Edexcel International A Level Chemistry Alcohols Exam Questions

About Edexcel International A Level Chemistry Alcohols

Alcohols in Unit 2 of Edexcel International A Level Chemistry focuses on the reactions of the hydroxyl group and on the practical techniques used to make and purify organic liquids. You classify alcohols as primary, secondary or tertiary and use this to predict their oxidation products with acidified potassium dichromate(VI). You also study reactions with sodium, halogenation using reagents such as PCl5, and dehydration to alkenes. The topic links to Halogenoalkanes, Mass spectra and IR, and later to Carbonyl compounds and Carboxylic acids in Unit 4. Examiners regularly test the colour change of dichromate, the choice between distillation and reflux, and the apparatus used to purify a product.

What you need to know

• Classification Primary alcohols have the OH carbon attached to one other carbon, secondary to two and tertiary to three. This decides the oxidation products.
• Combustion and sodium Alcohols burn to give CO2 and H2O. With sodium they produce hydrogen and a sodium alkoxide, for example 2C2H5OH + 2Na gives 2C2H5ONa + H2.
• Halogenation Alcohols are converted to halogenoalkanes using PCl5 (fumes of HCl form, a test for OH), concentrated HCl for tertiary alcohols, KBr with 50% sulfuric acid, or red phosphorus and iodine.
• Oxidation Acidified potassium dichromate(VI) turns from orange to green. Primary alcohols give aldehydes (distil off immediately) or carboxylic acids (heat under reflux with excess oxidant). Secondary alcohols give ketones. Tertiary alcohols are not oxidised.
• Dehydration Heating an alcohol with a concentrated phosphoric acid catalyst eliminates water to form an alkene, and some alcohols give more than one alkene isomer.
• Practical techniques Reflux prevents volatile reactants and products escaping. Purification uses a separating funnel, washing with sodium hydrogencarbonate solution to remove acid, drying with anhydrous calcium chloride or magnesium sulfate, and distillation, with boiling temperature used to check purity.

Common mistakes in Alcohols

• Writing that dichromate turns green to orange. It changes from orange to green as Cr2O7 2- is reduced to Cr3+.
• Saying a primary alcohol always gives a carboxylic acid. To isolate the aldehyde it must be distilled off as it forms.
• Drawing a reflux condenser sealed at the top. A reflux condenser must be open to avoid pressure build-up.
• Stating tertiary alcohols are oxidised to ketones. They are not oxidised by acidified dichromate because there is no hydrogen on the carbon bonded to OH.
• Using [O] without balancing. Check that oxidising a primary alcohol to an acid uses 2[O] and produces H2O.

Try three Alcohols exam-style questions