AQA A-Level Chemistry Paper 3, June 2022: Question 34

AQA A-Level Chemistry • Organic Chemistry • Multiple Choice

Dehydration of Alcohols to Form Alkenes

What this question tests

This question assesses fundamental recall of organic functional groups and their characteristic elimination reactions (AO1). Specifically, it tests your ability to identify that an alcohol undergoes acid-catalysed dehydration (elimination of water) to yield an alkene.

Question 34

Identifying the precursor for alkene synthesis via dehydration

Total: 1 Mark • Assessment Objective: AO1 (Recall and Understanding)

✅ Correct Answer

C: CH₃CH₂OH (Ethanol)

Reaction: Ethanol is an alcohol. Heating with an acid catalyst eliminates a molecule of water to produce ethene:
CH₃CH₂OH → CH₂=CH₂ + H₂O
Award 1 mark for choosing option C.

💡 Key Knowledge

Dehydration: A specific type of elimination reaction where a water molecule ( H₂O ) is removed from adjacent carbon atoms of an organic molecule.
Reagents & Conditions: Excess concentrated sulfuric acid ( conc. H₂SO₄ ) at ~170 °C or passing vapour over heated aluminium oxide ( Al₂O₃ ) / concentrated phosphoric acid ( conc. H₃PO₄ ).
Mechanism: Acid-catalysed elimination involving protonation of the –OH group followed by loss of water and a proton.

📐 Option-by-Option Breakdown

A: CH₃CHO (Ethanal): An aldehyde. It cannot undergo simple dehydration to yield an alkene.
B: CH₃COOH (Ethanoic acid): A carboxylic acid. Dehydrating it with P₄O₁₀ yields ethanoic anhydride, not an alkene.
C: CH₃CH₂OH (Ethanol): Primary alcohol possessing an adjacent –CH₃ group. Readily eliminates –H and –OH to form ethene ( CH₂=CH₂ ).
D: CH₃COOCH₃ (Methyl ethanoate): An ester. Does not undergo dehydration to form an alkene.

🧠 Exam Technique & Examiner Insight

Decode the Keyword: In AQA Organic Chemistry, "dehydration" almost exclusively refers to alcohols → alkenes.
Speed Strategy: Scan directly for the –OH group connected to a saturated carbon skeleton. Ethanol ( CH₃CH₂OH ) is instantly identifiable as the only alcohol in the list.
Check Carbon Count: Dehydration to an alkene requires at least two carbon atoms with a hydrogen atom on a carbon adjacent to the C–OH carbon. Methanol ( CH₃OH ) cannot form an alkene, but ethanol ( C₂ ) can.

❌ Common Errors & Misconceptions

Confusing Dehydration with Oxidation: Students sometimes confuse dehydrating ethanol (gives ethene) with oxidising ethanol with acidified potassium dichromate (gives ethanal, CH₃CHO , or ethanoic acid, CH₃COOH ).
Misidentifying Carbonyls: Confusing the aldehyde group ( –CHO ) with an alcohol group ( –CH₂OH ) due to hurried reading of molecular formulas.