Aldehyde Reactions: Carboxylic Acid Formation from Aldehyde using Chromate
Carboxylic acids can be formed from aldehydes and primary alcohols using chromates (H2CrO4, Na2Cr2O7, CrO3, etc…):
If a secondary alcohol is used, a ketone will be produced instead. Tertiary alcohols and ketones will not react with cromates:
The reaction mechanism is depicted below using H2CrO4 and an aldehyde, but other chromate salts would behave the same:
In the first step, the lone pair of electrons from the water molecule attack the aldehyde carbon atom, forming a bond between the water molecule and the ketone molecule. This also breaks the carbon-oxygen bond sending the electrons to the oxygen atom.
In the second step, the conjugate acid H3O+ is attacked by the negatively charged aldehyde-oxygen atom. Simultaneously, a water molecule attacks the newly attached H2O group, removing a proton and sending the lone pair of electrons to the oxygen atom.
In the third step, the free electrons from one of the two oxygen atoms attacks the chromate atom, breaking the chromate-oxygen double bond and sending the electrons to the oxygen.
In the fourth step, lone pair of electrons from a water molecule attack the proton attached to the chromate-oxygen atom, sending the electrons to the positively charged oxygen. Meanwhile, the negatively charged oxygen on the chromate moiety attacks an H3O molecule, removing the proton.
In the fifth step, lone pair of electrons from a water molecule attack the proton attached to the carbon atom between the two oxygen groups. This sends the free electrons from the hydrogen bond to the oxygen atom forming a double bond between the carbon an oxygen. This double bond forces the chromate-oxygen bond to break, sending the electrons to the chromate atom, which then sends electrons from the chromate-oxygen double bond to the oxygen atom.
The reagents used for this reaction can be H2CrO4, Na2Cr2O7, K2Cr2O7, and CrO3. PCC will not form carboxylic acids; it is not strong enough and will only result in aldehyde/ketone formation from primary/secondary alcohols.