Unit 10 Alkenes and alkynes
Key unit competence: Analyse the structure, properties and uses of alkenes and alkynes.
Key ideas
- Alkenes are unsaturated hydrocarbons with a C=C double bond, general formula CnH2n: ethene C2H4, propene C3H6, but-1-ene C4H8. Alkynes have a C≡C triple bond, general formula CnH2n-2: ethyne (acetylene) C2H2, propyne C3H4.
- Naming: longest chain containing the multiple bond; ending -ene or -yne; number the position of the multiple bond with the lowest number, e.g. but-1-ene CH2=CHCH2CH3 and but-2-ene CH3CH=CHCH3 (positional isomers).
- Preparation: ethene by dehydration of ethanol with concentrated H2SO4 at 170 °C (or over hot Al2O3), or by cracking alkanes; ethyne from calcium carbide and water: CaC2 + 2H2O → C2H2 + Ca(OH)2.
- Addition reactions of alkenes: with hydrogen (Ni catalyst, 150 °C) to form alkanes (C2H4 + H2 → C2H6); with bromine (C2H4 + Br2 → C2H4Br2); with hydrogen halides; with steam (H3PO4 catalyst) to form ethanol.
- Test for unsaturation: alkenes and alkynes decolourise bromine water (orange to colourless) and acidified potassium manganate(VII) (purple to colourless); alkanes do not.
- Uses: ethene makes polythene (addition polymerisation: n CH2=CH2 → –(CH2-CH2)n–), ethanol and ripens fruit; propene makes polypropene; ethyne burns in oxygen with a very hot flame (about 3000 °C) for welding and cutting metals.
Common misconceptions
- "Alkenes react by substitution like alkanes." No: Alkenes react mainly by ADDITION across the double bond.
- "Bromine water turns colourless with alkanes too." No: Only unsaturated compounds (C=C or C≡C) decolourise bromine water quickly.
- "Polymerisation produces a by-product like water." No: Addition polymerisation of alkenes gives only the polymer, no by-product.
Worked examples
Example 1. Describe the test to distinguish between ethane and ethene, giving the equation.
Solution
Bubble each gas through bromine water (orange).
Ethene decolourises it quickly: C2H4 + Br2 → CH2BrCH2Br (1,2-dibromoethane).
Ethane leaves the orange colour unchanged because it is saturated.
Example 2. Write the equation for the formation of polythene and explain why it is called addition polymerisation.
Solution
n CH2=CH2 → –(CH2-CH2)n–.
The double bond in each ethene molecule opens and the molecules join end to end.
Only one product (the polymer) forms, with no small molecule lost, so it is addition polymerisation.
Practice questions
Write the general formula of the alkynes.
Show answer
CnH2n-2.
Common mistake: Writing CnH2n.
Name CH2=CHCH3.
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Propene.
Common mistake: Writing propane.
Write the equation for the preparation of ethyne from calcium carbide.
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CaC2 + 2H2O → C2H2 + Ca(OH)2.
Common mistake: Writing CaO as product.
State the condition for the hydrogenation of ethene.
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Nickel catalyst at about 150 °C.
Common mistake: Saying UV light.
Give two uses of ethene.
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Making polythene, making ethanol, ripening fruit (any two).
Common mistake: Saying cooking gas.
Describe the colour change when propene is added to bromine water.
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Orange to colourless.
Common mistake: Saying colourless to orange.
Write the equation for ethene reacting with steam.
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C2H4 + H2O → C2H5OH.
Common mistake: Writing ethanoic acid.
Draw (describe) the positional isomers of C4H8 that are straight-chain alkenes.
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But-1-ene CH2=CHCH2CH3 and but-2-ene CH3CH=CHCH3.
Common mistake: Calling them chain isomers.
Why is ethyne more reactive than ethane?
Show answer
Its triple bond allows addition reactions; ethane has only single bonds.
Common mistake: Saying ethyne has more hydrogen.
How many moles of hydrogen are needed to convert 1 mol of ethyne to ethane?
Show answer
2 mol.
Common mistake: Saying 1 mol.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The test for an alkene is that it: a. ethyne, C2H2 2. The polymer made from ethene is: b. purple to colourless 3. Acidified potassium manganate(VII) with an alkene changes from: c. decolourises bromine water 4. The first member of the alkynes is: d. polythene Show answer
1 → c (decolourises bromine water) · 2 → d (polythene) · 3 → b (purple to colourless) · 4 → a (ethyne, C2H2)
- Bromine adds across C=C.
- Ethene molecules join by addition polymerisation.
- The alkene is oxidised.
- There is no alkyne with one carbon.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
water · polythene · ethane · propene, C3H6- A hydrocarbon X decolourises bromine water; 1 mol of X adds 1 mol of H2; X has Mr 42. X is .
- Ethyne is prepared from calcium carbide and .
- Ethene reacts with hydrogen in the presence of nickel to form .
Show answer
1: propene, C3H6 · 2: water · 3: ethane
- Step 1. Decolourises bromine: unsaturated.
Step 2. Adds 1 H2: one C=C.
Step 3. CnH2n with Mr 42: n = 3. - CaC2 + 2H2O → C2H2 + Ca(OH)2.
- This is hydrogenation, an addition reaction.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The formula of propyne is: a. ripen fruits such as bananas 2. The general formula of alkenes is: b. C3H6 3. Which compound is an alkene? c. C3H4 4. Ethene is used to: d. CnH2n Show answer
1 → c (C3H4) · 2 → d (CnH2n) · 3 → b (C3H6) · 4 → a (ripen fruits such as bananas)
- CnH2n-2 with n = 3.
- For example ethene C2H4.
- It fits CnH2n.
- Ethene is a plant hormone.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
positional isomers · dehydration with concentrated sulfuric acid · 1,2-dibromoethane · hydrogenation- But-1-ene and but-2-ene are .
- Ethene can be made from ethanol by .
- The reaction of ethene with bromine gives .
Show answer
1: positional isomers · 2: dehydration with concentrated sulfuric acid · 3: 1,2-dibromoethane
- The double bond is in different positions.
- Water is removed from ethanol.
- Each carbon gains one Br atom.