Chemistry • Year 12 • Module 7 • Lesson 7
Reactions of Alkynes & Alkanes
Build HSC Band 5–6 extended-response technique: evaluate experimental evidence, critique scientific claims, and justify reaction-pathway choices with mechanistic reasoning.
1. Evaluate reaction pathways using experimental data (Band 5–6)
8 marks Band 5–6
Scenario. A research chemist at CSIRO is developing a synthesis route from ethyne (HC≡CH) to produce three specific target molecules: (A) ethanal (CH3CHO), (B) 1,1,2,2-tetrabromoethane (CHBr2CHBr2), and (C) ethane (CH3CH3). The table below shows the yield and reaction time observed under different conditions for each pathway from ethyne.
| Pathway | Reagents & conditions | Reported yield (%) | Reaction time (h) | By-product noted |
|---|---|---|---|---|
| 1 → A | H2O, dilute H2SO4, HgSO4, 60°C | 82 | 2.0 | Hg metal deposits |
| 2 → A | H2O, dilute H2SO4, no Hg2+, 60°C | 4 | 24.0 | Unreacted ethyne |
| 3 → B | Br2 (2 eq), no catalyst, room temp | 91 | 0.5 | Trace CHBr=CHBr |
| 4 → B | Br2 (1 eq), no catalyst, room temp | 0 | 0.5 | CHBr=CHBr (major) |
| 5 → C | H2 (2 eq), Ni catalyst, 200°C | 97 | 1.0 | None |
| 6 → C | H2 (1 eq), Lindlar catalyst, room temp | 0 | 1.0 | Ethene (major) |
Q1. Analyse the data and evaluate which conditions best achieve each synthesis target. In your response you must:
- Identify the correct pathway for each target (A, B, C) and justify using the data.
- Explain, using reaction mechanism, why Pathway 2 (no Hg2+) gives only 4% yield for target A.
- Explain why Pathway 4 (1 eq Br2) gives 0% yield of target B, and identify the actual product formed.
- Evaluate the environmental trade-off in choosing Pathway 1 over Pathway 2 for industrial-scale synthesis.
- State which pathway you would recommend to synthesise ethane and justify your choice on two criteria from the data.
2. Source critique — evaluating a scientific claim (Band 5–6)
7 marks Band 5–6
“Acetylene (ethyne) is useful in both welding and as a PVC precursor because it is highly reactive with both halogens and water. When it reacts with water, it produces ethanol — a useful solvent — through the same Markovnikov addition mechanism that alkenes use. The UV light needed for the chlorination of alkane by-products from PVC manufacture acts as a catalyst, making the reaction faster without being consumed itself.”
— Student summary submitted for a chemistry assignment, 2024.
Q2. This passage contains two distinct scientific errors. For each error:
- Identify the error and quote or paraphrase the relevant sentence.
- Explain the correct chemistry, including the relevant equation or mechanism.
- For the second error (UV light as catalyst), describe how you would design a simple experiment that demonstrates UV light is an energy source rather than a catalyst.
Q1 — Marking criteria (8 marks)
Identify correct pathways (1 mark each ×3 = 3 marks): Target A: Pathway 1 (82% yield, 2 h; Pathway 2 fails without Hg2+). Target B: Pathway 3 (91% yield, 2 eq Br2; Pathway 4 stops at step 1 giving CHBr=CHBr, not target B). Target C: Pathway 5 (97% yield, Ni, 200°C; Pathway 6 stops at ethene using Lindlar).
Explain Pathway 2 mechanism (1 mark): Hg2+ is a Lewis acid catalyst that activates the C≡C toward nucleophilic attack by water. Without Hg2+, the activation barrier cannot be overcome at 60°C; the reaction barely proceeds, giving only 4% yield and leaving most ethyne unreacted.
Explain Pathway 4 (1 mark): Only 1 eq Br2 is provided; this is enough to open only the first pi bond (step 1), giving 1,2-dibromoethene (CHBr=CHBr). A second eq of Br2 is needed to open the second pi bond and give the target tetrabromoethane.
Environmental trade-off (1 mark): Pathway 1 gives 82% yield vs 4% for Pathway 2, so it is far more industrially efficient. However, Hg2+ generates Hg metal as a by-product, which is a severe environmental and health hazard (bioaccumulation, neurotoxicity — Minamata disease precedent). Industrial-scale use requires strict mercury capture and disposal protocols, increasing costs and risk.
Recommend ethane pathway with two criteria (1 mark): Pathway 5 recommended. Criteria: (i) highest yield (97% vs 0% for Pathway 6 which produces ethene, not ethane); (ii) shorter time (1 h) with no noted by-products, making it cleaner and more efficient.
Q2 — Marking criteria (7 marks)
Error 1 — Identify (1 mark): “it produces ethanol — a useful solvent” is incorrect. Alkyne hydration does not produce an alcohol. The enol intermediate (vinyl alcohol) instantly tautomerises to the more stable keto form.
Error 1 — Correct chemistry (2 marks): HC≡CH + H2O → [CH2=CHOH] → CH3CHO (ethanal, an aldehyde). Conditions: dilute H2SO4 + Hg2+, ~60°C. The product is ethanal, not ethanol; these are different functional group classes (aldehyde vs alcohol). Ethanol cannot form because the vinyl alcohol (enol) is thermodynamically unstable and rearranges. (1 mark equation; 1 mark explanation of tautomerism.)
Error 2 — Identify (1 mark): “UV light … acts as a catalyst, making the reaction faster without being consumed itself” is incorrect.
Error 2 — Correct chemistry (2 marks): A catalyst is matter (a substance) that lowers the activation energy of a reaction and is regenerated unchanged. UV light is electromagnetic radiation — energy, not matter. UV light is absorbed in the initiation step to break the Cl—Cl bond by homolysis, generating Cl• radicals. It is consumed (not regenerated) and does not lower activation energy through a catalytic cycle. Calling it a catalyst is factually wrong. (1 mark for catalyst definition; 1 mark for UV light mechanism explanation.)
Experimental design to distinguish energy source from catalyst (1 mark): Conduct two parallel reactions: (i) CH4 + Cl2 under UV light; (ii) CH4 + Cl2 in the dark at the same temperature. If UV light were a catalyst, the reaction in the dark would still proceed (just more slowly, as catalysts do not change whether a reaction occurs but only its rate). In reality, the dark reaction does not proceed at room temperature, showing UV provides the energy input needed to initiate radical formation — consistent with it being an energy source, not a catalyst.