PAG 5 Oxidation of an Alcohol: Distil or Reflux

PAG 5 asks you to oxidise a primary alcohol, ethanol, with acidified potassium dichromate(VI) and to steer the reaction to one of two products: the aldehyde, ethanal, or the carboxylic acid, ethanoic acid.

The chemistry is the same in both cases. What changes is the technique: distil the product off as it forms and you stop at the aldehyde; heat under reflux with an excess of oxidising agent and you go all the way to the acid.

The oxidising agent, orange dichromate(VI) ions, is reduced to green chromium(III) ions, so the colour change tells you the oxidation is happening.

Practical skills are assessed in the written papers (practical skills are examined in every paper) and the practical endorsement.

For this practical that means drawing and labelling reflux and distillation apparatus, and explaining every choice (why the water enters at the bottom, why the thermometer bulb sits level with the side arm, why the receiver is cooled, why there must be no naked flame).

It also means writing the equations with [O], reading a thermometer against the boiling points, and identifying the product from a pattern of test results.

The related organic preparation is PAG 5 (chlorination of 2-methylpropan-2-ol), and the test-tube reactions reappear in PAG 4 and PAG 7 (analysis of inorganic and organic unknowns).

The hazards here are real and the mark scheme expects you to name them.

Potassium dichromate(VI) is toxic, an oxidiser and a category 1B carcinogen: weigh it in a fume cupboard or with the sash down, avoid raising dust, wear gloves and goggles.

Concentrated sulfuric acid, from which the acidified solution is made, is corrosive; the dilute acid used in the flask is still an irritant.

Ethanol and ethanal are highly flammable, and ethanal boils at only 21 °C, so its vapour spreads quickly: heat with a water bath or an electric heating mantle, never a naked flame.

Ethanal is also an irritant, and ethanoic acid is corrosive with a sharp smell.

The aldehyde route uses the quantities below so that each drop of alcohol meets oxidant, reacts once and leaves.

The alcohol is diluted with water before it is added: this moderates the exothermic reaction and keeps the concentration of aldehyde near the oxidant low, so less of it is oxidised a second time.

Distillation set-up for ethanal: the alcohol is dripped onto the oxidant, the thermometer bulb sits level with the side arm and reads about 21 °C, and the open receiver stands in iced water.

The distillate is mostly ethanal but also contains a little water and ethanol. Left standing in air it slowly oxidises, so a faint acid test after a few days is not a fault in the method. It smells sweet and slightly sharp.

Drawing marks are cheap to lose. The apparatus is judged against a short checklist, and every item has a reason that can be asked as a separate question.

Reflux

• Condenser vertical, fitted directly into the flask, with no line drawn across the joint (a line reads as a bung).
• Top of the condenser open. Never stopper it: a sealed apparatus builds pressure and would burst.
• Water in at the lower side arm and out at the upper: the jacket fills completely, with no air gaps, and the coldest water meets the vapour at the exit, a counter-current flow that gives the most efficient cooling.
• Anti-bumping granules added before heating begins, never to a hot liquid: they provide nucleation sites so many small bubbles form instead of violent bumping.
• Heating mantle or water bath drawn beneath the flask; no Bunsen burner under a flammable liquid.

Distillation

• Thermometer bulb level with the side arm (the opening to the condenser), so it reads the temperature of the vapour that is being condensed, about 21 °C when ethanal is collected. A bulb dipped in the liquid reads the mixture, not the product.
• Condenser sloping downwards towards the receiver, water in at the lower end.
• Receiver open, standing in iced water. The apparatus is never sealed at the receiving end either.
• Reasonable proportions: the flask no more than half full, the condenser long enough to condense all the vapour.

The quantities in the methods are not arbitrary. Each dichromate(VI) ion supplies three [O], so you can check which reagent limits the reaction.

Moles of potassium dichromate(VI) in method A = 3.00 g ÷ 294.2 g mol⁻¹ = 0.0102 mol, giving 3 × 0.0102 = 0.0306 mol [O].

Mass of 1.5 cm³ of ethanol = 1.5 cm³ × 0.79 g cm⁻³ = 1.19 g; moles = 1.19 g ÷ 46.1 g mol⁻¹ = 0.0257 mol.

Oxidation to ethanal needs 0.0257 mol [O], so the oxidant is in slight excess (0.0306 mol available) and every drop of alcohol meets enough dichromate(VI) to react.

