Reactions of Alkenes

Q1 — Sample Band 6 response (8 marks), annotated

Markovnikov’s rule states that when HX adds to an unsymmetrical alkene, the H atom adds to the carbon of the C=C that already has the greater number of hydrogen atoms (the less-substituted carbon), and the X atom (halogen) adds to the more-substituted carbon. [1 — accurate definition]

But-1-ene (CH₃CH₂CH=CH₂) is an unsymmetrical alkene: C1 (=CH₂) bears 2 H atoms and C2 (=CH–) bears 1 H atom and an ethyl group. Because H can add to either C1 or C2 (with Br going to the other), two distinct products are possible. [1 — explains source of two products]

Product 1 (2-bromobutane, 87%) is the Markovnikov product: H adds to C1 (more H), Br adds to C2 (fewer H, more substituted). Product 2 (1-bromobutane, 13%) is the anti-Markovnikov product: H adds to C2, Br adds to C1. [1 — correctly identifies and justifies both products]

In the electrophilic addition mechanism, H&sup+; adds first to one carbon of the C=C, generating a carbocation intermediate on the other. When H&sup+; adds to C1, the positive charge forms at C2 — a secondary carbocation (stabilised by two alkyl groups via inductive electron donation). When H&sup+; adds to C2, the positive charge forms at C1 — a primary carbocation (only one alkyl group, less stable). The secondary carbocation at C2 forms more readily because it is more stable; Br¹¯ then attacks this carbocation to give 2-bromobutane as the major product. The less-stable primary carbocation at C1 forms less readily, so 1-bromobutane is the minor product. [2 — carbocation stability mechanism, secondary vs primary]

For but-2-ene (CH₃CH=CHCH₃), the two carbons of the C=C are equivalent: both are secondary carbons each bearing one H atom and one CH₃ group. Addition of HBr in either direction produces the same compound — 2-bromobutane — regardless of which carbon the H goes to. There is therefore only ONE product; Markovnikov’s rule is unnecessary (there is no selectivity to predict) and the product distribution becomes approximately 100% 2-bromobutane. [2 — symmetry argument correctly applied, correct prediction of single product]

The data (87% : 13%) are consistent with Markovnikov’s rule: the major product is 2-bromobutane, confirming H goes to the more-H carbon. The minor 13% yield of 1-bromobutane reflects the small but non-zero probability of the less-favoured primary carbocation pathway. [1 — links data to mechanism]

Marking criteria.

• 1 mark — Defines Markovnikov’s rule accurately (H to more-H carbon; X to more-substituted carbon).
• 1 mark — Explains that two products arise because H can add to either carbon of the unsymmetrical alkene’s C=C.
• 1 mark — Correctly identifies Markovnikov (2-bromobutane, H to C1, Br to C2) and anti-Markovnikov (1-bromobutane, H to C2, Br to C1) products with justification.
• 1 mark — States that a carbocation intermediate forms on C2 (secondary) when the Markovnikov pathway operates, and on C1 (primary) for the anti-Markovnikov pathway.
• 1 mark — Explains that the secondary carbocation (C2) is more stable than the primary carbocation (C1) due to greater alkyl group stabilisation.
• 1 mark — Links greater carbocation stability to higher rate of formation and therefore higher yield of the major product.
• 1 mark — Correctly predicts that but-2-ene gives only one product (2-bromobutane) because the two C=C carbons are equivalent (symmetrical alkene).
• 1 mark — Links the data table to Markovnikov’s rule: 87% major / 13% minor is consistent with secondary > primary carbocation stability.

Q2 — Sample Band 6 source critique (7 marks), annotated

Error 1 — Identifying but-2-yne as an alkene. But-2-yne (CH₃C≡CCH₃) is an alkyne, not an alkene. It contains a C≡C triple bond, not a C=C double bond. The student’s claim that “but-1-ene and but-2-yne are both alkenes” is incorrect. [1]

Error 2 — Overstating the specificity of the bromine water test. The bromine water test confirms the presence of unsaturation (C=C or C≡C) — it is NOT specific to alkenes. Both alkenes and alkynes decolourise Br₂(aq) by addition. The correct conclusion from decolourisation is: “the compound contains a C=C or C≡C bond” (confirmed unsaturation), not “the compound is an alkene.” [1]

Observations assessed: The colour changes described are correct. But-1-ene (C=C) does decolourise Br₂(aq) by halogenation. But-2-yne (C≡C) also decolourises Br₂(aq) by addition across the triple bond. Cyclohexane (fully saturated, no C=C or C≡C) correctly does not react. The observations themselves are valid. [1]

Conclusion validity: The observation data support only the conclusion that but-1-ene and but-2-yne both contain unsaturation; the data do NOT support the conclusion that both compounds are alkenes — this classification requires additional evidence (e.g. degree of unsaturation, infrared spectroscopy, or hydrogenation product analysis). The conclusion about cyclohexane being “confirmed saturated” is also too strong: a negative result for bromine water means no C=C or C≡C is present, but does not rule out other reactive functional groups. [2 — 1 for assessing alkene over-conclusion; 1 for cyclohexane caveat]

