IB DP Chemistry: SL复习笔记10.2.10 Alcohols - Oxidation

Oxidation of Primary Alcohols

  • The products of oxidation of alcohols depends on the class of alcohols
  • Here is a reminder of the three classes of alcohols:

3.4-Hydroxy-Compounds-Classifying-Alcohols_1The three classes of alcohols

Primary alcohols

  • Primary alcohols can be oxidised to form aldehydes which can undergo further oxidation to form carboxylic acids
  • The oxidising agents of alcohols include acidified K2Cr2O7 or acidified KMnO4
  • Acidified potassium dichromate(VI), K2Cr2O7, is an orange oxidising agent
    • Acidified means that that the potassium dichromate(VI) is in a solution of dilute acid (such as dilute sulfuric acid)
    • For potassium dichromate(VI) to act as an oxidising agent, it itself needs to be reduced
      • When alcohols are oxidised the orange dichromate ions (Cr2O72-) are reduced to green Cr3+ ions
  • Acidified potassium manganate(VII), KMnO4, is a purple oxidising agent
    • As with acidified KMnO4 the potassium manganate(VII) is in an acidic medium to allow reduction of potassium manganate(VII) to take place
      • When alcohols are oxidised, the purple manganate ions (MnO4-) are reduced to colourless Mn2+ ions

3.6-Carboxylic-Acids-Derivatives-Oxidation-of-Primary-Alcohols-and-Aldehydes

Further Oxidation

  • If the aldehyde is not distilled off, further oxidation with excess oxidising agent will oxidise it to a carboxylic acid
  • The reaction takes some time to complete and requires sustained heating

Test for alcohols

  • The oxidation using acidified dichromate provides the basis for the test for alcohols as the reaction gives a strong colour change from orange to green
  • Unfortunately, it does not work for tertiary alcohols, which cannot be oxidised

3.4-Hydroxy-Compounds-Tests-Alcohols

The test for primary and secondary alcohols

Oxidation of Secondary Alcohols

  • Secondary alcohols can be oxidised to form ketones only
  • To get a good yield of the ketone the reaction requires some sustained heating

3.4-Hydroxy-Compounds-Oxidation-of-Secondary-AlcoholsOxidation of Secondary Alcohols

  • Tertiary alcohols do not undergo oxidation
    • This is because there must be a hydrogen on the functional group carbon, which breaks off to form water
    • There are only C-C bonds on the functional group carbon in a tertiary alcohol

Distillation & Reflux

The difference between using distillation and heating under reflux

  • To  produce an aldehyde from a primary alcohol the reaction mixture must be heated
  • The aldehyde product has a lower boiling point than the alcohol ( since it has lost the H-bonding) so it can be distilled off as soon as it forms
  • Distillation can be carried out using a simple side arm arrangement which acts as an air condenser or the vapours can be made to pass through a condenser

3.4-Hydroxy-Compounds-Oxidation-of-Primary-Alcohols

Oxidation of ethanol by acidified K2Cr2O7 to form an aldehyde by distillation

Heating under reflux

  • For reactions that require sustained heating the apparatus has to be modified
  • To prevent loss of volatile reactants the apparatus includes a condenser in the vertical position which returns components back into the reaction flask
  • This is known as heating under reflux (reflux means re-boiling)

10.2.10-Heating-under-Reflux-1

Heating under Reflux Apparatus

 

 

 

 

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