers are possible from butene but-1-ene and but-2-ene differ in the position of the C=C, and 2-methylpropene is a branched isomer.
Bromine and alkenes
Bromine and alkenes bromine adds across the double bond in an addition reaction at room temperature, e.g. CH2=CH2 + Br2 → CH2BrCH2Br. The orange bromine water becomes colourless.
This is the standard test for unsaturation
This is the standard test for unsaturation alkenes decolourise bromine water rapidly, whereas alkanes do not because they only react with bromine by substitution in UV light.
Alkenes are much more reactive than alkanes and are valuable starting materials: they are made by cracking and used to make polymers and ethanol.
Pent-1-ene
The fourth alkene listed in the series is pent-1-ene, C5H10, with the C=C between carbons 1 and 2. Check it fits CnH2n: n = 5 gives 10 hydrogen atoms.
Other halogens add too
Any halogen adds across the C=C in the same way as bromine. For example, ethene and chlorine form 1,2-dichloroethane: C2H4 + Cl2 → C2H4Cl2, with one chlorine atom on each carbon.
Why bromine water?
Bromine water is used for the test instead of pure bromine because it is safer and easier to handle. The orange colour comes from dissolved bromine; it disappears when the bromine is used up by addition.
Why the C=C reacts
The double bond is a region of high electron density, which attracts molecules such as bromine. When it opens, each carbon forms a new single bond, so alkenes react far more readily than alkanes.
Part 4 of 7Process & sequence
1
Place a small volume of the hydrocarbon in a test tube.
2
Add a few drops of orange bromine water.
3
Stopper the tube and shake.
4
Observe: an alkene decolourises the bromine water quickly.
5
An alkane leaves the solution orange (no reaction without UV light).
6
The product with ethene is colourless 1,2-dibromoethane.
Testing for an alkenewrite these steps in this order; each step is a marking point.
1
Add bromine water to sample
2
Shake the test tube
3
Bromine adds across C=C
4
Orange turns colourless
5
Dibromoalkane formed
Part 5 of 7Worked examples
1Write the equation and name the product for propene reacting with bromine.
- CH2=CHCH3 + Br2 → CH2BrCHBrCH3. The product is 1,2-dibromopropane, formed by addition across the C=C.
256
An alkene has 8 hydrogen atoms. Find its molecular formula and relative formula mass (C = 12, H = 1).
- 2n = 8, so n = 4: C4H8. Mr = (4 × 12) + (8 × 1) = 56.
Part 6 of 7Summary
1
Alkenes are unsaturated hydrocarbons, CnH2n.
2
The C=C bond makes them undergo addition reactions.
3
Test: alkenes decolourise bromine water.
4
Alkenes are monomers for addition polymers.
Part 7 of 7Compare: Compare
| Feature | Alkanes | Alkenes |
|---|---|---|
| General formula | CnH2n+2 | CnH2n |
| Bond type | Only C–C single bonds (saturated) | Contains C=C (unsaturated) |
| Bromine water test | Stays orange | Decolourised |
| Typical reaction | Substitution (UV light) | Addition |
| Main source | Crude oil fractions | Cracking |
| Name | Formula | Structure |
|---|---|---|
| Ethene | C2H4 | CH2=CH2 |
| Propene | C3H6 | CH2=CHCH3 |
| But-1-ene | C4H8 | CH2=CHCH2CH3 |
| But-2-ene | C4H8 | CH3CH=CHCH3 |
There is no alkene with one carbon atom because a C=C needs two carbons.
Key terms
Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Alkene · Unsaturated · Addition reaction · Bromine water · Decolourise · Dibromoalkane · Monomer · Addition polymerisation · Position isomers
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. electronegativity, nucleophile, dynamic equilibrium, activation energy).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
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