Experimental detection of the flaws: To distinguish but-1-ene from but-2-yne using only the reactions covered in this lesson, the hydrogenation test would help: both but-1-ene and but-2-yne would add H₂ across their unsaturated bonds (addition across C=C or C≡C), but but-2-yne (an alkyne with a C≡C triple bond) would require two additions of H₂ to reach the fully saturated butane, while but-1-ene (one C=C) would require only one addition. If after one mole of H₂ added the product still decolourises bromine water, the original compound was an alkyne (still has a C=C after first hydrogenation). [1]

Defensible reformulation: “Adding bromine water to but-1-ene and but-2-yne resulted in decolourisation of the orange/brown Br₂(aq), confirming that both compounds contain unsaturated bonds (C=C or C≡C). Cyclohexane did not react, consistent with it being a saturated compound with no C=C or C≡C. The bromine water test alone cannot distinguish alkenes from alkynes; additional tests are required to confirm the nature of the unsaturation in each compound.” [1]

Marking criteria.

• 1 mark — Identifies Error 1: but-2-yne is an alkyne, not an alkene; contains a C≡C triple bond.
• 1 mark — Identifies Error 2: the bromine water test is not specific to alkenes; it confirms unsaturation (C=C or C≡C).
• 1 mark — Correctly assesses the observations as valid (but-1-ene and but-2-yne do decolourise; cyclohexane does not).
• 1 mark — States that the classification of both compounds as “alkenes” is not supported by the bromine water data alone.
• 1 mark — Notes a limitation in the cyclohexane conclusion or equivalent caveat about the test’s specificity.
• 1 mark — Describes an experimental method to distinguish alkene from alkyne using lesson-covered reactions (e.g. testing whether the product of a single H₂ addition still decolourises bromine water — if yes, the original compound was an alkyne with a C≡C triple bond requiring two additions to reach saturation).
• 1 mark — Provides a defensible reformulation of the conclusion that accurately describes what the bromine water test confirms and does not confirm.

Q3 — Sample Band 6 response (6 marks)

The claim is partly correct but substantially flawed. [1 — overall judgement]

What is defensible: Direct hydration of ethene does produce higher-purity ethanol (~95%) than fermentation (~15%), and it is a faster continuous process with higher throughput. These are genuine advantages for industrial fuel or solvent ethanol. [1 — acknowledges correct element]

What is wrong:

• “Always superior” — hydration requires ethene, derived from non-renewable crude oil. Countries without petrochemical infrastructure or significant oil reserves may have abundant cheap agricultural starch or sugar (e.g. Brazil with sugarcane, Australia with wheat at Manildra Nowra). For these countries, fermentation is cost-competitive or superior due to lower feedstock cost and renewable supply. [1 — refutes “always” with renewable/context argument]
• “Crude oil is still widely available, so no reason to prefer fermentation” — this ignores sustainability, carbon footprint, and future supply. Fermentation uses plant-derived glucose (renewable, carbon-cycle neutral on balance); direct hydration uses fossil-derived ethene (non-renewable, adds net CO₂ to the atmosphere). Policy, environmental regulation, and crude oil price fluctuations all make fermentation strategically important. [1 — refutes the fossil-fuel adequacy argument]
• “Behind the times” — a value judgment, not a scientific claim. Many high-volume bioethanol programs (Brazil’s sugar-ethanol industry; corn ethanol in the USA; wheat ethanol at Manildra in NSW) are expanding precisely because renewability is valued over the simplicity of direct hydration. [1 — refutes normative claim with real-world context]

Defensible reformulation: “Direct hydration of ethene produces higher-purity ethanol more rapidly than fermentation and is preferable where cheap fossil-derived ethene is available and renewable feedstocks are scarce. Fermentation from agricultural starch or sugars is preferable where renewable feedstocks are abundant, where sustainability is a priority, or where petrochemical infrastructure is limited. The choice between the two processes is context-dependent and should be evaluated against feedstock availability, purity requirements, environmental impact, and economic factors.” [1 — defensible reformulation, context-dependent framing]

Marking criteria.

• 1 mark — States an overall evaluative judgement (e.g. “partly correct but largely flawed”).
• 1 mark — Identifies the one defensible element: direct hydration does give higher purity and faster throughput than fermentation.
• 1 mark — Refutes “always superior” with the context-dependency argument (renewable feedstocks, country-specific infrastructure, cost).
• 1 mark — Refutes the fossil-fuel adequacy argument with sustainability / renewability / carbon footprint considerations.
• 1 mark — Refutes the normative “behind the times” claim with a real-world example of expanding fermentation-based programs.
• 1 mark — Provides a defensible reformulation framing the choice as context-dependent, integrating lesson content vocabulary (hydration, fermentation, renewable, conditions, catalyst).