ALDEHYDE (*dehydrogenated alcohol,' or alcohol which has been deprived of a portion of its hydrogen), a substance intermediate in composition between a primary alcohol and the corresponding acid. When an alcohol (q.v.) containing the molecular group CHLOH is acted upon by oxidizing agents, it loses two atoms of hydrogen from this group and becomes trans formed into a substance which no longer con tains the hydroxyl group (OH), and which is known as the *aldehyde* of the alcohol from which it was produced. In the laboratory al dehyde is formed by the oxidation of alcohol by air in the presence of platinum black or by manganese dioxide and sulphuric acid, and sub sequent rectifying with calcium chloride. It may also be prepared by passing alcohol through an iron tube heated to F. For commer cial use it is collected from the first vapors which come over in the rectification of alcohol which has previously been filtered through char coal. The largest yields are obtained from po tato alcohol and from alcohol made from the juice of the sugar beet. The aldehyde is sep arated from the alcohol in a special still con trived for the purpose. If the formula of the original alcohol is R.CH,.OH, that of the cor
responding aldehyde is R.CO.H. Aldehydes combine with bisulphites (or acid sulphites), producing compounds that are usually soluble m water, but insoluble in a solution of a bi sulphite. Hence if a solution containing an al dehyde is shaken with a saturated solution of a bisulphite (such as HNaS0.), the aldehyde is all thrown down in the form of an insoluble compound, from which the aldehyde itself may afterward be liberated by treatment with dilute sulphuric acid and distillation by steam. Alde hydes are easily oxidized into their correspond ing acids and on account of their affinity for oxygen • they act as powerful reducing agents. An aldehyde may also be reconverted into the alcohol from which it was obtained, by the action of sodium amalgam. About 50 aldehydes are known, nearly all of which are volatile liquids.
The general relation of the aldehydes to their corresponding alcohols and acids may be illus trated by the following examples : The formula of methyl alcohol is CH..OH or H.CHLOH. In the presence of platinum black, air oxidizes methyl alcohol in accordance with the following equation: