The two aldehydes of greatest importance in industry, formaldehyde and acetaldehyde, have complicated behavior because of their tendency to oligomerize or polymerize. Formaldehyde in particular is sold as the polymer paraformaldehyde ((C2H5O)"n", typically "n" = 50 to 100) as well as the trimer 1,3,5-trioxane (metaformaldehyde). In addition to the inconveniently low-boiling monomer (b.p. 21 °C), acetaldehyde is available as the trimer paraldehyde (a sedative and anticonvulsant) and tetramer metaldehyde (a slug and snail poison). In general, higher aliphatic aldehydes will accumulate a substantial amount of oligomer (mostly trimer) upon long-term storage and must be freshly distilled when a reaction calls for the monomeric starting material. They also tend to hydrate, forming the geminal diol. Formaldehyde is often sold as the aqueous solution, formalin, which is mostly 1,1-methanediol, with a small amount of methanol added as stabilizer. The oligomers/polymers and the hydrates exist in equilibrium with the parent aldehyde, and for "some" synthetic procedures, they can serve as substitutes for the anhydrous monomer.