Chemistry • Year 12 • Module 7 • Lesson 12
Reactions of Alcohols
Synthesise, evaluate, and critique complex claims about alcohol reactions at Band 5–6 level. This worksheet contains two extended-response tasks.
1. Designing an alcohol identification scheme — data and multi-criteria evaluation
A forensic chemist receives five colourless liquid samples labelled P–T. Each sample is one of the following: ethanol, propan-2-ol, 2-methylpropan-2-ol (tert-butanol), propanal, or propanone. The chemist performs three tests. Results are in the table below. 8 marks
| Sample | Test 1: K2Cr2O7/H+, distillation | Test 2: K2Cr2O7/H+, reflux | Test 3: dehydration attempt (conc. H2SO4, 170°C) |
|---|---|---|---|
| P | Orange → green; fruity distillate | Orange → green; acidic product | Gas produced; decolourises Br2/CCl4 |
| Q | Orange → green; no separate distillate collected | Orange → green; acidic product | No alkene detected |
| R | Orange → green; no separate distillate | Orange → green; neutral product | Gas produced; decolourises Br2/CCl4 |
| S | Stays orange | Stays orange | Gas produced; decolourises Br2/CCl4 |
| T | Stays orange | Stays orange | No alkene detected |
Note: propanal and propanone do not contain an –OH group and would not undergo dehydration under these conditions. “No alkene detected” means the gas produced did not decolourise Br2/CCl4.
Using the experimental data in the table, identify each sample P–T as one of the five compounds listed. For each identification, state the specific piece of evidence and the chemical reasoning that supports your conclusion. Your response must:
- correctly identify all five samples;
- use Test 1 and Test 2 data together to distinguish primary, secondary, and tertiary alcohols;
- explain why propanal and propanone give specific results in Tests 1 and 2;
- explain why 2-methylpropan-2-ol (tert-butanol) gives no alkene in Test 3 despite being an alcohol;
- reach a conclusion supported by the data for each sample.
2. Source critique — student study guide extract
Read the extract below from a student-produced study guide, then answer the question. 7 marks
Student Notes — Module 7 Revision (contains errors)
“Alcohols can be oxidised by K2Cr2O7/H2SO4. All three types of alcohol (primary, secondary, and tertiary) undergo this reaction because they all contain an –OH group. The colour change from orange to green always occurs. Primary alcohols always give carboxylic acids — you never get an aldehyde as a product because the reaction just goes all the way through. Breathalysers used by Australian police rely on this same reaction: ethanol in your breath is oxidised to ethanoic acid by K2Cr2O7, and you are charged if the solution turns green. The HX reactivity order is HCl > HBr > HI because chlorine is the most reactive halogen.”
The extract contains four distinct scientific errors. For each error:
- identify the incorrect claim;
- state the scientifically correct version;
- explain the chemistry that shows why the original claim is wrong.
For the breathalyser error, also explain what a real modern breathalyser measures and why the student’s description is an oversimplification, even if the chemistry direction is broadly correct.
Q1 — Identification scheme (8 marks)
Sample identifications:
- P = ethanol (1° alcohol): Test 1 orange→green + fruity distillate = aldehyde (ethanal) formed and removed by distillation. Test 2 orange→green + acidic product = ethanoic acid formed under reflux (excess oxidant). Test 3 gives ethene (decolourises Br2/CCl4).
- Q = propanal: Tests 1 and 2 both show orange→green because propanal (an aldehyde) IS oxidised by acidified dichromate to propanoic acid; the solution turns green in both test setups. No separate distillate is collected in Test 1 because propanal is not being generated from an alcohol — it is itself the starting material. Test 2 gives an acidic product (propanoic acid). Test 3 no alkene — propanal has no –OH group to undergo dehydration.
- R = propan-2-ol (2° alcohol): Test 1 orange→green but no separate distillate — propanone (ketone product) has higher BP than ethanal and is not collected separately. Test 2 orange→green; neutral product = propanone (ketone, neutral pH). Test 3 produces propene (decolourises Br2/CCl4).
- S = 2-methylpropan-2-ol (tert-butanol): Tests 1 and 2 stay orange (tertiary — no H on C–OH, cannot be oxidised). Test 3 does produce an alkene (2-methylpropene decolourises Br2/CCl4). Dehydration does not require oxidation so tertiary alcohols can still dehydrate.
- T = propanone: Tests 1 and 2 stay orange (propanone is already a ketone; no further oxidation). Test 3 no alkene — propanone has no –OH group.
Marking criteria:
- 1 mark per correctly identified sample supported by at least one piece of evidence (5 marks)
- 1 mark for explaining that propanal IS oxidised by dichromate (turns green, aldehyde → carboxylic acid), whereas propanone (a ketone) stays orange because ketones cannot be further oxidised under these conditions — no H on the carbonyl carbon
- 1 mark for explaining that primary alcohol gives aldehyde on distillation (lower BP, removed before further oxidation) vs carboxylic acid on reflux
- 1 mark for explaining why tert-butanol can dehydrate (dehydration is elimination, not oxidation; no H on C–OH only blocks oxidation, not dehydration)
Q2 — Source critique marking criteria (7 marks)
Error 1 — “All three types of alcohol undergo oxidation”: Incorrect. Tertiary alcohols do NOT undergo oxidation with K2Cr2O7/H+; the solution stays orange. The C–OH carbon in a tertiary alcohol bears no H atoms, so the C–H bond required for removal during oxidation is absent. (2 marks: 1 for identifying the error and giving correct rule; 1 for structural explanation.)
Error 2 — “Primary alcohols always give carboxylic acids, never aldehydes”: Incorrect. Whether a primary alcohol gives an aldehyde or a carboxylic acid depends on the equipment used. Distillation collects the aldehyde as it forms (before further oxidation), while reflux allows excess oxidant to convert the aldehyde all the way to a carboxylic acid. An aldehyde is always the intermediate product. (2 marks: 1 for stating distillation gives aldehyde; 1 for explaining the mechanism — aldehyde removed before further oxidation.)
Error 3 — Breathalyser charge criterion: The student claims that a simple green colour change is sufficient to charge a driver. This is incorrect. A qualitative colour change confirms only that ethanol is present (the dichromate was reduced). However, drivers are charged based on a quantitative measurement of ethanol concentration against the legal limit (e.g. 0.05 mg per 100 mL of breath for full-licence holders in Australia). The intensity of the colour change (absorbance decrease) is calibrated against known ethanol concentrations — the test is quantitative, not a binary pass/fail colour indicator. The product is ethanoic acid (correct), but the charge criterion requires a measured concentration to exceed the legal threshold, not merely a colour change. (1 mark for identifying that the test must be quantitative not binary colour; 1 mark for referencing the legal concentration limit as the basis for charging.)
Error 4 — HX reactivity order “HCl > HBr > HI”: Incorrect. The correct order is HI > HBr > HCl. Although Cl is the most electronegative and reactive halogen in many contexts, in substitution of alcohols the rate depends on bond strength in the product: C–I is weaker than C–Br or C–Cl, making the iodoalkane easier to form. Iodide is also a better nucleophile. The student has confused halogen reactivity in addition/oxidation reactions with halide reactivity in nucleophilic substitution. (2 marks: 1 for correct order; 1 for chemical reasoning distinguishing halogen vs halide reactivity.)