Figure 1: The simplified

Oxidation of Alcohols to Aldehydes

Oxidation of Alcohols

Reaction type: Oxidation-Reduction

Summary

  • The outcome of oxidation reactions of alcohols depends on the substituents on the carbinol carbon.
  • In order for each oxidation step to occur, there must be H on the carbinol carbon.
  • Primary alcohols can be oxidised to aldehydes or further to carboxylic acids.
  • In aqueous media, the carboxylic acid is usually the major product.
  • PCC or PDC, which are used in dichloromethane, allow the oxidation to be stopped at the intermediate aldehyde.
  • Secondary alcohols can be oxidised to ketones but no further:
  • Tertiary alcohols cannot be oxidised (no carbinol C-H)
  • Cr OXIDATION OF ALCOHOLS

    The mechanism is not trivial, so attention here is focussed on the actual oxidation step. Prior to this, the alcohol reacts to form a chromate ester (shown). A base (here a water molecule) abstracts a proton from the chromate ester, the C=O forms and a Cr species leaves.
    This demonstrates the importance of the carbinol H to this mechanism.
    QUESTIONS
    What type of reaction could the oxidation step be decribed as ? ANSWER
    What is the oxidation state of Cr in chromic acid ? ANSWER
    What is the oxidation state of the Cr in the chromate ester intermediate ? ANSWER
    What is the oxidation state of the Cr in the "leaving group", HCrO3- ? ANSWER

    Study Tip:
    If you see Cr reagents, you are probably looking at an oxidation reaction.
    Most likely the oxidation of an alcohol or an aldehyde

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