Methanediol, also known as formaldehyde monohydrate or methylene glycol, is an organic compound with chemical formula CH2(OH)2. It is the simplest geminal diol. In aqueous solutions it coexists with oligomers (short polymers). The compound is closely related and convertible to the industrially significant derivatives paraformaldehyde ((CH2O){n}), formaldehyde (H2C\dO), and 1,3,5-trioxane ((CH2O)3). Methanediol is a product of the hydration of formaldehyde. The equilibrium constant for hydration is estimated to be 103, CH2(OH)2 predominates in dilute (<0.1%) solution. In more concentrated solutions, it oligomerizes to HO(CH2O){n}H.

Occurrence

The dianion, methanediolate, is believed to be an intermediate in the crossed Cannizzaro reaction. Gaseous methanediols can be generated by electron irradiation and sublimation of a mixture of methanol and oxygen ices. Methanediol is believed to occur as an intermediate in the decomposition of carbonyl compounds in the atmosphere, and as a product of ozonolysis on these compounds.

Safety

Methanediol, rather than formaldehyde, is listed as one of the main ingredients of "Brazilian blowout", a hair-straightening formula marketed in the United States. The equilibrium with formaldehyde has caused concern since formaldehyde in hair straighteners is a health hazard. Research funded by the Professional Keratin Smoothing Council (PKSC), an industry association that represents selected manufacturers of professional-use only keratin smoothing products, has disputed the risk.