1. Test for functional groups

• Potassium dichormate (VI) is an oxidising agent. It can oxidise primary and secondary alcohols to form aldehydes and ketones respectively (tertiary alcohols can’t be oxidised)

  
• As primary and secondary alcohols are oxidised, potassium dichromate (VI) is reduced. This is accompanied by a colour change

  
• To test for presence of primary, secondary or tertiary alcohols:

  1. Add 10 drops of the alcohol to 2cm3 of acidified potassium dichromate solution in a test tube
  2. Warm mixture gently in hot water bath
  3. Watch the colour change - with primary and secondary alcohols, the orange solution turns green as aldehyde or ketone forms. No colour change is seen with tertiary alcohols
• Problems with test for alcohols:

  Test doesn’t help to work out whether alcohol is primary or secondary alcohol - as same result is given.
• Test whether unknown alcohol is primary or secondary using oxidised version of primary or secondary alcohol:

  • Oxidise alcohol under reflux and it tests positive for being Carboxylic acid, then its primary alcohol
  • oxidise alcohol under distillation conditions and it tests positive for being aldehyde, then its primary alcohol
  • Oxidise alcohol under reflux (or distillation) and it tests positive for being ketone, then its secondary alcohol
• Test for aldehyde / ketone with Benedict’s / Fehlings:

  1. Add 2cm3 of Fehlings or Benedict’s solution to test tube
  2. Add 5 drops of aldehyde or ketone to test tube
  3. Put test tube in hot water bath to warm for few minutes

  Benedicts and Fehlings solution will both reduce to brick red Cu2O precipitate when warmed with aldehyde. No reaction happens with ketones, so solution will stay blue

  
• Fehlings solution is blue solution of complexed copper (II) ions dissolved in sodium hydroxide. Benedicts solution is effectively same as Fehlings solution except the copper (II) ions are dissolved in sodium carbonate instead of sodium hydroxide.
• Tollens reagent contains [Ag(NH3)2] complex. It can be made using:

  1. Pur 2cm3 of 0.10 moldm-3 silver nitrate solution in test tube.
  2. Add few drops of dilute sodium hydroxide solution. A light brown precipitate should form
  3. Add drops of dilute ammonia solution until the brown precipitate dissolves completely
• Silver ions in Tollens reagent are reduced to silver metal when warmed with aldehyde, but not ketone. Silver will coat inside of apparatus to form silver mirror

  
• Aldehydes and ketones are flammable so they should always be warmed using water bath, rather than Bunsen burner, prevent catching alight
• Carboxylic acids form by oxidising aldehydes or primary alcohols:

  1. Add 2cm3 of solution that you want to test to test tube
  2. Add 1 small spatula of solid sodium carbonate (or 2cm3 of sodium carbonate solution)
  3. If solution begins to fizz, bubble gas that it produces through like water in second test tube If solution tested contains Carboxylic acid, carbon dioxide gas will be produced. When carbon dioxide is bubbles through limewater, limewater turns cloudy
• Testing for alkenes involves testing for presence of unsaturation:

  1. Add 2cm3 of solution that you want to test to test tube
  2. Add 2cm3 of bromine water to test tube
  3. Shake tube
  4. If alkene is present, bromine water will turn from orange to colourless