Chemistry • Year 12 • Module 7 • Lesson 7
Reactions of Alkynes & Alkanes
Lock in the core vocabulary and reaction types: two-step alkyne addition, alkyne hydration, free-radical substitution of alkanes, and combustion.
1. Term–definition match
Match each term to its definition. Write the correct term from the box in the right-hand column. 9 marks
Terms: two-step addition • Lindlar catalyst • enol tautomerism • geminal dihalide • free radical substitution • incomplete combustion • vicinal dihalide • homolysis • initiation
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The sequential addition of two equivalents of a reagent to a triple bond: alkyne → alkene → fully saturated product. | |
| 1.2 | A "poisoned" palladium catalyst that stops hydrogenation at the alkene stage, producing specifically the cis-alkene. | |
| 1.3 | The spontaneous rearrangement of a vinyl alcohol (enol) to the more stable carbonyl form (keto), explaining why alkyne hydration gives a ketone. | |
| 1.4 | A compound with two halogen atoms on the same carbon (e.g. CH3CHBr2). Formed from full hydrohalogenation of an alkyne. | |
| 1.5 | A chain reaction where UV light breaks a halogen molecule into radicals that replace hydrogen atoms on an alkane. | |
| 1.6 | Combustion with limited oxygen, producing CO and/or soot instead of CO2. | |
| 1.7 | A compound with two halogen atoms on adjacent carbons (e.g. CHBrCHBr). Formed from halogenation of an alkene or alkyne step 1. | |
| 1.8 | The bond-breaking process in which both electrons are shared equally between the two fragments, producing two radicals. | |
| 1.9 | The first step of a radical chain mechanism, where UV light generates reactive radical species from X2. |
2. Fill the blanks, reactions of alkynes and alkanes
Complete the passage using the word bank. Each word or phrase is used once. 10 marks
Word bank: two pi bonds • UV light • energy source • ketone • enol • Hg2+ • dilute H2SO4 • geminal dihalide • Lindlar • carbon monoxide
A triple bond contains , which means alkyne addition reactions proceed in two distinct steps. The first step opens one pi bond; the second step opens the other. When hydrohalogenation goes to completion with an excess of HBr, the product is a because both halogen atoms end up on the same carbon.
Alkyne hydration requires both AND as catalysts. The immediate product is a vinyl alcohol called an , which is thermodynamically unstable and instantly rearranges to a . Partial hydrogenation of an alkyne to a cis-alkene can be achieved using a catalyst.
Alkane halogenation requires to proceed. This is an , NOT a catalyst, because it is electromagnetic radiation that initiates radical formation by breaking the X, X bond. Incomplete combustion of alkanes is hazardous because it produces toxic .
3. True or false, with correction
Circle T or F. If false, write the corrected version on the line. 8 marks (1 for T/F, 1 for correction)
3.1 Alkyne hydration with dilute H2SO4 and Hg2+ produces a primary alcohol. T / F
3.2 An alkyne reacts with two equivalents of Br2 in full halogenation, decolourising bromine water twice. T / F
3.3 UV light is a catalyst for the reaction between methane and chlorine. T / F
3.4 Complete combustion of a hydrocarbon in excess oxygen always produces CO2 and H2O only. T / F
4. Function recall
Answer each in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
4.1 What is the role of the Lindlar catalyst, and how does it differ from a standard Ni/Pd/Pt catalyst in hydrogenation?
4.2 Why must both dilute H2SO4 AND Hg2+ be stated for alkyne hydration?
4.3 Explain why the substitution of one H in methane by Cl under UV light produces HCl as a second product.
4.4 State two health or environmental impacts of incomplete combustion products.
5. Build a concept map, alkyne reactions
Draw labelled arrows between the five terms below to show how they connect. Each arrow must carry a linking phrase (e.g. "produces", "requires", "tautomerises to"). Aim for at least 5 labelled arrows. 5 marks
Terms: alkyne (C≡C) • alkene (C=C) • ketone • enol (vinyl alcohol) • Br2 (halogenation)
Q1, Term–definition matches
1.1 two-step addition • 1.2 Lindlar catalyst • 1.3 enol tautomerism • 1.4 geminal dihalide • 1.5 free radical substitution • 1.6 incomplete combustion • 1.7 vicinal dihalide • 1.8 homolysis • 1.9 initiation
Q2, Cloze paragraph
In order: two pi bonds / geminal dihalide / dilute H2SO4 / Hg2+ / enol / ketone / Lindlar / UV light / energy source / carbon monoxide
Q3, True / false
3.1 False. Alkyne hydration produces a ketone (or ethanal from ethyne), not an alcohol. The enol intermediate tautomerises instantly to the keto form.
3.2 True.
3.3 False. UV light is an energy source, not a catalyst. A catalyst is matter (a substance) that lowers activation energy and is regenerated; UV light is electromagnetic radiation (energy) that initiates radical formation and is not regenerated.
3.4 True.
Q4, Function recall
4.1 The Lindlar catalyst is palladium poisoned with lead acetate and quinoline, deactivated so it stops after the first H2 addition (alkyne → cis-alkene) and cannot reduce the alkene to an alkane. A standard Ni/Pd/Pt catalyst has full activity and continues to the alkane.
4.2 Hg2+ is a Lewis acid catalyst that activates the C≡C toward nucleophilic attack by water; dilute H2SO4 provides the acidic medium. Either alone is insufficient, the reaction does not proceed without both.
4.3 In free radical substitution (CH4 + Cl2 → CH3Cl + HCl), the Cl radical abstracts one H atom from CH4; this H combines with a second Cl radical to form HCl. The substitution reaction replaces H with Cl, so HCl is a necessary co-product.
4.4 Any two: CO binds haemoglobin ≈250× more strongly than O2, blocking oxygen transport and causing toxicity. Soot (C, PM2.5) causes respiratory and cardiovascular disease and is a probable carcinogen.
Q5, Sample concept map
Award 1 mark per valid labelled arrow. Acceptable connections include: alkyne + Br2 (step 1) → alkene; alkyne + H2O/H2SO4/Hg2+ → enol; enol tautomerises to → ketone; alkene + Br2 (step 2) → tetrahaloalkane. Any biologically/chemically valid linking phrases accepted.