Potassium dichromate Tests for Primary, Secondary and Tertiary Alcohols

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Here are some ideas for some easy test tube reactions that cover redox, intermolecular forces, alcohols, ketones, aldehydes, carboxylic acids and chemical tests.

We can use acidified potassium dichromate (K2Cr2O7) solution to distinguish between primary, secondary and tertiary alcohols.

As with all the experiments listed on this website,

specific experimental details are not given.

It is your responsibility to research each method and to carry out a substantial risk analysis.

Primary alcohols are oxidised to aldehydes and the carboxylic acids

Secondary alcohols are oxidised to ketones.

Tertiary alcohols cannot be oxidised by the dichromate ions.

Dichromate ions (Cr2O7 2-) are orange. On reduction Cr 3+ ions are formed which are green.

Can you write a half equation to show this (Hint: The dichromate solution is acidified).

So you would expect to see the tertiary alcohol remaining an orange colour whilst the secondary and tertiary alcohols would be oxidised and in doing so, the dichromate ions would be reduced to give a green solution.

To distinguish between primary and secondary alcohols you would use either Tollen’s reagent or Fehlings solution.

The unknown alcohol would first need to be oxidised to either the ketone or the aldehyde.

Tollens or Fehlings would be added. Aldehydes will react. Ketones will not react.

For Tollens reagent, a positive result produces a Silver mirror.

 

For Fehlings solution, the liquid turns from a deep transparent blue colour to a brick red precipitate. The clip mentions that Glucose is added to Fehlings – this is because Glucose has an aldehyde group!

How could you ensure that a primary alcohol is not further oxidised to a carboxylic acid?

You would carry out the reaction by warming and collecting the vapour using a Lieberg condenser.

The primary alcohol will have a relatively high boiling point due to hydrogen bonds but the aldehyde only has dipole – dipole forces so it will evaporate / boil at a much lower temperature.

If you wanted to form the carboxylic acid you would need to reflux the mixture of primary alcohol and dichromate.

6 Comments
  • Ahmed
    June 2, 2011

    Thanks that was useful 🙂

    note:i thought its a liebig not lieberg condenser .

  • lalo
    January 5, 2014

    how far does the potassium dichromate detect alcohol (timewise)in a breath test of a .02?

  • lalo
    January 5, 2014

    im referring about the reditest alcohol breath test

  • anan
    February 27, 2016

    Wouldn’t acidified potassium dichromate, being a strong oxidising agent ,oxidise the primary alcohol to carboxylic acid before you get the chance to test for the aldehyde?

    • triplea_da
      March 5, 2016

      Hi Anan,

      Yes, this is correct. However, if you wish to obtain the aldehyde you can distill it, otherwise you can reflux to obtain the acid.

      All the best,

      Dave

  • Apenji
    June 8, 2018

    Thanks.Very helpful.

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