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.