CCEA A Level Chemistry 2.6 Alcohols Exam Questions

About CCEA A Level Chemistry 2.6 Alcohols

Alcohols are section 2.6 of Unit AS 2. You need to explain their relatively high boiling points and solubility in water in terms of hydrogen bonding, classify them as primary, secondary or tertiary, and prepare them from halogenoalkanes using aqueous alkali. CCEA also covers complete and incomplete combustion and alcohols as alternative fuels, and reactions with sodium, hydrogen bromide and phosphorus pentachloride. Oxidation with acidified potassium dichromate is the key reaction: primary alcohols give aldehydes then carboxylic acids, secondary alcohols give ketones, and tertiary alcohols resist oxidation. This links to IR spectroscopy (2.7) and to aldehydes, ketones and carboxylic acids in A2 1.

What you need to know

• Physical properties The O-H group allows hydrogen bonding, so alcohols have higher boiling points than alkanes of similar Mr. Short chain alcohols are soluble in water because they hydrogen bond with water; solubility falls as the non-polar chain gets longer.
• Classification Primary alcohols have the OH on a carbon bonded to one other carbon (for example ethanol), secondary to two (propan-2-ol) and tertiary to three (2-methylpropan-2-ol).
• Combustion and fuels Alcohols burn cleanly to CO2 and H2O, for example C2H5OH + 3O2 gives 2CO2 + 3H2O. Ethanol from fermentation is a renewable alternative fuel.
• Reactions With sodium: 2C2H5OH + 2Na gives 2C2H5ONa + H2 (effervescence). With HBr (from NaBr and concentrated H2SO4): a bromoalkane forms. With PCl5: C2H5OH + PCl5 gives C2H5Cl + POCl3 + HCl, with steamy fumes, used as a test for OH.
• Oxidation Acidified potassium dichromate changes from orange to green. Primary alcohols give an aldehyde, collected by distillation as it forms, or a carboxylic acid on heating under reflux with excess oxidant. Secondary alcohols give ketones. Tertiary alcohols are not oxidised, so the solution stays orange. Oxidant is written as [O].

Common mistakes in 2.6 Alcohols

• Saying dichromate turns from orange to blue. It turns green as Cr2O7 2- is reduced to Cr3+.
• Mixing up reflux and distillation. Distil immediately to collect the aldehyde; reflux to oxidise fully to the carboxylic acid.
• Saying alcohols are acidic enough to turn litmus red. They react with sodium but are not acidic in water in the way carboxylic acids are.
• Forgetting to use [O] in oxidation equations. Write, for example, CH3CH2OH + [O] gives CH3CHO + H2O.
• Calling propan-2-ol a primary alcohol because it has three carbons. Classification depends on the carbon bearing the OH.

Try three 2.6 Alcohols exam-style questions