Oxidation to ethanoic acid would need 2 × 0.0257 = 0.0514 mol [O], which is why method B doubles the dichromate(VI) to 6.0 g (0.0612 mol [O]): a clear excess that drives the reaction to completion.

Percentage yield questions follow the same route. If method B gave 0.90 g of ethanoic acid after purification, the theoretical mass is 0.0257 mol × 60.1 g mol⁻¹ = 1.54 g, so the yield is 0.90 ÷ 1.54 × 100 = 58 % (2 significant figures). Losses come from the aqueous distillate, evaporation of the volatile intermediate and transfers between vessels.

The product is a few cubic centimetres of liquid, so small losses matter, and the apparatus errors change which product you get rather than just how much.

Percentage uncertainty in the measurements, using the actual uncertainty of each piece of apparatus:

• Mass of potassium dichromate(VI), 3.00 g on a 2 d.p. balance: two readings, ±0.005 g each, so ±0.01 g; percentage uncertainty = (0.01 ÷ 3.00) × 100 = 0.33 %.
• Volume of ethanol, 1.5 cm³ in a 10 cm³ measuring cylinder, ±0.1 cm³: (0.1 ÷ 1.5) × 100 = 6.7 %. This is the dominant uncertainty; a 2 cm³ graduated pipette (±0.02 cm³) cuts it to (0.02 ÷ 1.5) × 100 = 1.3 %.
• Thermometer reading of 21 °C, ±0.5 °C: (0.5 ÷ 21) × 100 = 2.4 %.
• Volume of distillate, 2.0 cm³ in the same measuring cylinder: (0.1 ÷ 2.0) × 100 = 5.0 %.

Adding them gives an overall measurement uncertainty of about 12 %, yet real yields are often only 40 to 60 %. The difference is not measurement error: it is loss of volatile product, incomplete reaction and the aqueous distillate. Say so when a question asks you to compare the two.

• Writing “the mixture turns reddish-brown” for Fehling’s solution: the answer is a brick-red precipitate of copper(I) oxide.
• Drawing the reflux condenser with a bung in the top, or with the water going in at the top.
• Putting the thermometer bulb in the liquid, or drawing a line across the joint between flask and condenser.
• Explaining the choice of distillation without a single boiling point. Quote 21 °C and 78 °C.
• Using one [O] for the acid, or leaving out H₂O in the aldehyde equation.
• “Dichromate turns green” without naming the species or the oxidation numbers (+6 to +3).
• Calling the acid distillate “pure ethanoic acid” when it is an aqueous solution that co-distilled with water.
• Saying “propan-2-ol would give an acid on longer heating”: a secondary alcohol stops at the ketone, propanone.
• Suggesting a Bunsen burner for gentle heating of a highly flammable liquid.

Other specifications run this practical with propan-1-ol instead of ethanol, and exam questions may use either. Nothing in the method changes except the numbers.

Propan-1-ol (b.p. 97 °C) is oxidised to propanal (b.p. 49 °C) by distillation and to propanoic acid (b.p. 141 °C) by reflux: CH₃CH₂CH₂OH(l) + [O] → CH₃CH₂CHO(l) + H₂O(l) and CH₃CH₂CH₂OH(l) + 2[O] → CH₃CH₂COOH(l) + H₂O(l).

Propanal boils at 49 °C, so it is easier to collect cleanly than ethanal, and the thermometer reads about 49 °C while it is collected.

The tests are identical: the aldehyde gives a silver mirror and a brick-red precipitate, the acid effervesces with magnesium and with sodium hydrogencarbonate. If a question uses the other alcohol, swap the names, formulae and boiling points and keep every reason the same.

Say

“Ethanal (b.p. 21 °C) is more volatile than ethanol (b.p. 78 °C), so it distils off as soon as it forms and cannot be oxidised further.”

“Under reflux the condensed aldehyde returns to the excess acidified dichromate(VI) and is oxidised to ethanoic acid.”

“Brick-red precipitate with Fehling’s solution; silver mirror with Tollens’ reagent.”

Do not say

“Reddish-brown solution” for Fehling’s solution. “Reflux stops the smell.” “The thermometer measures the temperature of the liquid.” “Concentrated sulfuric acid is used as the oxidising agent” (it acidifies; dichromate(VI) oxidises).

Watch for

Diagrams: open condenser top, water in at the bottom, bulb level with the side arm, no line across the joint, granules drawn, heating mantle not Bunsen.

Equations: [O] count (one for the aldehyde, two for the acid), H₂O in the first step, state symbols.

Product identity: always argue from a pattern of tests.