Oxidation of alcohols

Alcohols can be oxidised using sodium or potassium dichromate (VI) acidified with dilute sulfuric acid. If oxidation occurs, the orange solution containing dichromate ions is reduced to a green solution containing chromium (III) ions. Since tertiary alcohols cannot be oxidised, there is no colour change when the oxidising agent is added. This reaction can therefore be used to distinguish tertiary alcohols from primary and secondary alcohols.

Primary, secondary and tertiary alcohols

Alcohols can be classified as primary, secondary or tertiary depending on the number of R groups the carbon with the -OH attached is bonded to. By ‘R group’ we mean anything other than hydrogen.

Primary alcohols contain a carbon atom attached to the alcohol group (-OH), and at least two hydrogen atoms.

Secondary alcohols contain a carbon attached to the alcohol group (-OH), two R groups and one hydrogen.

Tertiary alcohols contain a carbon attached to the alcohol group (-OH) and three R groups. There is no hydrogen atom bonded to the carbon atom.

Oxidation of primary alcohols

Primary alcohols can be partially oxidised to form aldehydes or fully oxidised to form carboxylic acids. For the formation of aldehydes, two hydrogen atoms are removed and combine with oxygen from the oxidising agent to form water. A double bond forms between carbon and oxygen and the (-CHO) functional group is formed. When aldehydes are completely oxidised into carboxylic acids, oxygen from the oxidising agent inserts itself between carbon and hydrogen to form the (-COOH) functional group.

Partial oxidation of primary alcohols can be achieved by using an excess of alcohol and distilling off the aldehyde as soon as it forms. For full oxidation, the alcohol is heated under reflux with excess oxidising agent.

Oxidation of secondary alcohols

Secondary alcohols can be oxidised to form ketones. Two hydrogen atoms combine with oxygen from the oxidising agent to form water. A double bond forms between carbon and oxygen to form the (-C=O) ketone functional group.

Oxidation of tertiary alcohols

Nothing happens when you mix a tertiary alcohol with an oxidising agent such as potassium dichromate (VI). If you look at the reactions above, two hydrogen atoms were removed from each alcohol which were used in the formation of water. In tertiary alcohols there is only one hydrogen atom (the one in the -OH alcohol group), so tertiary alcohols are unable to be oxidised.

Tests to distinguish between aldehydes and ketones

Observing whether a colour change occurs upon addition of an alcohol to a suitable oxidising agent is fine for distinguishing a tertiary alcohol, but how can we tell whether we have a primary or secondary alcohol on our hands? The way to distinguish between these alcohols is to perform (partial) oxidation to produce aldehydes or ketones and then observe how they react with Tollens reagent or Fehling’s solution. Both of these reactions rely on the fact that aldehydes can be further oxidised but ketones can’t.

Addition of Tollen’s reagent to aldhydes oxidises the aldehyde, causing formation of a carboxylic acid. Diammimesilver ions are reduced to form a silver precipitate, or a ‘silver mirror’ on the bottom of the test tube. No reaction occurs when a ketone is added.

Fehling’s solution can also oxidise aldehydes into carboxylic acids. Copper ions within the solution are reduced to copper oxide, causing a colour change from blue to red. This is essentially the same reaction as that of the Benedict’s test for the presence of reducing sugars.