Polyhydric alcohols

Alcohols with several –OH groups are polyhydric alcohols. The dihydric member is ethylene glycol HOCH₂–CH₂OH (ethane-1,2-diol), the trihydric one is glycerol HOCH₂–CH(OH)–CH₂OH (propane-1,2,3-triol). Each –OH sits on a separate carbon: two –OH on one carbon is unstable and loses water to give an aldehyde. They are sweet viscous liquids, very soluble in water; ethylene glycol is poisonous (never to be drunk), glycerol is used in cosmetics and medicine. Like monohydric alcohols they react with sodium: HOCH₂CH₂OH + 2Na → NaOCH₂CH₂ONa + H₂; HCl replaces –OH by chlorine. Qualitative test for polyhydric alcohols: the blue precipitate of freshly prepared copper(II) hydroxide Cu(OH)₂ dissolves in them to give a clear deep-blue solution (a complex). Glycerol with nitric acid forms nitroglycerin (an ester); it is explosive, and in tiny doses is used in medicine as a heart drug. Preparation: hydrolysis of 1,2-dichloroethane, oxidation of ethylene with KMnO₄, and glycerol by hydrolysis of fats. Ethylene glycol is used as antifreeze and as a raw material for polymers (lavsan).