The synthesis of Benzil

In the first step, the oxygen atom of the hydroxyl group attacks the electrophilic nitrogen of a nitric acid molecule (or of an activated species generated by protonation of nitric acid), leading to an intermediate that, through a series of proton transfers (represented here as intramolecular for simplicity), eliminates water to afford the corresponding nitrate ester. This intermediate can then evolve via the transfer of the hydrogen attached to the α-carbon to the nitrate moiety, accompanied by the reduction of the latter and the formation of a nitrous species, represented here as nitrous acid (HONO). Such species are unstable under the reaction conditions and may participate in further redox and disproportionation processes, ultimately leading to nitrogen oxides such as NO and NO₂.

Before moving on to the experimental part, let me add a small remark for the reader regarding the synthesis of benzil. Although using concentrated nitric acid is simple and widely adopted, it is worth noting that this reaction is not always the cleanest or most controllable. The final product can often be contaminated with unreacted benzoin [1] or, if harsher conditions are applied, over-oxidation and side reactions may occur. A cleaner and more controlled alternative utilizes copper(II) as an oxidant, either in stoichiometric quantities [1] or, more conveniently, in catalytic amounts. In the latter case, a secondary oxidizing agent is employed to re-oxidize the copper(I) species generated during the reduction cycle. Some authors have utilized the direct air oxidation of copper sulfate in pyridine [2], while others suggest using copper(II) acetate in glacial acetic acid in the presence of ammonium nitrate [3]. The US2658920A patent even suggests introducing a catalytic amount of an alkaline nitrite (such as sodium, potassium, or ammonium nitrite) to assist in the conversion of any copper(I) oxide formed during the reaction.

Experimental Part

In a 100 mL flask equipped with a magnetic stir bar, 2 g of benzoin (9.42 mmol, 1 eq.) were suspended in a mixture of 9.3 mL of 65% nitric acid (134.3 mmol, 14.2 eq.) and 0.7 mL of deionized water. The water was added to tame the reaction, as performing it directly with pure 65% nitric acid can be quite vigorous. The mixture was heated for 1 h in a boiling water bath until no trace of brown nitrogen oxide fumes was visible. This step is critical; interrupting the reaction too early has a negative impact on the yield. Having performed this reaction before, I previously stopped after only 40 minutes when only a faint presence of brown gas was observed, which noticeably reduced the final yield. After one hour of heating, the mixture was slowly poured into 100 mL of cold water under constant, vigorous stirring, resulting in the immediate formation of a yellowish precipitate. The mixture was allowed to cool, and the solid was recovered by vacuum filtration and washed a few times with cold water. The crude product was dried over anhydrous calcium chloride, yielding a final mass of 1.89 g.