How Can We Tell Whether the Oxidation of Benzoin Was Complete or Not?

Benzoin oxidation requires the use of a catalytic oxidant. An exhaustive list of catalysts has been applied to this reaction, with varying effects. The end product of an efficient benzoin oxidation is the compound benzyl, which has industrial uses. Benzoin is quantified in the end product via spectroscopic methods or a color change reaction to determine whether the oxidation reaction has reached full completion.

Catalytic Oxidants

• Using an effective catalyst for benzoin oxidation is important. The range of catalysts generally used includes oxidants such as nitric acid, copper sulfate, copper acetate, bismuth nitrate and vanadium oxide. Although the benzyl yield may be sufficient, particularly with nitric oxide and copper sulfate, most of these compounds are highly toxic and pose disposal problems. Less toxic oxidants such as sodium dichromate in acetic acid produce a lower yield as some benzoin is converted to benzaldehyde.

Alumina and Air

• Researchers have reported on a novel method for oxidizing benzoin, which involves exposing the compound to air in the presence of aluminum oxide, also known as alumina. This is an attractive option, when you consider the environmental and economic benefits of using air and the wide availability of alumina. In this process, air acts as the oxidant and alumina serves as the catalyst. The major drawback is the large amounts of alumina needed to complete the reaction.

Spectroscopy

• Spectroscopy measures the wavelength at which functional groups in various compounds absorb radiation, whether ultraviolet or infrared. In the ultraviolet spectrum, benzoin absorbs at 247 nm, owing to its phenyl ketone group, which consists of a carbonyl group attached to a benzene ring. In the infrared spectrum, benzoin has an absorption band at 1664 cm-1, attributed to the same carbonyl group. Thus, any benzoin present in the end product will be detected by either of these techniques.

Color Change

• The oxidation of benzoin yields an intermediate compound that is purple in color. Testing for this compound determines whether the reaction is complete or not. A small amount of the benzyl product is dissolved in an alcohol such as ethanol or methanol, and a drop of sodium hydroxide is added. A purple color indicates the reaction did not reach completion, but if no color change is visible, the benzoin has been fully oxidized.