Chemistry · Year 12 · Module 7 · Lesson 20
HSC Exam Practice
Organic Reactions Mastery — Conditions, Pathways & Band 6 Responses
Short answer
1.Short answer — conditions and mechanisms
Define esterification and identify the catalyst and type of reaction arrow required in its balanced equation.
Distinguish between the conditions used to produce an aldehyde versus a carboxylic acid when oxidising a primary alcohol with acidified K2Cr2O7. Explain why the different conditions give different products.
Identify the role of UV light in the free-radical halogenation of alkanes. Explain why it is incorrect to label UV light as a catalyst in this reaction.
Describe the observations you would make when each of the following compounds is treated with acidified K2Cr2O7 solution, and name the organic product formed (if any):
(a) propan-1-ol (b) propan-2-ol (c) 2-methylpropan-2-ol (d) propanal
Outline the conditions required to produce ethanol from ethene on an industrial scale. Include the catalyst, temperature, pressure, and type of equipment used. Explain the purpose of using high pressure.
Compare the substitution and elimination reactions of 1-bromopropane with sodium hydroxide. In your answer, state the specific reagent form required for each reaction type, write a balanced equation for each, and name each organic product.
Data response
2.Data response — multi-step synthesis yield data
A research group at the University of New South Wales investigated the yield of ethyl propanoate from a three-step synthesis starting from 1-chloropropane (Step 1: substitution; Step 2: oxidation; Step 3: esterification). They repeated the synthesis six times, varying the oxidation conditions in Step 2. Their results are recorded in the table below.
| Trial | Step 2 conditions | K2Cr2O7 amount (eq) | Step 2 equipment | Overall yield of ester (%) | Major Step 2 product identified |
|---|---|---|---|---|---|
| 1 | K2Cr2O7/H2SO4, heat | 1.0 | Distillation | trace (<5%) | Propanal |
| 2 | K2Cr2O7/H2SO4, heat | 2.5 | Reflux | 47% | Propanoic acid |
| 3 | K2Cr2O7/H2SO4, heat | 2.5 | Reflux; Na2CO3 wash | 61% | Propanoic acid (purified) |
| 4 | K2Cr2O7/H2SO4, heat | 1.5 | Reflux | 39% | Mixture: propanal + propanoic acid |
| 5 | KMnO4/H2SO4 | 2.5 | Reflux | 44% | Propanoic acid |
| 6 | No oxidant added | 0 | — | 0% | None (propan-1-ol unreacted) |
Table 2.1. Overall ester yield from six trials varying Step 2 oxidation conditions. All trials used the same Step 1 and Step 3 procedure. Adapted from teaching laboratory data.
(a) Identify the trend between the amount of K2Cr2O7 used and the overall ester yield in Trials 1, 2, and 4. Explain this trend using your knowledge of oxidation conditions in Module 7. (3 marks)
(b) Account for the difference in yield between Trial 2 (47%) and Trial 3 (61%), using your understanding of purification steps in multi-step synthesis. (2 marks)
(c) Trial 6 (no oxidant) gives 0% ester yield. Explain, using the three-step pathway, why there is zero yield and why simply adding more methanol at Step 3 would not fix the problem. (2 marks)
An unknown compound X is tested as follows:
- Tollens’ reagent: no reaction.
- Bromine water: decolourises immediately.
- K2Cr2O7/H2SO4: solution stays orange.
- Sodium carbonate solution: vigorous fizzing, CO2 released.
(a) Identify the two functional groups present in compound X. Justify each identification with reference to the specific test result. (4 marks)
(b) Suggest a specific structural formula consistent with all four test results. Give the IUPAC name of your compound. (2 marks)
Extended response
3.Extended response — multi-step synthesis
Analyse and evaluate the following proposed synthesis pathway, identifying all errors. Then propose the correct, complete synthesis pathway with equations, conditions, and product names for each step.
