prepration of di and poly hydric alcohols

Polyhydric Alcoholsalcoholsofthefattyseriesthatcontainseveral—OHgroupsinthemolecule.Polyhydricalcohols,aswellasotherpolyatomiccompoundscontainingmorethanonefunctionalgrouppermolecule,areclassifiedasdihydric(glycols),trihydric(glycerols),tetrahydric(tetritols),pentahydric(pentitols),hexahydric(hexitols),andsoon.Themostimportantalcoholsthatcontainnofewerthanfour—OHgroupsarepentaerythritol,C(CH2OH)4,andthepentitols(forexample,xylitol,adonitol,andarabitol)andhexites(mannitol,sorbitol,anddulcitol),whicharegeneticallyrelatedtomonosaccharides.

Polyhydricalcoholsarecolorlesscrystallinesubstanceswithasweettastethatarereadilysolubleinwater.Manyofthemaresynthesizedinplants.Alargenumberofstereoisomersareknowntoexistforeachtypeofalcohol.Polyhydricalcoholsexhibitallthepropertiescharacteristicofmonohydricalcohols(forexample,theyreadilyundergoesterificationandoxidation).Nitratesofpolyhydricalcoholsareexplosive.

Polyhydricalcoholsareusuallypreparedcommerciallybythereductionofaldosesandketoses.Theyareusedinthemanufactureofpolymers(pentaerythritolandxylitol)andexplosives,andalsoassugarsubstitutesfordiabetics(sorbitolandxylitol).Theyarealsousedasmoisteningagentsinthemanufactureofcosmeticsandpharmaceuticals;theirethersareusedasemulsifiers.
Diols are prepared from alkenes byoxidationwith reagents such as osmium tetroxide, potassium permanganate, or hydrogen peroxide. However, ethylene glycol is made on a commercial scale from oxacyclopropane, which in turn is made by airoxidation of etheneat high temperatures over a silver oxide catalyst.

Ethylene glycol has important commercial uses. It is an excellent permanent antifreeze for automotive cooling systems because it is miscible with water in all proportions and a 50% solution freezes at —34° (—29°F). It also is used as a solvent and as an intermediate in the production of polymers (polyesters) and other products.

The trihydric alcohol, 1,2,3-propanetriol (glycerol), is a nontoxic, water-soluble, viscous, hygroscopic liquid that is used widely as a humectant (moistening agent). It is an important component of many food, cosmetic, and pharmaceutical preparations. At one time, glycerol was obtained on a commercial scale only as a by-product of soap manufacture through hydrolysis of fats, which are glyceryl esters of long-chain alkanoic acids. The major present source is bysynthesis from propene.