AS Chemistry: Alcohols — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Alcohols revision notes, covering classification, oxidation, dehydration and the distinguishing tests for Cambridge AS & A Level Chemistry 9701.

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Questions

1. Classify propan-1-ol, propan-2-ol and 2-methylpropan-2-ol as primary, secondary or tertiary, explaining the basis of the classification. [4]

2. Explain why ethanol has a much higher boiling point than propane, despite similar relative molecular masses. [3]

3. Propan-1-ol is oxidised with acidified potassium dichromate(VI).

(a) State the product and conditions if you distil. [2] (b) State the product and conditions if you reflux. [2] (c) State the colour change observed. [1] (d) State what happens if 2-methylpropan-2-ol is treated the same way, and explain. [2]

4. Ethanol can be dehydrated to ethene.

(a) State the reagent and conditions. [2] (b) Name the type of reaction. [1]

5. Describe the iodoform (tri-iodomethane) test.

(a) State the reagents. [1] (b) State the positive observation. [1] (c) State which structural feature gives a positive result. [2]

6. (Fermentation is prior IGCSE knowledge, not itself an AS 9701 syllabus point — 16.1.1 lists six AS production routes for alcohols and fermentation is not one of them. Included here for context and comparison only; no AS marks are awarded for fermentation recall alone, only for the hydration-of-ethene side.) Compare the two industrial routes to ethanol — hydration of ethene and fermentation — on rate, purity and sustainability. [6]

7. Ethanol is added separately to (i) a small piece of sodium and (ii) phosphorus(V) chloride (PCl₅).

(a) State the observation in each case. [2] (b) Write an equation for the reaction with sodium. [1] (c) Explain why the reaction with PCl₅ is used as a test for the -OH group. [1]

8. Ethanol reacts with ethanoic acid in the presence of a small amount of concentrated sulfuric acid.

(a) Name the type of reaction and the organic product. [2] (b) State the role of the concentrated sulfuric acid. [1]

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Answers

1. Primary — the carbon bearing the OH is attached to one other carbon: propan-1-ol [1]. Secondary — attached to two: propan-2-ol [1]. Tertiary — attached to three: 2-methylpropan-2-ol [1]. The classification depends on the number of alkyl groups on the carbon carrying the OH group [1].

2. Ethanol molecules form hydrogen bonds with each other [1] because O–H is present [1]; propane has only weak induced dipole–induced dipole forces, so far less energy is needed to separate its molecules [1].

3. (a) Propanal; K₂Cr₂O₇/H₂SO₄, warm and distil off the product as it forms [1] [1]. (b) Propanoic acid; K₂Cr₂O₇/H₂SO₄, heat under reflux [1] [1]. (c) Orange to green [1]. (d) No reaction / no colour change [1], because a tertiary alcohol has no hydrogen on the carbon bearing the OH group, so it cannot be oxidised without breaking a C–C bond [1].

4. (a) Either concentrated sulfuric acid, heated to about 170 °C (elimination in the liquid phase) [1], or alcohol vapour passed over a heated aluminium oxide (Al₂O₃) catalyst at about 300–400 °C [1]. (b) Elimination (dehydration) [1].

5. (a) Iodine and sodium hydroxide (alkaline aqueous iodine) [1]. (b) A pale yellow precipitate with an antiseptic smell [1]. (c) The presence of a CH₃CH(OH)– group [1] or a CH₃CO– group [1].

6. (Background context; marks apply to the hydration-of-ethene content, which is the AS syllabus route.) Rate: hydration of ethene is fast and continuous; fermentation is slow and batch [1] [1]. Purity: hydration gives a pure product; fermentation gives a dilute aqueous mixture requiring fractional distillation [1] [1]. Sustainability: hydration uses ethene from crude oil, which is finite and non-renewable [1]; fermentation uses renewable plant material and is close to carbon neutral, but competes with food production for land [1].

7. (a) With sodium: effervescence (bubbles of gas) as the sodium reacts and gradually disappears [1]. With PCl₅: misty/steamy white fumes (of HCl) are produced [1]. (b) 2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂ [1]. (c) PCl₅ substitutes the hydroxyl group, replacing –OH with –Cl to give a chloroalkane [1], and HCl gas is released as a by-product, which fumes visibly (misty/steamy white fumes) on contact with moist air — so the appearance of these fumes shows that an –OH group was present and has reacted.

8. (a) Esterification (condensation) [1]; the product is ethyl ethanoate [1]. (b) It acts as a catalyst for the reaction [1].

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Where marks are usually lost

• Confusing distil with reflux, so the aldehyde over-oxidises.
• Saying tertiary alcohols “react slowly” — they do not react at all under these conditions.
• Giving only one product for the iodoform-positive structural feature.
• Evaluating the ethanol routes on one dimension only.
• Forgetting to balance the sodium/alcohol equation — two moles of alcohol react with two moles of sodium to produce one mole of hydrogen gas.
• Describing the PCl₅ test result as “bubbles” rather than the distinctive misty/steamy white fumes of HCl gas.
• Naming the ester product the wrong way round, or forgetting that concentrated sulfuric acid here is acting purely as a catalyst rather than being consumed in the reaction.

The full set of alcohol reactions, in one place

Beyond oxidation, an alcohol’s -OH group takes part in several other reaction types worth holding together as a set: with sodium, it behaves as a weakly acidic O-H group, releasing hydrogen gas and forming a sodium alkoxide; with concentrated sulfuric or phosphoric acid at about 170°C, it undergoes elimination (dehydration) to form an alkene; with a carboxylic acid and an acid catalyst, it undergoes condensation (esterification) to form an ester; and with PCl₅, it substitutes the -OH for chlorine, producing the characteristic misty fumes used as a simple confirmatory test for the -OH group in an unknown compound. For the full reaction scheme and oxidation-product table by alcohol class, see the Alcohols revision notes.