Phenylglyoxal
# Phenylglyoxal

Phenylglyoxal is the organic compound with the formula $C_6H_5C(O)C(O)H.$ It contains both an aldehyde and a ketone functional group. It is yellow liquid when anhydrous but readily forms a colorless crystalline hydrate. It has been used as a reagent to modify the amino acid, arginine. It has also been used to attach chemical payload (probes) to the amino acid citrulline and to peptides/proteins.

## Properties

Like some other aldehydes, phenylglyoxal polymerizes upon standing, as indicated by solidification of the liquid. Upon heating, this polymer "cracks" to give back the yellow aldehyde. Dissolution of phenylglyoxal in water gives crystals of the hydrate:

    $C_6H_5C(O)COH$ + $H_2O$ → $C_6H_5C(O)CH(OH)_2$

Upon heating, the hydrate loses water and regenerates the anhydrous liquid.

## Preparation

Phenylglyoxal was first prepared by thermal decomposition of the sulfite derivative of the oxime:

    $C_6H_5C(O)CH(NOSO_2H)$ + 2 $H_2O$ → $C_6H_5C(O)CHO$ + $NH_4HSO_4$

More conveniently, it can be prepared from methyl benzoate by reaction with $KCH_2S(O)CH_3$ to give $PhC(O)CH(SCH_3)(OH),$ which is oxidized with copper(II) acetate. Alternatively, it can also be prepared by oxidation of acetophenone with selenium dioxide.

