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Chemistry · Practice book · Senior 5 · Unit 3 of 15

Alkenes and alkynes

Senior 5 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 3 Alkenes and alkynes

Key unit competence: Relate the structure of alkenes and alkynes to their physical and chemical properties and uses.

Key ideas

  • Alkenes (CnH2n) contain C=C (one σ + one π bond; sp2 carbons, planar, 120°). Alkynes (CnH2n-2) contain C≡C (one σ + two π; sp carbons, linear, 180°). The π electrons make them reactive towards electrophiles.
  • Preparation of alkenes: dehydration of alcohols (conc. H2SO4, 170 °C, or Al2O3); dehydrohalogenation of halogenoalkanes with ethanolic KOH (CH3CH2Br + KOH → CH2=CH2 + KBr + H2O); cracking. Alkynes: CaC2 + 2H2O → C2H2 + Ca(OH)2; double dehydrohalogenation of dihalides.
  • Electrophilic addition: H2 (Ni), Br2 (decolourises), HBr, H2O (steam, H3PO4 catalyst), conc. H2SO4. Mechanism with HBr: the π bond attacks H, forming a carbocation, then Br− attacks.
  • Markovnikov's rule: when HX adds to an unsymmetrical alkene, H attaches to the carbon already bearing more H atoms, because the more stable (more substituted) carbocation forms (tertiary > secondary > primary).
  • Oxidation: cold dilute acidified/alkaline KMnO4 gives diols (purple to colourless/brown: test for C=C); hot concentrated KMnO4 cleaves the double bond; ozonolysis gives aldehydes/ketones. Polymerisation gives polythene, polypropene, PVC.
  • Alkynes add two molecules of H2, Br2 or HX. Terminal alkynes are weakly acidic: they react with sodium or ammoniacal silver nitrate ([Ag(NH3)2]+, white ppt) and copper(I) chloride (red ppt), which distinguishes them from alkenes and internal alkynes.

Common misconceptions

  • "HBr adds randomly to propene giving equal amounts of both products." No: The major product is 2-bromopropane (Markovnikov), via the more stable secondary carbocation.
  • "All alkynes give a precipitate with ammoniacal silver nitrate." No: Only terminal alkynes (with ≡C–H) react.
  • "The π bond is stronger than the σ bond." No: The π bond is weaker and more exposed, which is why alkenes react by addition.

Worked examples

Example 1. Describe the mechanism of electrophilic addition of HBr to propene and explain the major product.

Solution

H–Br is polar; the π electrons of C=C attack Hδ+, and the H–Br bond breaks heterolytically.

H adds to the end CH2 carbon, forming the secondary carbocation CH3CH+CH3 (more stable than the primary one).

Br− attacks the carbocation, giving 2-bromopropane (major product, Markovnikov's rule).

Example 2. Give two chemical tests to distinguish ethene from ethyne.

Solution

Ammoniacal silver nitrate: ethyne gives a white precipitate (silver ethynide); ethene does not react.

Ammoniacal copper(I) chloride: ethyne gives a red-brown precipitate; ethene does not.

(Both decolourise bromine water, so that test cannot distinguish them.)

Practice questions

  1. State Markovnikov's rule.

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    When HX adds to an unsymmetrical alkene, H attaches to the carbon with more H atoms.

    Common mistake: Saying X attaches there.

  2. Write the equation for the dehydration of ethanol.

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    C2H5OH → C2H4 + H2O (conc. H2SO4, 170 °C).

    Common mistake: Writing ethane.

  3. What is observed when propene is shaken with cold, dilute acidified KMnO4?

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    Purple colour disappears.

    Common mistake: Saying it turns orange.

  4. Why are tertiary carbocations more stable than primary?

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    More alkyl groups donate electrons, spreading the positive charge.

    Common mistake: Saying they are larger.

  5. Name the product of ethyne with 1 mol of HCl.

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    Chloroethene.

    Common mistake: Writing chloroethane.

  6. Write the equation for calcium carbide with water.

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    CaC2 + 2H2O → C2H2 + Ca(OH)2.

    Common mistake: Writing CaO.

  7. Give one use each of ethene and ethyne.

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    Ethene: polythene/ethanol/fruit ripening; ethyne: welding.

    Common mistake: Swapping the uses.

  8. Name the type of reaction when ethene forms polythene.

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    Addition polymerisation.

    Common mistake: Saying condensation.

  9. Explain why but-1-yne reacts with sodium metal.

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    Its terminal ≡C–H is weakly acidic; sodium replaces H giving hydrogen.

    Common mistake: Saying alkynes are strong acids.

  10. What is the product of ozonolysis of ethene (with Zn/water work-up)?

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    Methanal (two molecules).

    Common mistake: Saying ethanal.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. PVC is made from:a. chloroethene
    2. The major product of HBr with propene is:b. sideways overlap of p orbitals
    3. Terminal alkynes react with ammoniacal silver nitrate to give:c. 2-bromopropane
    4. The π bond is formed by:d. a white precipitate
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    1 → a (chloroethene) · 2 → c (2-bromopropane) · 3 → d (a white precipitate) · 4 → b (sideways overlap of p orbitals)

    • Vinyl chloride polymerises.
    • Markovnikov's rule.
    • Silver acetylide.
    • Above and below the plane.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    elimination only  ·  electrophilic addition  ·  ethyne  ·  accepts a pair of electrons
    1. Copper(I) chloride in ammonia gives a red precipitate with .
    2. An electrophile is a species that .
    3. Alkenes undergo mainly .
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    1: ethyne · 2: accepts a pair of electrons · 3: electrophilic addition

    • Copper(I) acetylide.
    • Electron-pair acceptor.
    • The π bond attracts electrophiles.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Ethene is made from bromoethane by heating with:a. Ammoniacal silver nitrate
    2. Ozonolysis of but-2-ene gives:b. ethane
    3. Ethyne reacts with excess hydrogen (Ni) to give:c. ethanolic KOH
    4. Which reagent distinguishes but-1-yne from but-2-yne?d. ethanal
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    1 → c (ethanolic KOH) · 2 → d (ethanal) · 3 → b (ethane) · 4 → a (Ammoniacal silver nitrate)

    • Elimination.
    • C=C is split into two CH3CHO.
    • Adds two H2.
    • Only the terminal alkyne reacts.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    180°  ·  120°  ·  ethane-1,2-diol  ·  concentrated sulfuric acid at 170 °C
    1. The bond angle in ethyne is .
    2. Ethanol is dehydrated to ethene using .
    3. Cold dilute KMnO4 converts ethene into .
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    1: 180° · 2: concentrated sulfuric acid at 170 °C · 3: ethane-1,2-diol

    • sp hybridised, linear.
    • Elimination of water.
    • Two OH groups add.

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