Mr Cole Chemistry

Oxidation of Alcohols Practical Questions

Required Practical 5: Oxidation

Alcohols Worksheet

1. Partial Oxidation (Distillation)

You are oxidising propan-1-ol to form propanal.
a) When conducting the experiment, you mix the propanol and acid first before adding the sodium dichromate. Explain why you do it in this order, rather than mixing the sodium dichromate and the sulfuric acid first before adding the alcohol.

You have to prevent there from being excess oxidising agent (when compared to the alcohol) or you risk fully oxidising the alcohol to a carboxylic acid.

b) Draw a diagram of the apparatus that you would use to separate the product from the reactants.
Distillation Apparatus Setup

Key Checks:

  • There should be no gaps in the apparatus except for the gap between the end of the condenser and the boiling tube.
  • Check the water in (bottom) and water out (top).
  • Check that the top of the apparatus is sealed.
  • Check that you have placed the thermometer at the right height (bulb in line with the condenser side-arm).
c) Propanol and propanal boil at 97°C and 46°C respectively. State the temperature that should be observed on the thermometer.

46°C

Only the propanal should reach this point and distil over.

d) State the method that you would use to heat the mixture, and explain why you would use that method.

Either a water bath or an electric heating mantle.

These allow you to control the temperature more carefully, and do not involve an open flame (organic compounds like propanol are flammable).

e) Your product is not pure; it contains some propanoic acid. Describe a method that you would use to remove the propanoic acid impurities. Include an equation in your answer.

Mix the product with sodium carbonate solution in a separating funnel. Add a little at a time. This will react with the propanoic acid in the following reaction:

2C2H5COOH + Na2CO3 → 2C2H5COO–Na+ + H2O + CO2

Don’t forget to remove the stopper / open the tap from time to time to relieve the pressure from the carbon dioxide gas produced. Once the reaction has stopped fizzing, stop adding the sodium carbonate. Leave the two layers to settle and gradually let the lower (aqueous) layer out, leaving just the organic layer.

f) After this process, you risk having some water left over in your product. Describe the process that you would use to remove the water.

Add a drying agent, such as anhydrous calcium chloride or anhydrous magnesium sulfate. Then, filter to remove the solid drying agent.

2. Full Oxidation (Reflux)

You are oxidising propan-1-ol to propanoic acid.
a) Draw a diagram of the apparatus used to ensure that this reaction occurs.
Reflux Apparatus Setup

Key Checks:

  • There should be no gaps in the apparatus except at the very top (it must remain open to prevent pressure buildup, but vertical).
  • Check the water in (bottom) and water out (top) on the vertical condenser.
b) Explain how this process works.

The reactants can be heated for a long time without losing volatile substances. The mixture boils and the gases move up the condenser. They cool and condense before reaching the top, falling back into the round-bottomed flask where any unreacted reactants can continue to react to form products.

c) Once you have completed the experiment, you separate it from the reactants using a different set of apparatus. Name the process that you would use to do this.

Fractional Distillation (or simply Distillation, though Fractional is better for separation based on boiling points).

d) Once you have your propanoic acid, you test a sample it to determine whether there is any propanal. Describe one test that you could use.

Option 1: Mix a small amount with Tollens’ reagent and gently warm. If there is any propanal (aldehyde), it will form a silver mirror.

Option 2: Mix a small amount with Fehling’s solution and heat. If propanal is present, it forms a brick-red precipitate.

e) Your test confirms that there is no propanal in the mixture. The next step is to remove any excess water. Why would you not use anhydrous sodium carbonate to as a drying agent in this reaction?

Anhydrous sodium carbonate is a base. It would react with the product (propanoic acid), effectively neutralising the acid you just produced. Use an anhydrous salt (like calcium chloride or magnesium sulfate) instead.

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