Reaction pathways are the highest-mark skill in HSC Chemistry — and the gap between Band 4 and Band 6 is almost entirely about conditions recall and structured annotation. This consolidation lesson gives you the complete reference table, the spot-the-error drill, the Band 6 template, and a 7-mark model answer to build genuine exam fluency.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
A Band 6 response to a 7-mark pathway question typically contains: seven balanced equations, fourteen conditions boxes, seven named intermediates, and correct arrows — all written under exam conditions in under 12 minutes.
Before reading: open your reaction map from L19 and time yourself. How long does it take you to write, from memory, the reagents and conditions for every reaction in Module 7? That time is your current exam readiness.
Wrong: Chemical equations can be balanced by changing subscripts in formulas.
Right: Chemical equations must be balanced by changing coefficients only. Subscripts in chemical formulas define the identity of the compound — changing them creates a different substance. If you cannot balance an equation with whole-number coefficients, check that your formulas are correct.
This is the authoritative reference table. Study it, then cover it and reproduce it from memory using the blank table in Card 4. ★ Priority rows (highlighted) are the most frequently penalised.
| Reaction | Starting material | Reagent | Catalyst | Conditions / temp | Equipment | Arrow | Product(s) |
|---|---|---|---|---|---|---|---|
| Hydrogenation (full) | Alkene or alkyne | H₂ (gas; 2 eq for alkyne) | Ni, Pd, or Pt | ~150–200°C, high pressure | Pressure vessel | → | Alkane |
| Partial hydrogenation | Alkyne | H₂ (gas, 1 eq) | Lindlar (poisoned Pd) | Mild, room temp | Standard glassware | → | Alkene (cis) |
| Halogenation | Alkene or alkyne | Br₂ or Cl₂ | None | Room temperature | Fume cupboard | → | Dihalo (alkene) or tetrahalo (alkyne) |
| Hydrohalogenation | Alkene or alkyne | HCl, HBr, or HI | None | Room temperature | Fume cupboard | → | Monohalo (Markovnikov); geminal dihalo from alkyne |
| ★ Hydration of alkene | Alkene | H₂O (steam) | H₃PO₄ or dil. H₂SO₄ | ~300°C, HIGH PRESSURE (~65 atm) | High-pressure reactor | ⇌ | Alcohol (Markovnikov) |
| ★ Hydration of alkyne | Alkyne | H₂O | dil. H₂SO₄ AND Hg²⁺ (BOTH required) | ~60°C | Heated glassware | → | Ketone (or ethanal from HC≡CH) |
| ★ Halogen subst. (alkane) | Alkane | Cl₂ or Br₂ | UV light (energy source — NOT catalyst) | Room temp, UV light | Transparent glassware | → | Haloalkane + HX (mixture) |
| Dehydration | Alcohol | conc. H₂SO₄ or H₃PO₄ | As above (acid cat.) | ~170–230°C, atmospheric P | Distillation apparatus | → | Alkene + H₂O |
| Haloalkane → alcohol | Haloalkane | NaOH(aq) — AQUEOUS | None | Reflux | Reflux condenser | → | Alcohol + NaX |
| Alcohol → haloalkane | Alcohol | HCl, HBr, or HI | None (ZnCl₂ sometimes) | Reflux | Reflux condenser | → | Haloalkane + H₂O |
| ★ 1° alcohol → aldehyde | Primary alcohol | K₂Cr₂O₇/H₂SO₄ | H₂SO₄ (acidified) | Gentle heat; DISTILLATION | Distillation apparatus | → | Aldehyde + H₂O (orange → green) |
| ★ 1° alcohol → carb. acid | Primary alcohol | K₂Cr₂O₇/H₂SO₄ (excess) | H₂SO₄ | Heat; REFLUX | Reflux condenser | → | Carboxylic acid + H₂O (orange → green) |
| 2° alcohol → ketone | Secondary alcohol | K₂Cr₂O₇/H₂SO₄ | H₂SO₄ | Reflux | Reflux condenser | → | Ketone + H₂O (orange → green) |
| 3° alcohol | Tertiary alcohol | K₂Cr₂O₇/H₂SO₄ | — | Any | — | — | NO REACTION (stays orange) |
| Aldehyde → carb. acid | Aldehyde | K₂Cr₂O₇/H₂SO₄ (excess) | H₂SO₄ | Reflux | Reflux condenser | → | Carboxylic acid (orange → green) |
| ★ Esterification | Carboxylic acid + alcohol | Carboxylic acid + alcohol | conc. H₂SO₄ (catalyst) | Heat, reflux | Reflux condenser | ⇌ | Ester + H₂O (yield <100%) |
| Ester hydrolysis (acid) | Ester + H₂O | H₂O + dil. H₂SO₄ | H₂SO₄ | Reflux | Reflux condenser | ⇌ | Carboxylic acid + alcohol |
| Saponification | Ester (fat/oil) | conc. NaOH(aq) or KOH(aq) | None (NaOH = reagent) | Reflux | Reflux condenser | → | Carboxylate salt + alcohol (glycerol for fat) |
| Amide formation | Carboxylic acid + amine | Amine (R-NH₂) | None (or acid activation) | Heat | Heated flask | → | Amide + H₂O |
| Fermentation | Glucose solution | Glucose (C₆H₁₂O₆) | Yeast (zymase enzyme) | ~35°C, ANAEROBIC | Sealed vessel | → | Ethanol + CO₂ |
By Lesson 20 you have built every individual reaction in Module 7. This card assembles them into a single connected system — every functional group as a node, every transformation as a labelled arrow — so you can see the shortest path between any two points at a glance.
