AS-Level Alcohols

  • For organic chemistry – oxidation is a carbon atom gaining a bond to an oxygen atom or losing a bond to a hydrogen atom.  

  • To oxidise an alcohol an oxidising agent (usually acidified potassium dichromate) is used and the alcohol is heated. 

  • Primary alcohols can be oxidised to an aldehyde and then to a carboxylic acid. To isolate the aldehyde, the products must be distilled from the reaction mixture. If a carboxylic acid is desired, the mixture must be heated under reflux conditions.

  • Secondary alcohols can only be oxidised to form ketones. 

  • Fehling’s solution and Tollens’ reagent are used to distinguish between aldehydes and ketones. Adehydes react to form a brick red precipitate with Fehling’s solution and a silver solid (silver mirror) with Tollens’ reagent. Ketones do not react with either.

QUICK NOTES

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Alcohols (Oxidation)

 

In organic chemistry, oxidation describes a carbon atom in a molecule gaining a bond to an oxygen atom or the losing of a bond to a hydrogen atom (and electrons).

 

Primary and secondary alcohols can be oxidised and new products formed. These reactions are very important in organic chemistry.

 

An oxidising agent is needed, usually potassium dichromate (to provide Cr O  ions) and concentrated sulphuric acid (to provide H⁺ ions).

 

Oxidation of a primary alcohol

 

A primary alcohol can be oxidised to an aldehyde, which can then (when the reaction is carried out in reflux conditions) be oxidised into a carboxylic acid.

 

 

 

 

 

 

 

 

Oxidation of a secondary alcohol

 

A secondary alcohol can be oxidised to a ketone. There is no further oxidation possible.

 

 

 

Oxidation of a tertiary alcohol

 

A tertiary alcohol cannot be oxidised, as there is no hydrogen atom to be lost from the carbon atom that the alcohol group is attached to.

 

 

 

Distinguishing between a primary and secondary alcohol

 

One easy way of distinguishing between a primary and a secondary alcohol is to test for the initial products of oxidation for each alcohol – aldehydes and ketones.


Tollens’ reagent and Fehling’s solution both give positive test results for an aldehyde, but not for a ketone. So, if you oxidise separate samples of the primary and secondary alcohols in the same conditions (to ensure no carboxylic acid is produced from further oxidisation of the aldehyde), the sample that gives a positive result for an aldehyde must have contained the primary alcohol to start with.

 

Tollens’ reagent:

 

 

Heat aldehyde with Tollens’ reagent and a silver mirror precipitate is formed.

 

Fehling’s solution:

 

Heat aldehyde with Fehling’s solution and a brick red precipitate is formed.

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