# 1911 Encyclopædia Britannica/Aldehydes

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​ALDEHYDES, a class of chemical compounds of the general formula R·CHO (R＝an alkyl or an aryl group). The name is derived from alcohol dehydrogenatum in allusion to the fact that they may be prepared by the oxidation of alcohols. The lower members of the series are neutral liquids possessing a characteristic smell; they are soluble in water and are readily volatile (formaldehyde, however, is a gas at ordinary temperatures). As the carbon content of the molecule increases, they become less soluble in water, and their smell becomes less marked with the increase in boiling point, the highest members of the series being odourless solids, which can only be distilled without decomposition in vacuo.
The aldehydes may be prepared by the careful oxidation of primary alcohols with a mixture of potassium dichromate and sulphuric acid,—3R·CH$_2$OH＋K$_2$Cr$_2$O$_7$＋4H$_2$SO$_4$＝K$_2$SO$_4$＋Cr$_2$(SO$_4$)$_3$＋7H$_2$O＋3R·CHO; by distilling the calcium salts of the fatty acids with calcium formate; and by hydrolysis of the acetals. L. Bouveault (Bull. soc. chim., 1904 [3], 31, p. 1306) prepares aldehydes by the gradual addition of disubstituted formamides (dissolved in anhydrous ether) to magnesium alkyl haloids, the best yields being obtained by the use of diethyl formamide. Secondary reactions take place at the same time, yielding more particularly hydrocarbons of the paraffin series. G. Darzens (Comptes Rendus, 1904, 139, p. 1214) prepares esters of disubstituted glycidic acids, by condensing the corresponding ketone with monochloracetic ester, in the presence of sodium ethylate. These esters on hydrolysis yield the free acids, which readily decompose, with loss of carbon dioxide and formation of an aldehyde,
| OC | R R$_1$ | ＋Cl·CH$_2$·COOC$_2$H$_5$ → O | CRR$^1$ | CH·COOC$_2$H$_5$ | → O | CRR$^1$ | CH·COOH | → CO$_2$＋CHRR$^1$·CHO. |
In the German Patent 157573 (1904) it is shown that by the action of at least two molecular proportions of an alkyl formate on two molecular proportions of a magnesium alkyl or aryl haloid, a complex addition compound is formed, which readily decomposes into a basic magnesium salt and an aldehyde,
C$_6$H$_5$MgBr＋HCOOR → RO·CH·C$_6$H$_5$·OMgBr → MgBr·OR＋C$_6$H$_5$CHO.
The aldehydes are characterized by their great chemical reactivity. They act as reducing agents, silver nitrate in the presence of ammonia being rapidly reduced to the condition of metallic silver. They are easily oxidized to the corresponding fatty acid, in many cases simply by exposure to air. Nascent hydrogen reduces them to primary alcohols, and phosphorus pentachloride replaces the carbonyl oxygen by chlorine. They form many addition compounds, combining with ammonia to form aldehyde ammonias of the type R·CH(OH)·NH$_2$. These are colourless crystalline compounds, which are most readily prepared by passing ammonia gas into an ethereal solution of the aldehyde. With sodium bisulphite they form the so-called bisulphite compounds R·CH(OH)·SO$_3$Na, which are readily resolved into their components by distillation with dilute acids, and are frequently used for the preparation of the pure aldehyde.
With hydrocyanic acid aldehydes form the cyanhydrins R·CH(OH)·CN. They react with hydroxylamine and phenylhydrazine, with the formation of aldoximes and hydrazones. (For the isomerism of the aldoximes see Oximes). The hydrazones are crystalline substances which are of value in the characterization of the aldehydes. Both oximes and hydrazones, on boiling with dilute acid, regenerate the parent aldehyde. The hydrazones are best prepared by mixing the aldehyde with phenylhydrazine in dilute acetic acid solution, in the absence of any free mineral acid. Semioxamazid, NH$_2$·CO·CO·NH·NH$_2$, has also been employed for the identification of aldehydes (W. Kerp and K. Unger, Berichte, 1897, 30. p. 585). Aldehydes are converted into resins by the action of caustic alkalies. On heating with alcohols to 100° C. they form acetals, and they also form condensation products with para-amido-di-methyl-aniline (A. Calm, Berichte, 1884, 17, p. 2939). They react with the zinc alkyls to form addition products, which are decomposed by water with formation of secondary alcohols (K. Thurnlach, Annalen, 1882, 213, p. 369) thus:—
| | Zn(C$_2$H$_5$)$_2$ | | | H$_2$O |
| CH$_3$·CHO | → | CH$_3$·CH | C$_2$H$_5$ | →CH$_3$·CH | C$_2$H$_5$＋ZnO＋C$_2$H$_6$. |
| CH$_3$·CHO | → | CH$_3$·CH | OZnC$_2$H$_5$ | →CH$_3$·CH | OH |
The reaction is a general one for all aldehydes with zinc methyl and zinc ethyl, but not with the higher zinc alkyls. V. Grignard (Comptes Rendus, 1900 et seq.) showed that aldehydes combine with magnesium alkyl iodides (in absolute ether solution) to form addition products, which are decomposed by water with the ​formation of secondary alcohols, thus from acetaldehyde and magnesium methyl iodide, isopropyl alcohol is obtained.
