Run-of-the-mill liquid bleach, aqueous NaOCl, is an attractive green option for industrial oxidations. It’s cheap, doesn’t tend toward explosive reactions like hydrogen peroxide, doesn’t require metal catalysts, and its waste product is table salt. But it is tough to work with and inefficient on a large scale. Now researchers have determined that an extra-pure, crystallized bleach—sodium hypochlorite pentahydrate—oxidizes more efficiently, often with higher yield and selectivity, than dilute aqueous bleach (Org. Process Res. Dev. 2017, DOI:10.1021/acs.oprd.7b00288).
Aqueous bleach readily breaks down, so every use requires a titration to determine its current strength. Large-scale reactions also require huge volumes because NaOCl can’t be used at concentrations higher than 13% by weight; higher concentrations degrade too quickly to be useful. Finally, its high pH 13 slows down many reactions.
A few years ago, researchers at Nippon Light Metal (NLM) figured out a way to manufacture a more concentrated and stable form of NaOCl. Sodium hypochlorite pentahydrate was first described in 1919, but not until Nippon researchers found a way to make it on an industrial scale in 2013 did it become practical for industry. The light-yellow crystal is 44% NaOCl by weight, has a pH of around 11, and is stable for a year when stored in the refrigerator.
In new research funded by the company, Yoshikazu Kimura of Iharanikkei Chemical Industry Company, Masayuki Kirihara of Shizuoka Institute of Science & Technology, and coworkers, including at NLM, explored using sodium hypochlorite pentahydrate versus aqueous bleach in a variety of industrial oxidations.
The crystal is “a superior substitute” to aqueous bleach, Kimura says, because of its higher efficiency, stability, and ease of use. The catalyzed oxidation of bulky secondary alcohols to ketones is “an especially remarkable example,” Kimura adds. When catalyzed by tetrabutylammonium hydrogen sulfate and a nitroxyl-radical-based catalyst known as TEMPO, oxidation of 2-octanol to 2-octanone with liquid bleach yields only 11% after 22 hours. But with the crystalline form, the yield was 97% after just one hour. The crystal also made possible some reactions that don’t work with aqueous bleach at all, such as oxidative cleavage of trans-diols to diketones.