Highly Selective and Catalyst-Free Oxidation of Benzyl Alcohol With Nitric Acid Under Mild Conditions

Abstract

Benzaldehyde is a useful starting material of pharmaceuticals and spice. Benzaldehyde was synthesized via the oxidation of benzyl alcohol with concentrated nitric acid at 10°C–39°C, and the selectivity and yield were up to 99.7% and 98.9%, respectively. The selectivity was reduced at higher reaction temperatures. This is an intensely exothermic reaction in the absence of catalyst and organic solvent, and the composition of products was very simple. Most of benzyl alcohol was transformed into benzaldehyde within 2 min, showing high production efficiency. The oxidation reaction of benzaldehyde followed pseudo-zeroth-order kinetics with activation energy of 30.5 kJ/mol, so the accumulation of benzaldehyde would not accelerate its oxidation reaction. The high selectivity to benzaldehyde at low temperatures was attributed to the higher activation energy of oxidation of benzaldehyde compared to oxidation of benzyl alcohol. The reaction process was monitored by in-situ UV-Vis. The oxidation rate with concentrated HNO₃ was much higher than that with dilute HNO₃. NO₂ and NO were the dominant product in the presence of concentrated and dilute HNO₃, respectively. This paper presents a feasible scheme to selectively transform benzyl alcohol into benzaldehyde with a new oxidant instead of catalyst.