To show the reaction between ethanol and acidified potassium manganate (VII)

Theory

• Acidified potassium permanganate, Fehling’s reagent and ammoniacal silver nitrate are all oxidising agents

• They will cause aldehydes to be oxidised to a carboxylic acid

• Ketones will not be oxidised under the same conditions

Note: These reactions test for aldehydes/distinguish between aldehydes and ketones i.e. prove a certain substance is an aldehyde and not a ketone

Important: If asked to show that an aldehyde can be ‘EASILY’ oxidised, a weak oxidising agent must be chosen i.e. Fehling’s reagent or ammoniacal silver nitrate

Procedure

• The ethanal (aldehyde) is placed in a test tube in a warm water bath

• Using a dropper, acidified dilute potassium manganate (VII) is added to the ethanal

Result: The purple colour of potassium manganate (VII) turns colourless

Note: When repeated with propanone (ketone) in place of ethanal, no colour change is observed

Write the half reactions that occur when acidified potassium manganate (VII) is added to ethanal (aldehyde)

1) image

2) MnO4- + 8H+ + 5e- —[R]→ Mn2+ + 4H2O

Mn7+: purple

Mn2+: colourless

Explain the colour change that occurs when acidified potassium permanganate is reacted with ethanal

• MnO4- contains Mn7+ ions causing a purple colour

• When reacted with ethanal in an acidic environment, Mn7+ ions are reduced (gain 5 e –) to Mn2+ ions

• Mn2+ ions are colourless