A marker reading 30 HSC responses to the same pathway question sees the same errors repeatedly. This card presents all five, embedded in a realistic flawed pathway — so you recognise and fix them before the exam finds them.
A Band 6 pathway response is not just chemically correct — it is structured, annotated, and gives the marker every component needed to award full marks on the first read.
The difference between 65% and 85% in Module 7 is almost entirely about conditions recall — the chemistry is correct, but missing "high pressure" or "aqueous NaOH" or "distillation" costs marks systematically across multiple questions.
For each reaction below, write the reagent, catalyst, conditions/temperature, equipment, and correct arrow type from memory. Do NOT look at the reference table until you have attempted every row. Then check against the table at the top of this lesson.
| Reaction | Reagent | Catalyst | Conditions / Temp | Equipment | Arrow |
|---|---|---|---|---|---|
| Hydrogenation (full) | |||||
| Partial hydrogenation | |||||
| ★ Hydration of alkene | |||||
| ★ Hydration of alkyne | |||||
| ★ Alkane halogenation | |||||
| Dehydration | |||||
| Haloalkane → alcohol | |||||
| ★ 1° alcohol → aldehyde | |||||
| ★ 1° alcohol → acid | |||||
| ★ Esterification | |||||
| Saponification | |||||
| Fermentation |
★ Priority rows most frequently left incomplete in HSC exams.
Product: ethanoic acid. Observable: K₂Cr₂O₇ orange → green. Excess oxidant + reflux ensures complete oxidation past ethanal to ethanoic acid.
Product: ethyl ethanoate. Arrow: reversible (⇌). Yield ~65–67% at equilibrium. Note: ethanol plays two roles — partially oxidised to ethanoic acid (Step 1), and used directly as the alcohol component for esterification (Step 2). Split the batch.
Product: propan-1-ol (primary alcohol). Explanation: NaOH must be aqueous — aqueous OH⁻ acts as a nucleophile, displacing Cl⁻ and producing the alcohol. Alcoholic NaOH would cause elimination → propene. Reflux is required because the reaction is slow at room temperature; the condenser keeps volatile 1-chloropropane in the flask.
Product: propanoic acid (carboxylic acid). Observable: K₂Cr₂O₇ orange → green. Explanation: Excess dichromate + reflux ensures the propanal intermediate is fully oxidised to propanoic acid. If distillation were used, propanal would be removed before further oxidation — giving propanal, not the target propanoic acid.
Product: methyl propanoate (ester). Arrow: reversible (⇌) — equilibrium yield ~67% for equimolar reactants. Explanation: Conc. H₂SO₄ performs two roles: (1) acid catalyst — H⁺ activates the carboxylic acid for nucleophilic attack by methanol; (2) dehydrating agent — absorbs water produced, shifting equilibrium right (Le Chatelier's Principle) to increase ester yield. Reflux retains volatile methanol in the flask.
Q1. A student reacts butan-2-ol with excess K₂Cr₂O₇/H₂SO₄ under reflux. What is observed, and what is the organic product?
Q2. Which correctly identifies the single error in this equation: "CH₃CH₂COOH + CH₃OH → CH₃CH₂COOCH₃ + H₂O (conditions: H₂SO₄, reflux)"?
Q3. A student wants to synthesise propanal from propan-1-ol. Which conditions exactly achieve this — no more, no less?
Q4. In the Spot the Error pathway (Card 2), which single error would cause the entire synthesis to produce zero yield of target product — not just reduce yield or produce impure product?
Q5. Compound X (formula C₄H₁₀O) gives no colour change with K₂Cr₂O₇/H₂SO₄. Compound Y (formula C₄H₈) decolourises bromine water. Which two-step sequence converts X to Y?
Q6. (4 marks) Outline a two-step synthesis of ethyl ethanoate starting from ethene only. For each step, write the balanced equation, state all conditions, and name the compound produced.
Q7. (5 marks) Starting from 1-bromopropane and ethanol, outline a three-step synthesis of ethyl propanoate. For each step, write the balanced equation, conditions, and name the intermediate. Explain in one sentence why distillation is used in one step and reflux in another during the oxidation sequence.
Q8. (6 marks) A student proposes the pathway: "butan-2-ol → butanone → butanoic acid → butyl butanoate." (a) Identify the error in this pathway and explain why it is not achievable in Module 7. (b) Propose a valid four-step alternative starting from butan-1-ol that achieves the same target product. Write equations and conditions for each step.
At the start of this lesson you timed yourself reproducing all Module 7 reaction conditions from memory. After working through the full reference table, the Spot the Error analysis, the Band 6 template, and the blank table drill — time yourself again on the blank conditions table. How much faster are you now? Any rows that still require more than 5 seconds of recall are your priority targets for the days before the exam.