Alcohol Reactions: Carboxylic Acid and Ketone Formation from Alcohols using Chromate

Alcohols reacted with chromates (H2CrO4, Na2Cr2O7, CrO3, etc…) will result in the formation of carboxylic acids (primary alcohols) or ketones (secondary alcohols):

This reaction will also proceed using an aldehyde, resulting in the formation of a carboxylic acid. Tertiary alcohols and ketones will not react with cromates:

The reaction mechanism is depicted below using H2CrO4 and a primary alcohol, but other chromate salts would behave the same:

In the first step, the lone pair of electrons from the alcohol molecule attack the chromate atom, forming a bond between the chromate molecule and the alcohol molecule. A free electron pair from one of the chromate-oxygen atoms attacks H2SO4, taking a proton.

In the second step, a water molecule attacks one of the protons attached to the carbon atom on the alcohol group. This sends the free electrons from that bond to the alcohol-oxygen bound to the chromate. This causes the oxygen-chromate bond to break, releasing the chromate moiety from the molecule.

In the third step, a water molecule attacks the alcohol-carbon, attaching itself to the molecule. This breaks the carbon-oxygen double bond, sending the lone pair to the oxygen atom.

In the fourth step, the newly added water group is deprotonated (not pictured) and one of the 2 alcohol groups reacts with the chromate molecule. This follows the same electron flow as Step 1.

In the fifth step, a water molecule removes the final proton from the original carbon atom, sending the electrons to the oxygen atom, releasing the chromate from the molecule.

In the sixth step, the conjugate base HSO4- removes the proton attached to the double bonded oxygen.

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.