Aldehyde, , is a volatile fluid produced by the oxidation and destructive distillation of alcohol and other organic compounds. There are many modes of obtaining it; the following is the method described by Liebig: A mixture of 2 lb. of strong alcohol, 2 lb. of water, and 3 lb. of sulphuric acid, is distilled into a receiver kept cool by ice. As soon as the distillate reddens litmus-paper, the operation is stopped. The product in the receiver, weighing about 3 lb., is then twice rectified over chloride of calcium, being reduced by these operations to about 12 oz. This is then mixed with twice its volume of ether, and saturated with ammonia gas. After cooling, crystals of aldehyde ammonia, , are formed, which are mixed with dilute sulphuric acid, and distilled at a low temperature. The hydrated aldehyde thus obtained is dried with chloride of calcium and again rectified by distillation. The aldehyde thus prepared is a thin, transparent, colourless liquid, very inflammable, burning with a blue flame, and having a spec. gr. of .800, a boiling-point of about (), and a pungent, suffocating odour. It mixes in all proportions with water, alcohol, and ether, and dissolves sulphur, phosphorus, and iodine. As is shown in the article ALCOHOL, it constitutes an intermediate stage in the oxidation of alcohol into acetic acid. When potassium is gently heated with aldehyde, one atom of H is replaced by one of K, the resulting compound being aldehydiate of potash, . Various compounds of this kind may be formed, of which the most important is aldehydiate of ammonia, or aldehyde-ammonia, , which is obtained in transparent shining crystals, and is a compound that has led chemists to the discovery of a large number of very remarkable derivatives.
ALDEHYDES are a class of organic compounds intermediate between primary alcohols and acids. Each aldehyde is derived from the corresponding alcohol by the abstraction of two atoms of hydrogen, and each aldehyde is converted into its corresponding acid by the addition of one atom of oxygen.
Ten aldehydes of the series , corresponding to , and 16, are at present known, the simplest being formic aldehyde, , and the highest being palmitic aldehyde, .
Amongst aldehydes not connected with the preceding group may be mentioned various organic compounds which have been recently shown to belong to this class—thus, acrolein, , is acrylic aldehyde; camphor, , is campholic aldehyde; bitter-almond oil, , is benzoic aldehyde; oil of cumin, , is cuminic aldehyde; oil of cinnamon, , is cinnamic aldehyde. Most of these aldehydes are obtained directly from plants, and either exist in them ready formed, or are given off as volatile oils on distillation with water. Owing to their great tendency to oxidise into their corresponding acids, the aldehydes are powerful reducing agents. They reduce the silver in silver salts to the metallic state. On the other hand, by the action of nascent hydrogen upon the aldehydes, the corresponding alcohols are regenerated. Thus ordinary alcohol may be obtained from ordinary aldehyde.
With the acid sulphites of the alkalies the aldehydes form sparingly soluble crystalline compounds. When treated with caustic alkali, many of the aldehydes are converted into the corresponding alcohols, and the potassium salt of the corresponding acid. Thus benzoic aldehyde yields benzyl alcohol and benzoate of potash:
The aldehydes have a great tendency to form polymeric compounds. Thus ordinary aldehyde passes readily into two polymeric modifications (see ISOMERISM): (1) Par-aldehyde, a liquid which boils at (); (2) Metaldehyde, a solid body which sublimes at (), and is converted back into ordinary aldehyde by heating to () for a few hours in a closed vessel.