Methanediol

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. It is not encountered in pure form, but as aqueous solutions, where it coexists with oligomers (short polymers). The compound is well characterized but of less technological significance than related paraformaldehyde ((CH2O)n), formaldehyde (H2C=O), 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)nH.
Occurrence.
The dianion, methanediolate, is believed to be an intermediate in the crossed Cannizzaro reaction.
The compound is of some relevance to astrochemistry.
Safety.
Methanediol, rather than formaldehyde, is listed as one of the main ingredients of "Brazilian blowout", a hair-straightening formula marketed in the US. The equilibrium with formaldehyde has caused concern since formaldehyde in hair straighteners is a health hazard, but the risk has been disputed.