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Inorganic Chemistry

The structure of bleach

X-ray crystallography of sodium hypochlorite plugs long-standing gap in literature

by Mark Peplow, special to C&EN
July 31, 2021 | A version of this story appeared in Volume 99, Issue 28


The crystal structure of sodium hypochlorite
Credit: Angew. Chem., Int. Ed.
A crystal of sodium hypochlorite contains alternating layers of hydrated Na+ ions (purple octahedral) and chains of water molecules (oxygen = red; hydrogen = white) flanked by ClO ions (chlorine = green), held together by hydrogen bonds (yellow dashes).

Sodium hypochlorite has been used as a bleaching agent and disinfectant for over 200 years, and manufacturers produce more than 2 billion gallons of aqueous NaOCl every year. Yet despite that long history, chemists have only just determined its crystal structure (Angew. Chem., Int. Ed. 2021, DOI: 10.1002/anie.202108843). “I think it’s one of those things that was hiding in plain sight, that somehow slipped through the cracks,” says McGill University’s Tomislav Friščić, who led the work. Solid sodium hypochlorite is highly hydrated (NaOCl∙5H2O) and rapidly liquefies at room temperature, so Friščić’s team carried out X-ray diffraction on a single crystal of the solid at –100 °C. This revealed alternating layers of hydrated sodium (Na+) and hypochlorite (ClO) ions in the crystal. Each Na+ is surrounded by an octahedron of six water molecules, while the other layer contains a chain of water molecules flanked by two rows of ClO ions. “There is a ton of hydrogen bonds in the structure, which hold everything together,” Friščić says. Interestingly, the chlorine atoms play little part in all this. The team found that sodium hypobromite (NaOBr) has a very similar structure. Both structures were in line with density functional theory analysis and Raman spectroscopy measurements. So could these structures help build a better bleach? “Probably not,” Friščić says with a laugh. “It’s not a breakthrough, but it’s really cute.”


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