ALDEHYDES, an important class of chemical compounds having the general formula R.CHO, where R is an alkyl (aliphatic) or aryl (aromatic) radical. The name is derived from alcohol dehydrogenatus (J. Liebig), indicating that these compounds may be obtained from primary alcohols by removal of hydrogen RCH2 • OH — 2H = R • CHO.
The group CHO which is to be found in all aldehydes contains the most active form of the carbonyl radical, CO, known to chemists, and it is to the intense reactivity of this radical that the aldehydes owe their importance as organic synthetic agents both in nature and in the chemical laboratory.
Aldehydes are generally named specifically after the acids into which they are converted by oxidation, thus formaldehyde, acetal dehyde and benzaldehyde furnish formic, acetic and benzoic acids, respectively. In more complicated cases, the termination al is em ployed to denote aldehydic functions, according to the Geneva convention of chemists, thus geranial, the aldehyde of lemon-grass oil, is the oxidation product of the alcohol, geraniol, contained in geranium oil (see TERPENES)..
Many aldehydes are known to chemistry, some being produced artificially, whereas numerous others are found in the vegetable kingdom either free (e.g., cinnamaldehyde in cinnamon oil) or combined in certain glucosides (e.g., benzaldehyde in amygdalin, see GLUCOSIDES).
Acetaldehyde, prepared by oxidation of ethyl alcohol and by direct synthesis from carbon monoxide and hydrogen (v. infra), is a volatile liquid boiling at 2I ° C. and miscible in all proportions with water. It is readily polymerized catalytically by traces of zinc chloride, hydrochloric acid, etc., at the ordinary temperature to para-acetaldehyde or "paraldehyde" a liquid boiling at 124° and only moderately soluble in water. Paraldehyde, em ployed medicinally as a sedative without action on the heart, is administered intravenously as an anaesthetic or hypnotic. When the foregoing polymerization of acetaldehyde takes place in a freezing mixture, metaldehyde (meta-acetaldehyde) is produced as a white crystalline solid subliming at 112-115° C. This sub stance is insoluble in water and when heated in a sealed tube at is reconverted into the monomeric form. Metaldehyde, which is produced commercially, has been recommended as a smokeless fuel, replacing the spirit lamp for toilet purposes.
The simultaneous chlorination and oxidation of ethyl alcohol leads to trichloroacetaldehyde which in its hydrated state is the well-known drug Chloral Hydrate (q.v.).
Aldolisation.—Thecondensation of formaldehyde and acet aldehyde into their polymeric forms is a chemical change which is readily reversible by thermal dissociation. But in addition to this reversible change, aldehydes have the capacity for undergoing a polymerization in which the carbon atoms of separate molecules become directly attached so that the process becomes irreversible. This mode of polymerization, which is termed aldolisation, un doubtedly plays an important part in the building up of carbon compounds in the green plant, following on the assimilation of carbon dioxide by photosynthesis. Aldol (f-hydroxybutaldehyde) is formed by a polymerization of two molecules of acetaldehyde: By loss of water aldol passes into the unsaturated crotonaldehyde, : CH•CHO.
This adding of carbon to carbon occurs in a marked degree in the case of formaldehyde and the process is instrumental in the vegetable kingdom in the building up of complex carbohydrates (sugars, starches, cellulose) from formaldehyde, the simplest car bohydrate, The aldolisation of formaldehyde has been effected in the laboratory; a sixfold polymerization in the presence of magnesia (Loew) furnished a- and 0-acroses, two synthetic sugars which have since been identified as inactive fructose and inactive sorbose respectively.
Resinification.—Acetaldehydeand certain of its homologues are converted into resins under the influence of alkalis. Acrolein, : CH•COH, the simplest of the unsaturated aldehydes, is ob tained by abstraction of water from glycerine by the agency of alkali bisulphates and phosphoric or boric acid. It is a colourless liquid boiling at 52.4° C. with a disagreeable, tear-exciting odour. In the presence of traces of acids, it readily undergoes polymeriza tion to form a solid resinous material, metacrolein (see RESINS, SYNTHETIC). Acrolein forms a dibromide from which E. Fischer by the action of baryta water obtained the synthetic sugars, s and (3-acroses.
Benzaldehyde, the most important commercially of the aro matic aldehydes, is usually made from the chlorinated deriva tives of toluene. Benzyl chloride, heated with que ous lead or cupric nitrate furnishes benzaldehyde, which is also prepared by heating benzal or benzylidene chloride, with milk of lime. As prepared by either method, benzaldehyde is contaminated with traces of chlorinated benzaldehydes, and when chlorine-free benzaldehyde is needed it is prepared by the direct oxidation of toluene by chemical means (manganese di oxide and sulphuric acid) or by electrolytic means (an acetone sulphuric acid solution with platinum electrode).
Benzaldehyde is a colourless, highly refractive liquid boiling at 179-18o° with the characteristic odour of oil of bitter almonds of which oil it is the essential constituent, being derived from the hydrolysis of the glucoside, amygdalin, present in the almond. it readily absorbs oxygen from the air, becoming oxidized to benzoic acid. With aqueous caustic alkalis it undergoes the Cannizzaro reaction, = yielding mole cular proportions of benzyl alcohol and benzoic acid. With alco holic potassium cyanide, it gives benzoin, Benzaldehyde is also an important intermediate in the pro duction of colouring matters (see DYES, SYNTHETIC, and MALA CHITE). When condensed with acetic anhydride and dry sodium acetate, it yields cinnamic acid, whereas cinnamaldehyde arises from a condensation between benzaldehyde and acetaldehyde in the presence of sodium ethoxide.
aromatic cyclic aldehyde (see CHEMISTRY : Organic, Hetero cyclic), formerly obtained by the hydrolysis of bran with dilute mineral acid and now manufactured in the U.S.A. by the action of steam and dilute acid on oat hulls and corn cobs and stalks contained in large digestors. The volatile furfuraldehyde (b.p. 16o-162 °) is isolated and purified by fractional distillation.