ALDEHYDES - GENERAL METHODS OF PREPARATION I. Direct synthesis. Arising from a study of chemical reactions under high pressure it has been found that when carbon monoxide and hydrogen are heated together at 36o-38o° C. under a pressure of 200 atmospheres in the presence of a mixed catalyst consisting of the oxides of cobalt, copper, maaganese and zinc, a mixture of aliphatic aldehydes is produced among which formaldehyde, acetaldehyde, propionaldehyde and higher aldehydes have been identified (see PRESSURE CHEMISTRY).
2. Action of ozone on olefines. During an investigation of the action of ozone on various organic substances, C. D. Harries discovered that this active form of oxygen has a specific action on carbon compounds containing a double or ethylenic linkage. A molecular proportion of ozone attaches itself to the carbon atoms of the double linking forming an ozonide, which on treat ment with water yields an aldehyde and an aldehyde peroxide, the latter becoming transformed into an acid. In the simplest case of ethylene this gas furnishes an explosive ozonide which subse quently gives rise to formaldehyde and formic acid, A similar ozonization applied to more complex olefines has led to aldehydes which are otherwise prepared only with difficulty: 3. Dehydrogenation of primary alcohols. The primary alcohols may be dehydrogenated either thermally or by oxidation. In the former process the alcohol is passed over a heated metallic cata lyst, preferably copper, although nickel, palladium or platinum or such oxides at vanadium pentoxide, may be employed. The oxidation of a primary alcohol to an aldehyde is generally ef fected by aqueous chromic acid (a solution of sodium bichromate and dilute sulphuric acid), but other oxidizing agents such as manganese dioxide and dilute sulphuric acid, or even atmospheric oxygen in presence of platinum black or bone black, may be employed.
4. Synthesis through the Grignard reagent (q.v.). A method of wide application for either aliphatic or aromatic aldehydes depends on the employment of the well-known Grignard reaction with ethyl formate, an ester which it will be noticed contains the characteristic aldehydic group HCO. Thus when the formate is treated successively with magnesium ethyl iodide (the Grignard reagent from magnesium and ethyl iodide) and water, propionaldehyde is produced. A similar condensation on ethyl formate with magnesium phenyl bromide leads to benzalde hyde. In general the Grignard synthesis of aldehydes may be r by the following equations, when R is an organic tadical and X a halogen (usually iodine or bromine) : 5. Thermal decomposition of a-hydroxycarboxylic acids. This process, discovered by H. R. Le Sueur, has been worked out chiefly with the higher fatty acids. For example, stearic acid is converted successively into a- bromostearic and a-hydroxystearic acids. The latter acid, on heating at 270° C., yields margaric aldehyde CHO and formic acid. Incidentally this method forms a means of passing from common acids such as stearic and palmitic acids, which are readily obtained from natural sources, to acids such as margaric acid, C16H33 • COOH, which do not occur in nature.
6. The metallic formates contain the aldehydic group CHO and another general method for aldehydes depends on the heating to gether of an intimate mixture of calcium formate and the calcium salt of some other acid.
7. Aromatic aldehydes from phenols. A long-known process for the conversion of phenols into hydroxy-aryl-aldehydes is asso ciated with the names of C. L. Reimer and F. Tiemann. It consists in treating the phenolic compound with chloroform in presence of dilute alkalis. With phenol itself the reaction proceeds as follows: Two isomerides are produced, parahydroxybenzaldehyde and sali cylaldehyde (orthohydroxybenzaldehyde). The latter isomeride occurs in the volatile oils of Spiraea and in the glucoside, salicin, of the willow.
8. Syntheses by the aid of compounds containing bivalent car bon. The two syntheses described under this heading, which are due to L. Gattermann, are, like those of the preceding section, applicable only to the aromatic series. The first depends on the use of carbon monoxide, this gas and hydrogen chloride being passed into a solution of an aromatic hydrocarbon in a dry sol vent in the presence of cuprous and aluminium chlorides. In the case of toluene the condensation gives rise to p-tolualdehyde. In all probability the carbon monoxide absorbed by the cuprous chloride forms an intermediate compound, H•COC1, with the hydrogen chloride and this nascent formyl chloride under the in fluence of aluminium chloride condenses with the hydrocarbon to give rise to the aldehyde : The second Gattermann synthesis involves the use of hydrogen cyanide (anhydrous prussic acid), and since this reagent is now procurable on a manufacturing scale (see PRUSSIC ACID) the synthesis offers industrial possibilities. The aromatic substance, which may be a hydrocarbon or a phenol, is dissolved in a dry sol vent containing anhydrous aluminium or zinc chloride and to this mixture hydrogen cyanide is added while hydrogen chloride is bubbled in to saturation when the two hydrides interact poten tially as HN :CHC1: The aldehyde-imine thus formed as an intermediate product is hydrolysed when boiled with water into ammonia and the corre sponding aldehyde, In this connection may be mentioned the synthesis of benzaldoxime by R. Scholl who treated benzene with mercury fulminate and partially hydrated aluminium chloride. The condensation may be regarded as occurring between fulminic acid (q.v.) and the hydrocarbon, The three preceding reactions, including the two Gattermann syntheses and Scholl's condensation, represent the intervention of bivalent carbon as exemplified by carbon monoxide, CO, hydro gen cyanide, C : NH, and fulminic acid, C : NOH, respectively.
General Reactions of Aldehydes. The aldehydes combine addi tively with sodium hydrogen sulphite to furnish bisulphite com pounds, These compounds, being crystalliz able and easily hydrolysed by dilute acid, are frequently used in the purification of the aldehydes. Sodium hydrosulphite also interacts with aldehydes giving rise to compounds of the general formula R•CH (OH) The formaldehyde compound is the important reducing agent, Rongalite, employed in calico printing for dis charging the colour of dyes.
Aldehydes react with hydrogen cyanide to form cyanohydrins, R•CH(OH)•CN, which are hydrolysable to a-hydroxycarboxylic acids containing one more carbon atom than the original aldehyde.
Aldehydes combine with hydroxylamine to form oximes (q.v.), and with phenylhydrazine to phenylhydrazones and with semicar bazide to crystallisable semicarbazones (see CHEMISTRY, Or ganic). Nascent hydrogen reduces aldehydes to primary alco hols, whereas with Grignard reagents they are converted into sec ondary alcohols. Formaldehyde condenses with aqueous ammonia to form hexamethylenetetramine, a valuable diuretic (hexamina B.P.) whereas the other aliphatic aldehydes combine with ethereal ammonia to give aldehyde-ammonias (hydroxya mines) of the type from which the aldehyde can be regenerated by distillation with dilute acid. Aldehydes in gen eral are characterized by the colour reaction of H. Schiff, this be ing the reddish-violet coloration developed with a solution of magenta or fuchsine which has been decolorized by sulphurous acid. (G. T. M.)