Testing for functional groups

​​In a nutshell

Functional groups such as alcohols, alkenes and carboxylic acids can be identified through simple test tube reactions. These reactions cause colour changes which indicate the presence or absence of a particular functional group.

Testing to identify the type of alcohol 

Alcohols can be oxidised and the product formed depends on which type of alcohol is present. Primary alcohols such as ethanol are oxidised to form aldehydes, which can be further oxidised to form carboxylic acids. Secondary alcohols such as $$2$$​-propanol are oxidised to form ketones. Tertiary alcohols such as $$2$$​-methylpropan-$$2$$​-ol are not easily oxidised.  

These oxidation reactions are used to identify which type of alcohol is present. Acidified potassium dichromate is an oxidising agent which can be used for this reaction.

Potassium dichromate reaction 

Adding $$10$$​ drops of alcohol to $$2 \, cm^{3}$$ of acidified potassium dichromate(VI) and warming the mixture gently will form a colour change if oxidation has taken place. This is because the orange dichromate(VI) ions are reduced to green chromium(III) ions.

• Primary alcohol - orange solution turns green (aldehyde forms)
  
• Secondary alcohol - orange slowly turns green (ketone forms)
• Tertiary alcohol - no colour change (no oxidation)

Aldehyde vs ketone 

The above reaction shows that there is the same colour change expected in the formation of both aldehydes and ketones therefore further testing is needed to differentiate between the two.

The products of the oxidation reaction need to be distilled immediately and tested. There are three main reagents which are used, Fehling's solution, Benedict's solution and Tollens' reagent. All of these tests involve gentle oxidation of aldehydes to carboxylic acids. Ketones cannot be changed by gentle oxidation.

Tollens' reagent

Tollens' test is also called the silver mirror test. Tollens' is a solution of silver nitrate in aqueous ammonia which oxidises an aldehyde but will not oxide a ketone. 

Tollens' test

​​​​procedure

As the aldehyde oxidises the colourless silver(I) complex ions, containing $$Ag^{+}$$ are reduced to metallic silver. These will form a 'silver mirror' on the inside of the test tube. If a ketone is present, nothing will happen.

Fehling's solution and Benedict's solution

Fehling's reagent is also a gently oxidising agent. This agent contains blue copper(II) complex ions which will also oxidise aldehydes but not ketones. If an aldehyde is present, the blue copper(II) complex ions will reduce to a red precipitate of copper(I) oxide. If a ketone is present, nothing will happen.

Benedict's solution behaves the exact same way.

Fehling's/Benedict's solution test 

​​​​PROCEDURE

Upon warming, the blue to red colour change will indicate the presence of an aldehyde.

Testing for alkenes

​​Bromine water

Bromine water can be used to test for alkenes by testing for the presence of a double bond. Bromine will be added to the double bond of an alkene, by electrophilic addition and causes a change in solution colour. 

Bromine water test

​​​​PROCEDURE

Testing for carboxylic acids 

​​Sodium carbonate

Carboxylic acids will react with a carbonate to form a salt, water and carbon dioxide ($$CO_2$$). The presence of $$CO_2$$ will cause a solution limewater to fizz, and can therefore be used to test for the presence of a carboxylic acid. 

PROCEDURE