REQUIRED PRACTICAL 6

• For this practical, you need to be able to give the tests and positive results for the following functional groups:
  • Alkenes
  • Alcohols
  • Aldehydes
  • Carboxylic Acids

   

Testing for an Alkene

• Halogens can be used to test if a molecule is?unsaturated?(i.e. contains a double bond)
• Br2(aq) is an orange-yellow solution, called?bromine water
• The unknown compound is?shaken?with the bromine water
• If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise

The?bromine water test is the standard test for unsaturation in alkenes

Testing an Alcohol

• Alcohols can be classified as either primary, secondary or tertiary, depending on the placement of the -OH group
• Primary and secondary alcohols can both be oxidised, but tertiary alcohols cannot
• To test for the alcohol functional group, add a small amount (1 cm3) of the substance to a test tube using a pipette
• Then, add a small amount (1 cm3) of a suitable oxidising agent to the sample using a different pipette
  • The most commonly used oxidising agent for this test is acidified potassium dichromate solution (K2Cr2O7, acidified with H2SO4)

   

• Add a stopper to the test tube and shake well
• Place in a hot water bath (heated to around 60?oC) for a few minutes
• If a primary or secondary alcohol are present, then the colour will change from orange to green
  • If a tertiary alcohol is present, then nothing will happen - the solution will remain orange

   

Positive test results of the oxidation of a primary, secondary and tertiary alcohol

Testing for an Aldehyde

Fehling’s solution

• Fehling’s solution?is an alkaline?solution containing copper(II) ions which act as the oxidising agent
• When?warmed?with an aldehyde, the aldehyde is oxidised to a carboxylic acid and the?Cu2+?ions are reduced to Cu+?ions
  • In the alkaline conditions, the carboxylic acid formed will be neutralised to a carboxylate ion (the -COOH will lose a proton to become -COO–?)
  • The carboxylate ion (-COO–) will form a salt with a positively charged metal ion such as sodium (-COO–Na+)

   

• The?clear blue?colour of the solution turns?opaque?red?due to the formation of a?copper(I) oxide?precipitate
• Ketones?cannot be oxidised and therefore give a?negative test?when warmed with Fehling’s solution

The?Ag+?ions in Tollens’ reagent are?oxidising agents,?oxidising the aldehyde to a carboxylic acid and getting reduced themselves to silver atoms

Testing for a Carboxylic Acid

• Carboxylic acids in solution have a pH of around 3, so measuring the pH is a way of testing for the presence of the carboxylic acid functional group in an organic sample
• The end of a glass rod could be dipped into the solution and then carefully dripped onto indicator paper
• Or, a pH probe could be used, which would give you an exact pH
• Since carboxylic acids are acids, they will react with a carbonate solution to produce carbon dioxide gas
• 1-2 cm3?of sodium carbonate (Na2CO3) or sodium hydrogen carbonate solution (NaHCO3) could be added using a pipette
• If bubbles of gas are seen, this is a good indicator that the solution is a carboxylic acid
  • If an exam question asks you to simply distinguish between different types of organic compound, and the carboxylic acid is the only organic compound present which would react in this way with a carbonate solution, then this is enough

   

• The gas produced could then be bubbled into limewater
• If the limewater turns milky or cloudy, then this proves that the gas produced was carbon dioxide