Part 7: Alcohol reactions and Core Practical 5

Use alcohol classification to predict oxidation, choose distillation or reflux for the intended product and connect CP5 observations to evidence for ethanoic acid.

The OH-bearing carbon determines the class

Alcohols contain an OH group bonded to a saturated carbon. Count the carbon groups attached to that carbon: one gives a primary alcohol, two a secondary and three a tertiary. Butan-1-ol is primary, butan-2-ol secondary and 2-methylpropan-2-ol tertiary. A molecule with three OH groups is a triol, not necessarily a tertiary alcohol.

A primary alcohol has the local pattern RCH₂OH; secondary is R₂CHOH; tertiary is R₃COH. This classification predicts oxidation because forming C=O while retaining the carbon skeleton requires removal of H from both O–H and the OH-bearing carbon. A tertiary alcohol has no H on that carbon. Methanol is a special zero-carbon-neighbour example that follows the primary oxidation pattern.

Alcohol molecules can hydrogen-bond to one another and to water. For similar size, alcohols generally boil at higher temperatures than alkanes because more energy is needed to separate their molecules. Longer hydrocarbon chains reduce water solubility because the growing non-polar part does not form favourable hydrogen bonds with water.

Four families of alcohol reaction

PCl₅ with an alcohol gives steamy/misty HCl fumes in moist air. Water also reacts with PCl₅, so it is not a uniquely selective alcohol test in a wet unknown. With bromide, the stated acid conditions generate HBr while limiting unwanted oxidation. Red phosphorus and iodine provide PI₃ for iodination; replacing this with iodide and concentrated sulfuric acid is unsuitable because iodide/HI can be oxidised.

Combustion and oxidation with dichromate both oxidise the alcohol, but have very different products and purposes. The specific alcohol mechanisms in 6.38 are not required; select the reagent, conditions and product accurately.

Primary, secondary and tertiary alcohols behave differently

A primary alcohol first gives an aldehyde. Limited oxidant and gentle warming with immediate distillation remove the volatile aldehyde from the oxidising mixture and reduce further oxidation. Excess acidified dichromate and reflux retain the organic material in contact with oxidant, producing a carboxylic acid. Merely writing “heat” leaves the selectivity unresolved.

A secondary alcohol gives a ketone under oxidation with acidified dichromate, often with reflux. The ketone is not readily further oxidised under these usual conditions. A tertiary alcohol does not undergo the corresponding oxidation without breaking a C–C bond, so it gives no normal orange-to-green test change. Avoid the absolute claim that tertiary alcohols can never be oxidised by any conditions.

[O] represents one oxygen-equivalent supplied by the oxidising agent; it does not mean a free oxygen atom was tipped into the flask. Note that aldehyde-to-acid oxidation adds oxygen without the extra water formed in alcohol-to-aldehyde oxidation. Check the hydrogens instead of adding H₂O automatically to every oxidation equation.

Test for an aldehyde using a chemical change

Warm a small separate sample with Benedict’s or Fehling’s solution in a water bath. A simple aliphatic aldehyde reduces the blue Cu(II) reagent to a red/orange or brick-red precipitate of Cu₂O; the aldehyde is oxidised to a carboxylate in the alkaline reagent. A ketone or the original simple alcohol does not give that positive test.

State reagent, warming and precipitate. “Turns red” does not distinguish a red solution from a solid. In Pearson 8CH0/02 June 2023 Q7(c)(iii), omitted warming and omitted precipitate were common issues. That question concerned butanal versus butan-1-ol; the lesson applies the underlying redox chemistry to ethanal as a new example.

An orange-to-green dichromate result alone cannot prove that an unknown was an alcohol: an aldehyde also reduces dichromate. Combine the response with other tests and infrared evidence. A negative Benedict’s/Fehling’s test after complete ethanol oxidation is consistent with an acid product rather than retained ethanal.

Core Practical 5: oxidise ethanol, reflux, then distil

The official Pearson CP5 worksheet prepares an ethanoic-acid-containing distillate by complete ethanol oxidation. It uses acidified sodium dichromate, whereas specification reaction recall names potassium dichromate(VI) in dilute sulfuric acid; both supply dichromate(VI) ions in acidic solution. This worksheet is not a method for collecting ethanal as the main product.

Assemble a small flask with anti-bumping granules and a vertical reflux condenser. The method cools the oxidant in an ice bath and adds ethanol slowly, allowing the exothermic reaction to subside after each addition. Controlled addition prevents an excessive temperature rise and loss of volatile ethanol. After addition, warm as directed and heat under reflux; the Pearson method uses a boiling water bath for 20 minutes so oxidation can proceed further.

Reflux condenses vapour and returns it to the reaction flask, allowing prolonged heating with reduced loss of volatile material. Cooling water enters the lower condenser connection and leaves the upper one, keeping the jacket full. The top must remain open to the atmosphere. Add anti-bumping granules before heating to provide nucleation sites for smooth boiling; adding them to an already hot liquid can cause violent boiling.

Allow the apparatus to cool before rearranging for distillation. Heat with an appropriate controlled source and collect the directed colourless distillate. Distillation separates volatile material from the dichromate/chromium-containing mixture; the distillate contains water and ethanoic acid and must not automatically be treated as pure ethanoic acid. Use a fume cupboard as directed, eye protection and suitable gloves because dichromate(VI) is carcinogenic/oxidising and the acid corrosive. Keep flammable ethanol away from ignition sources and collect chromium waste correctly.

Test the distillate and interpret a pattern

On separate portions, an acidic pH and effervescence with carbonate support an acidic product. Confirm evolved CO₂ using limewater, which becomes cloudy. No further dichromate colour change and no red precipitate with warmed Fehling’s solution support extensive oxidation rather than unchanged ethanol or accumulated ethanal.

For a clean ethanol/ethanal/ethanoic-acid system these observations fit ethanoic acid, but acidity alone does not identify it uniquely: transferred sulfuric acid would also be acidic. Prevent splashing during distillation and combine the acid test with the organic reaction history and, if available, spectroscopy. Use a separate portion for each test to avoid one reagent changing the result of the next.

Worked stoichiometry and the meaning of a yield

If 1.50 g ethanol is the limiting reactant, n = 1.50/46.0 = 0.0326087 mol. Full oxidation forms one mole ethanoic acid per mole ethanol, so theoretical pure acid mass = 0.0326087 × 60.0 = 1.96 g to three significant figures. The mass of an aqueous distillate is not the mass of pure acid: multiplying the entire solution mass into a yield would overestimate it.

If analysis instead finds 0.0250 mol ethanoic acid in the collected material, molar yield = 0.0250/0.0326087 × 100 = 76.7%. Loss during transfer or incomplete collection reduces isolated yield; water contamination can inflate a mass-based apparent yield. Target an improvement at the actual cause rather than claiming all errors reduce the answer.

Alcohols can supply alkenes without starting from crude oil

Concentrated phosphoric acid and heat remove water from an alcohol. Ethanol forms ethene; butan-2-ol can give but-1-ene and but-2-ene, with E/Z possibilities for the latter. The OH-bearing carbon and a neighbouring carbon must form the double bond, so check where a neighbouring H is available.

Dehydrating bioethanol provides a potential renewable ethene feedstock for polymer manufacture. This links reaction chemistry to life-cycle analysis: the feedstock can be renewable while the resulting poly(ethene) remains persistent. In a route, hydration and dehydration are opposite functional-group changes, but the selected conditions determine which process is useful.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official Edexcel mark allocations.

Sources