Thursday, 19 May 2011

Oxidation of alcohols

Students have started studying organic chemistry.

Today the chemistry students did an investigation to establish the oxidation reaction patterns of primary, secondary and tertiary alcohols, using oxidising agent acidified potassium dichromate. 

If oxidation occurs, the orange solution containing the dichromate(VI) ions is reduced to a green solution containing chromium(III) ions.

In the case of a primary or secondary alcohol, the orange solution turns green. With a tertiary alcohol there is no colour change.
Orange  Acidified Potassium Dichromate
Green Chromium ions


Primary alcohols can be oxidised to either aldehydes or carboxylic acids depending on the reaction conditions. In the case of the formation of carboxylic acids, the alcohol is first oxidised to an aldehyde which is then oxidised further to the acid.

Partial oxidation to aldehydes

You get an aldehyde if you use an excess of the alcohol, and distil off the aldehyde as soon as it forms. The excess of the alcohol means that there isn't enough oxidising agent present to carry out the second stage.

Ethanol  a primary alcohol  would produce the aldehyde ethanal, CH3CHO.

Full oxidation to carboxylic acids

You need to use an excess of the oxidising agent and make sure that the aldehyde formed as the half-way product stays in the mixture. The alcohol is heated under reflux with an excess of the oxidising agent. When the reaction is complete, the carboxylic acid is distilled off.

The full equation for the oxidation of ethanol to ethanoic acid is:

Secondary alcohols are oxidised to ketones
Heating the secondary alcohol propan-2-ol with acidified potassium dichromate(VI) solution  produces propanone.

Tertiary alcohols are not oxidised by acidified potassium dichromate(VI) solution. There is no reaction.

For the next few weeks the SMAF class are fortunate to have an extra practicing teacher Dr. Paul K. Bowyer. Dr. Bowyer has a background in organic chemistry. He undertook a PhD in 1992 at UNSW on heterocyclic chemistry and metal complexation, part of which was completed at the University of Cambridge Chemical Laboratory in the United Kingdom after obtaining a Young Endeavour Science Award in 1992.  Three years of post-doctoral research in pure chemistry followed at the Australian National University (Canberra, Australia) and the Universit├Ąt Basel (Basel, Switzerland).   In 2011 Dr Bowyer joined Blue H2O Filtration as the Regional Manager for SA. 
Dr. Paul Bowyer chatting to students about the oxidation of alcohols.
Paul looking at a students result.

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