Proposed pathway to synthesise butyl butanoate, starting from butan-1-ol only:
Step 1: butan-1-ol + K2Cr2O7/H2SO4, reflux → butanal
Step 2: butanal + K2Cr2O7/H2SO4 (excess), distillation → butanoic acid
Step 3: butanoic acid + butan-1-ol, H2SO4, reflux → butyl butanoate + H2O
Marking criteria — do not open before attempting
Q1.1 — Esterification definition (3 marks)
1 mark: a reversible condensation reaction between a carboxylic acid and an alcohol. 1 mark: catalyst = conc. H2SO4 (a few drops; it is a catalyst — not consumed). 1 mark: reversible arrow ⇌ is required (equilibrium; yield <100%). Common errors: writing → (irreversible); writing H2SO4 as a reagent not a catalyst; omitting yield comment.
Q1.2 — Distillation vs reflux for aldehyde vs carboxylic acid (4 marks)
Aldehyde conditions (2 marks): K2Cr2O7/H2SO4, limited oxidant, DISTILLATION. Explanation: distillation continuously removes the aldehyde (lower boiling point than starting alcohol) as it forms, preventing further oxidation by excess Cr2O72−. Without removal, the aldehyde remains in contact with oxidant and is oxidised to the acid.
Carboxylic acid conditions (2 marks): K2Cr2O7/H2SO4, excess oxidant, REFLUX. Explanation: reflux keeps all species in the flask; with excess oxidant, the aldehyde intermediate is completely converted to carboxylic acid. Observable: orange → green in both cases.
Q1.3 — Role of UV light (2 marks)
1 mark: UV light provides the activation energy to initiate the free-radical chain reaction by homolytically cleaving the Cl—Cl (or Br—Br) bond, producing two chlorine (or bromine) radicals. 1 mark: UV light is an energy source (initiator), not a catalyst, because it IS consumed in the process (energy is absorbed to break the halogen bond) and is not regenerated during the chain reaction. A catalyst lowers activation energy without being consumed and is regenerated; UV light does neither.
Q1.4 — Observations with K2Cr2O7/H2SO4 (4 marks)
(a) propan-1-ol (1° alcohol): orange → green. With limited oxidant + distillation → propanal; with excess + reflux → propanoic acid. (1 mark)
(b) propan-2-ol (2° alcohol): orange → green. Product = propanone (ketone). Cannot be further oxidised. (1 mark)
(c) 2-methylpropan-2-ol (3° alcohol): orange stays orange. NO colour change. No product (dead end — no C–H on the OH-bearing carbon to oxidise). (1 mark)
(d) propanal (aldehyde): orange → green. Product = propanoic acid (further oxidation of aldehyde to carboxylic acid, K2Cr2O7 excess, reflux). (1 mark)
Q1.5 — Industrial hydration of ethene (4 marks)
1 mark: Reagent = H2O (steam); catalyst = H3PO4 (or dil. H2SO4); temperature = ~300°C; pressure = HIGH (~65 atm); equipment = high-pressure reactor/vessel. 1 mark: balanced equation: CH2=CH2 + H2O ⇌ CH3CH2OH. 1 mark: reversible arrow required (equilibrium, not complete). 1 mark: high pressure drives the equilibrium toward the alcohol product (Le Chatelier’s principle — fewer moles of gas on the product side); also increases rate of gas-phase reaction. Common error: writing → or omitting pressure.
Q1.6 — Substitution vs elimination with NaOH (4 marks)
Substitution (2 marks): NaOH(aq) — aqueous NaOH. CH3CH2CH2Br + NaOH(aq) → CH3CH2CH2OH + NaBr. Product: propan-1-ol. Conditions: reflux.
Elimination (2 marks): NaOH(alc) — alcoholic NaOH. CH3CH2CH2Br + NaOH(alc) → CH3CH=CH2 + NaBr + H2O. Product: propene. Conditions: reflux. Key distinction: aqueous OH− is a nucleophile (SN2 substitution); alcoholic OH− is a base (E2 elimination). Writing “NaOH” without specifying aqueous/alcoholic loses marks.
Q2.1 — Data response: ester yield table (7 marks)
(a) 3 marks: Trend: as K2Cr2O7 amount increases from limited (Trial 1, 1.0 eq, distillation) to intermediate (Trial 4, 1.5 eq, reflux) to excess (Trial 2, 2.5 eq, reflux), ester yield increases (trace → 39% → 47%). Trial 1 distillation removes propanal before it can be oxidised to propanoic acid, so there is no carboxylic acid for esterification — hence near-zero ester yield. With excess oxidant and reflux (Trial 2), full conversion to propanoic acid occurs, giving a higher ester yield. Trial 4 (limited oxidant, reflux) gives a mixed product: some propanal and some propanoic acid, reducing ester yield.
(b) 2 marks: Trial 3 adds a Na2CO3 wash step before esterification. This removes residual propanoic acid and H2SO4 that would otherwise compete in or inhibit the esterification equilibrium, and removes water. Purer propanoic acid entering Step 3 shifts the esterification equilibrium further right, increasing ester yield by ~14 percentage points (from 47% to 61%).
(c) 2 marks: Without oxidation in Step 2, propan-1-ol (the intermediate from Step 1 substitution) is never converted to propanoic acid. Esterification requires BOTH a carboxylic acid AND an alcohol; Step 3 only has the alcohol (propan-1-ol or ethanol). Adding more methanol at Step 3 provides only an additional alcohol; without the acid component (propanoic acid), no ester can form.
Q2.2 — Unknown compound X (6 marks)
(a) 4 marks (2 per functional group):
Functional group 1: C=C double bond (alkene). Evidence: bromine water decolourises immediately — Br2 adds across the C=C bond (addition reaction). The orange-to-no-change with K2Cr2O7 is consistent (alkenes don’t give colour change under these conditions).
Functional group 2: carboxylic acid (–COOH). Evidence: vigorous fizzing with Na2CO3 — CO2 is produced, indicating acidic –COOH group reacting with carbonate: RCOOH + Na2CO3 → RCOONa + CO2 + H2O. K2Cr2O7 stays orange confirms no alcohol or aldehyde present. Tollens’ negative confirms no free aldehyde.
(b) 2 marks: A compound with both C=C and –COOH, e.g. but-3-enoic acid (CH2=CH–CH2–COOH) or but-2-enoic acid (CH3CH=CHCOOH). IUPAC name: but-3-enoic acid (or but-2-enoic acid / crotonic acid). Award 1 mark for correct structural formula with both functional groups shown; 1 mark for correct IUPAC name.
Q3.1 — Pathway evaluation and correction (7 marks)
Error identification (3 marks, 1 each):
Error 1 (Step 1): Reflux is stated for producing butanal. This is incorrect — reflux keeps butanal in contact with oxidant, causing over-oxidation to butanoic acid. DISTILLATION is required to produce butanal (remove it as it forms before over-oxidation).
Error 2 (Step 2): Distillation is stated for producing butanoic acid from butanal. This is incorrect — distillation removes the intermediate and prevents complete oxidation. REFLUX with excess K2Cr2O7 is required to fully oxidise butanal to butanoic acid.
Error 3 (Step 3): Single arrow (→) is used for esterification. Esterification is a reversible equilibrium; the reversible arrow (⇌) is required. Also, “H2SO4” should be specified as “conc. H2SO4 (catalyst)”; yield is <100%.
Corrected pathway (4 marks, 1 per equation+conditions):
Note: to make butyl butanoate from butan-1-ol, the student must split the butan-1-ol batch: some oxidised → butanoic acid; the remainder used as the alcohol for esterification.
Step 1: CH3CH2CH2CH2OH + [O] → CH3CH2CH2CHO + H2O. Conditions: K2Cr2O7/H2SO4 (limited), heat, DISTILLATION. Product: butanal. (Observable: orange → green.) [1 mark]
Step 2: CH3CH2CH2CHO + [O] → CH3CH2CH2COOH. Conditions: K2Cr2O7/H2SO4 (excess), heat, REFLUX. Product: butanoic acid. (Observable: orange → green.) [1 mark]
Step 3: CH3CH2CH2COOH + CH3CH2CH2CH2OH ⇌ CH3CH2CH2COOCH2CH2CH2CH3 + H2O. Conditions: conc. H2SO4 (catalyst), heat under reflux. Arrow: ⇌ (reversible). Product: butyl butanoate. Yield <100%. [1 mark]
Award final mark (1) for including: (a) note that distillation vs reflux is the key distinction between Steps 1 and 2, OR (b) conc. H2SO4 acting as dehydrating agent in esterification to increase yield, OR (c) noting that butan-1-ol must be split between Step 2 (as alcohol for oxidation) and Step 3 (as alcohol for esterification). Band 6 responses will address all three.