Question

\(\alpha\)-Hydroxyketones and \(\alpha\)-hydroxyaldehydes are also oxidized by treatment with periodic acid. It is not the \(\alpha\)-hydroxyketone or aldehyde, however, that undergoes reaction with periodic acid, but the hydrate formed by addition of water to the carbonyl group of the \(\alpha\)-hydroxyketone or aldehyde. Write a mechanism for the oxidation of this \(\alpha\)-hydroxyaldehyde by \(\mathrm{HIO}_{4}\).

Short answer

Answer: After the oxidation of an α-hydroxyaldehyde by periodic acid (HIO₄), the products formed are a carboxylic acid, an aldehyde, and iodinic acid (HIO₃).

Step-by-step solution

Formation of the Hydrate

In the presence of water, the carbonyl group of the α-hydroxyaldehyde will form a hydrate. This happens when a molecule of water adds to the carbonyl group, forming a diol. The mechanism for this reaction involves the nucleophilic attack of the partially negative oxygen atom from a water molecule to the partially positive carbon atom of the carbonyl group, followed by the protonation of the negatively charged oxygen formed.

Reaction with Periodic Acid

Next, the hydrate will react with the periodic acid (HIO₄). In this reaction, periodic acid acts as an oxidizing agent, cleaving the C-O bonds in the diol. The mechanism involves the nucleophilic attack of the two oxygen atoms from the diol to the two iodine atoms in the periodic acid. In the process, an unstable intermediate is formed, which then decomposes through the release of iodinic acid (HIO₃) and the formation of an aldehyde or a ketone and a carboxylic acid.

Products of the Reaction

The final products of the reaction will be a carboxylic acid and an aldehyde, along with iodinic acid (HIO₃). The aldehyde product is formed from the cleavage of the C-O bond involving the carbonyl carbon atom while the carboxylic acid product is formed from the cleavage of the C-O bond involving the α-carbon atom of the α-hydroxyaldehyde. Therefore, after the reaction with periodic acid, the α-hydroxyaldehyde is converted into a carboxylic acid and an aldehyde.