Topic 10 · page 24 of 27

Core Practical 7: propan-1-ol to propanal and propanoic acid

Part A distils propanal out of the oxidation mixture as it forms. Part B heats the alcohol under reflux with excess oxidising agent to reach propanoic acid. The tests and the boiling temperatures show what was made.

14 min read10.2010.1810.19

Read first: Telling the products apart, Distil to stop, reflux to go on

Aim and reactions 10.20

Core Practical 7 is the oxidation of propan-1-ol to produce propanal and propanoic acid. Both come from the same alcohol and the same oxidising agent, acidified potassium dichromate(VI). Only the apparatus and the amount of oxidising agent change.

CHX3CHX2CHX2OH+[O]→CHX3CHX2CHO+HX2O \ce{CH3CH2CH2OH + [O] -> CH3CH2CHO + H2O}
CHX3CHX2CHX2OH+2 [O]→CHX3CHX2COOH+HX2O \ce{CH3CH2CH2OH + 2[O] -> CH3CH2COOH + H2O}
Two flow charts side by side. Part A: propan-1-ol and anti-bumping granules in a flask set up for distillation, acidified dichromate added drop by drop from a dropping funnel while warming, propanal distils at about 49 degrees Celsius into a receiver in ice, then test with Benedict's solution for a brick-red precipitate. Part B: propan-1-ol with excess acidified dichromate and granules heated under reflux for about 20 minutes, then re-arranged for distillation and the liquid boiling near 141 degrees Celsius collected, then test with sodium carbonate for fizzing.
The two parts of Core Practical 7. Part A stops at the aldehyde. Part B goes on to the acid.

Part A: stop at propanal 10.2010.1810.19

PracticalMethod (distillation)

  1. Put propan-1-ol and anti-bumping granules in a round-bottomed flask set up for distillation. Place the receiving flask in a beaker of ice.
  2. Add acidified potassium dichromate(VI) solution drop by drop from a dropping funnel while warming the flask gently.
  3. Propanal distils off as it forms. Collect it, keeping the thermometer near 49 °C.
  4. Test a little of the distillate with Benedict's or Fehling's solution, warmed. Blue to brick-red precipitate shows an aldehyde.

The thermometer stays near 49 °C because only propanal is boiling. The alcohol (97 °C) stays in the flask. The oxidising agent is added gradually so that it is never in excess, and the receiver is in ice because propanal is volatile and flammable.

Part B: on to propanoic acid 10.2010.18

PracticalMethod (reflux, then distil)

  1. Put propan-1-ol, excess acidified dichromate(VI) and anti-bumping granules in the flask. Fit a vertical condenser and heat under reflux for about 20 minutes.
  2. Re-arrange the apparatus for distillation. Collect the liquid that boils near 141 °C.
  3. Test a little of the product with sodium carbonate solution. Fizzing (CO₂) shows an acid.

The colour in the flask changes from orange to green as Cr₂O₇²⁻ becomes Cr³⁺. Do not start with distillation: the aldehyde would escape before it could be oxidised again. Distil after reflux, so that the acid is separated from the water, sulfuric acid and chromium salts. Test the distillate, not the flask: the sulfuric acid in the flask would also fizz with carbonate.

Results and evaluation 10.20

Part Product Boiling temperature / °C Test Result
A propanal 49 Benedict's, warmed blue to brick-red precipitate
B propanoic acid 141 sodium carbonate fizzing

Worked examplePercentage yield of propanal (illustrative)

5.0 g of propan-1-ol gives 2.1 g of propanal. Calculate the percentage yield.

  1. MrM_\text{r} of CH₃CH₂CH₂OH = 3(12.0) + 8(1.0) + 16.0 = 60.0.
  2. MrM_\text{r} of CH₃CH₂CHO = 3(12.0) + 6(1.0) + 16.0 = 58.0.
  3. n(alcohol) = 5.0 ÷ 60.0 = 0.08333 mol. The equation is 1 : 1, so the maximum amount of propanal is 0.08333 mol.
  4. Maximum mass = 0.08333 × 58.0 = 4.83 g.
  5. Percentage yield = 2.1 ÷ 4.83 × 100 = 43% (2 s.f.).

Why the yield is low: some propanal is oxidised to the acid before it distils; some alcohol distils over unchanged with it; and propanal is volatile, so some is lost as vapour.

Weakness Effect Improvement
Propanal is volatile the yield is low keep the receiver in ice; use a well-fitting condenser
Some propanal is oxidised further the yield of aldehyde falls add the oxidising agent slowly; distil gently
Some propan-1-ol distils over the distillate is impure keep the thermometer near 49 °C; collect only that fraction
Sulfuric acid fizzes with carbonate a false positive in Part B distil the acid from the mixture and test the distillate

Safety 10.20

Check yourself

  1. Write the equations for the two parts of the practical, using [O].
  2. Describe the apparatus and conditions for Part A, and give two reasons why propanal is collected in a receiver in ice.
  3. Describe the apparatus and conditions for Part B, and explain why distillation follows reflux.
  4. Give the test and observation for each product.
  5. Calculate the percentage yield of propanal if 7.5 g of propan-1-ol gives 3.9 g of propanal.
  6. State two hazards and a precaution for each.

Answers cover these until you have tried the questions

  1. A: CHX3CHX2CHX2OH+[O]→CHX3CHX2CHO+HX2O\ce{CH3CH2CH2OH + [O] -> CH3CH2CHO + H2O}. B: CHX3CHX2CHX2OH+2 [O]→CHX3CHX2COOH+HX2O\ce{CH3CH2CH2OH + 2[O] -> CH3CH2COOH + H2O}.
  2. Distillation apparatus, warm gently, add the dichromate drop by drop, thermometer near 49 °C. The ice keeps the volatile, flammable propanal liquid and stops it escaping as vapour.
  3. Heat under reflux with excess dichromate for about 20 minutes, so that the aldehyde cannot escape and is oxidised. Then distil to separate the propanoic acid (141 °C) from the water, sulfuric acid and chromium salts, ready for testing.
  4. A: Benedict's or Fehling's solution, warmed: blue to brick-red precipitate. B: sodium carbonate: fizzing, CO₂.
  5. n(alcohol) = 7.5 ÷ 60.0 = 0.125 mol; maximum mass of propanal = 0.125 × 58.0 = 7.25 g; yield = 3.9 ÷ 7.25 × 100 = 54%.
  6. For example: propanal and propan-1-ol are flammable, so use an electrical heater or water bath and no flames; dichromate(VI) is toxic, so wear gloves and goggles; sulfuric acid is corrosive, so add it slowly with eye protection.