Preparation of ethanal

        This preparation produces ethanal in aqueous solution. It is invariably contaminated with a small amount of ethanoic acid.


  • Place 50 cm3 of water in a 500 cm3 round-bottomed flask, and add slowly with shaking 17 cm3 of concentrated sulphuric acid: then add some anti-bumping granules.
  • Why must the sulphuric acid be added slowly and with shaking?
  • The reaction of sulphuric acid with water is dangerously exothermic. Sulphuric acid is almost twice as dense as water, so if two layers are allowed to form and are then mixed it is possible to generate steam which will spray the acid about.
  • What are anti-bumping granules and what is their purpose?
  • The granules are silica, and prevent the sudden formation of large gas bubbles that lead to 'bumping' or succussion.
  • Assemble the flask containing the sulphuric acid into a distillation apparatus. The stillhead carries a tap funnel instead of a thermometer, and the receiving flask should be surrounded by an iced water bath..
  • Why is the receiver surrounded by iced water? The answer should make a specific chemical point.
  • The boiling temperature of ethanal is 21oC, so the cooling reduces evaporation of the ethanal.
  • Dissolve 50 g of sodium dichromate(VI) in 50 cm3 of water contained in a small beaker, and add 40cm3 of ethanol. Stir thoroughly, and place this mixture in the tap funnel.
  • Why isn't the ethanol oxidised by this mixture?
  • The oxidation requires hydrogen ions: the mixture as made is not acidic.

Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H2O

  • Why is sodium dichromate(VI) used rather than the commoner potassium dichromate(VI)?
  • Sodium dichromate(VI) is much more soluble in ethanol than is the potassium salt. Sodium dichromate(VI) is not used in volumetric analysis since it is deliquescent, so cannot be weighed to make a primary standard solution. Potassium dichromate(VI) does not suffer from this disadvantage.
  • Heat the dilute acid in the flask until it begins to boil gently, and then remove the flame and run the alcohol/dichromate(VI) solution very slowly into the flask. Directly the solution enters the hot acid in the flask a vigorous reaction occurs, and a mixture of ethanal and water containing a little ethanoic acid distils over. The reaction mixture becomes green. The addition of the alcohol/dichromate(VI) mixture should take about 20 minutes; towards the end of this time it will be necessary to replace the flame under the distilling flask to maintain gentle boiling.
  • Why is the alcohol/dichromate(VI) mixture added slowly to the hot acid?
  • Rapid addition would lead to a large amount of oxidising agent being present in the reaction mixture and therefore significant oxidation of the ethanal produced to ethanoic acid. Excess of the oxidising agent must be avoided.
  • Why does the distillate always contain some ethanoic acid?
  • It is not possible to prevent oxidation of ethanal completely, since it is easily oxidised.
  • Why does the mixture turn green?
  • The orange dichromate(VI) ion is reduced to hexaquachromium(III) in this reaction. [Cr(H2O)6]3+ is green.
  • When the addition of the alcohol/dichromate(VI) mixture is complete, a moderately concentrated aqueous solution of ethanal will have collected in the receiver.
  • Suggest two reasons why the industrial manufacture of ethanal employs vapour phase oxidation of ethanol using air over a heated silver catalyst.
  • Firstly the use of an expensive reagent like sodium dichromate(VI) would be prohibitively expensive on an industrial scale unless the product was of high value; secondly an aqueous solution of ethanal would be formed and this would have to be separated, needing further expense. Vapour phase oxidation gives the pure aldehyde. Industrial processes are seldom a simple scaling-up of laboratory reactions.