Quick answer: Primary alcohols are oxidised to aldehydes and then carboxylic acids, secondary alcohols to ketones, and tertiary alcohols resist oxidation; acidified dichromate turns from orange to green. This Chemistry 11-12 lab on oxidation of alcohols gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Can a few test-tube reactions identify five clear, colourless liquids? You will oxidise alcohols with dichromate, then test the carbonyl products with 2,4-DNPH and Tollens’ reagent. The pattern of results shows which liquids are primary, secondary or tertiary alcohols and which are aldehydes or ketones.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.3.5
Aim
To identify five unlabelled liquids (propan-1-ol, propan-2-ol, 2-methylpropan-2-ol, propanal and propanone) using acidified dichromate, 2,4-DNPH and Tollens’ reagent, and to test the products of oxidation.
Equipment
- five liquids labelled A to E (1 cm³ each)
- acidified potassium dichromate(VI), 0.1 mol/dm³
- 2,4-DNPH solution; freshly prepared Tollens’ reagent
- test tubes and rack, beaker for a hot-water bath at 60 °C, dropping pipettes, thermometer
- small distillation set-up for 5 cm³ (teacher-assisted)
Safety
Wear goggles and gloves. Dichromate is toxic and a suspected carcinogen; keep it off the skin. 2,4-DNPH is irritant and flammable when dry. Make Tollens’ reagent fresh and wash it away with dilute nitric acid immediately, because stored mixtures can become explosive. Propanone and alcohols are flammable: use a hot-water bath, never a naked flame.
Method
- Place 1 cm³ of dichromate in five tubes, add 5 drops of a liquid to each and warm in the 60 °C bath for 3 minutes. Note the colour.
- Add 3 drops of each liquid to 2 cm³ of 2,4-DNPH in a clean tube and record any precipitate.
- Add 3 drops of each liquid to fresh Tollens’ reagent and warm for 5 minutes; record any mirror.
- For each alcohol that turned the dichromate green, repeat the oxidation on 5 cm³ and distil off a few drops of product.
- Test each distillate with 2,4-DNPH and with Tollens’ reagent.
Results
Sample observations:
| Liquid | Dichromate (warm) | 2,4-DNPH | Tollens’ | Distillate: DNPH / Tollens’ |
|---|---|---|---|---|
| A | green | no precipitate | no mirror | precipitate / mirror |
| B | green | no precipitate | no mirror | precipitate / no mirror |
| C | stays orange | no precipitate | no mirror | not tested |
| D | green | orange precipitate | silver mirror | not tested |
| E | stays orange | orange precipitate | no mirror | not tested |
Analysis questions
Question 1 Identify the liquids
Use the table to identify A to E.Question 2 Separating A and B
Explain how the distillate tests separate A from B.Question 3 Why C stays orange
Explain the result for C.Question 4 D and E with dichromate
Explain why D turns the dichromate green but E does not.Question 5 Equation
Write an equation using [O] for the reaction of A, then name the final acid if refluxed.Question 6 Silver from propanal
A student reacts 0.58 g of propanal (M = 58.0 g/mol) with excess Tollens’ reagent. Calculate the mass of silver formed (2 Ag per molecule, Ag = 107.9 g/mol).Question 7 Safety and improvement
Explain why Tollens’ reagent is never stored and suggest one improvement to the procedure.