| | | H$_2$O |
| CH$_3$·CHO＋CH$_3$MgI → CH$_3$·CH | CH$_3$ OMgI | → (CH$_3$)$_2$CH·OH＋MgI·OH. |
The lower members of the aliphatic series are characterized by their power of polymerization (see Formalin, and the account of Acetaldehyde below), and also by the so-called “aldol” condensation, acetaldehyde in this way forming aldol, CH$_3$·CHOH·CH$_2$·CHO. These aldols generally lose the elements of water readily and pass into unsaturated compounds; aldol itself on distillation at ordinary atmospheric pressure gives crotonaldehyde, CH$_3$·CH : CH·CHO.
Aldehydes are characterized by the reddish-violet colour which they give with a solution of fuchsine that has been decolorized by sulphurous acid (H. Schiff, Ann., 1866, 140, p. 131). With diazobenzene sulphonic acid in the presence of alkali and a trace of sodium amalgam, a reddish-violet coloration is formed on standing (E. Fischer, Ber., 1883, 16, p. 657). A. Angeli (Gazz. chim. Ital., 1896, 22, ii, 17) has shown that aldehydes in the presence of nitrohydroxylaminic acid form hydroxamic acid. The aldehydes condense readily with acetoacetic ester in the presence of ammonia, to pyridines (see Pyridine), whilst O. Doebner and W. v. Miller (Ber., 1892, 25, p. 2864; 1896, 29, p. 59) have shown that in the presence of aniline and sulphuric acid they give substituted quinolines. (See also C. Beyer, Ber., 1887, 20, p. 1908). The chief aldehydes are shown in the following table:—
For formaldehyde see Formalin. Acetaldehyde, CH$_3$·CHO, was first noticed by C. Scheele in 1774 and isolated and investigated by J. v. Liebig (Annalen, 1835, 14, p. 133). It is prepared by oxidizing ethyl alcohol with dilute sulphuric acid and potassium bichromate, and is a colourless liquid of boiling point 20·8° C., possessing a peculiar characteristic smell. Its specific gravity is 0·8009 (0° C.). It is miscible in all proportions with alcohol, ether and water. It is readily polymerized, small quantities of hydrochloric acid, zinc chloride, carbonyl chloride, &c. converting it, at ordinary temperatures, into paraldehyde, (C$_2$H$_4$O)$_3$, a liquid boiling at 124° C. and of specific gravity 0·998 (15° C.). Paraldehyde is moderately soluble in water, and when distilled with sulphuric acid is reconverted into the ordinary form. Metaldehyde, (C$_2$H$_4$O)$_3$, is produced in a similar way to paraldehyde, but at lower temperatures (e.g. in presence of a freezing mixture). It is a crystalline solid, which sublimes at 112°—115° C. It is insoluble in water, and is only slightly soluble in alcohol and ether. When heated in a sealed tube at 120° C. it is completely converted into the ordinary form. Paraldehyde is oxidized by dilute nitric acid, with formation of much glyoxal, (CHO)$_2$. (For trichloracetaldehyde see Chloral.)
By the action of acetaldehyde on alcohol at 100° C., acetal, CH$_3$·CH(OC$_2$H$_5$)$_2$, is produced. It may also be prepared by oxidizing ethyl alcohol with manganese dioxide and sulphuric acid (A. Wurtz). It is a colourless liquid of specific gravity 0·8314 (20°/4°) (J. W. Brühl) and boiling point 104° C. Dilute acids readily transform it into alcohol and aldehyde, and chromic acid oxidizes it to acetic acid. Chlor- and brom-acetals have been described.
Thioaldehydes are also known, and are obtained by leading sulphuretted hydrogen into an aqueous solution of acetaldehyde. By this means a mixture is obtained which by distillation or the action of hydrochloric acid yields trithioaldehyde, (C$_2$H$_4$S)$_3$. For the constitution of these substances see E. Baumann and E. Fromm (Berichte, 1891, 24, p. 1426). Aldehyde ammonia, CH$_3$·CH(OH)·NH$_2$, is formed when dry ammonia gas is passed into an ethereal solution of acetaldehyde. It crystallizes in glistening rhombohedra, melting at 70°–80° C., and boiling at 100° C. It is completely resolved into its components when warmed with dilute acids.
The higher aldehydes of the series resemble acetaldehyde in their general behaviour. Unsaturated aldehydes are also known, corresponding to the olefine alcohols; they show the characteristic properties of the saturated aldehydes and can form additive compounds in virtue of their unsaturated nature. The simplest member of the series is acrolein, C$_3$H$_4$O or CH$_2$ : CH·CHO, which can be prepared by the oxidation of allyl alcohol, or by the abstraction of the elements of water from glycerin by heating it with anhydrous potassium bisulphate. It is also produced by the action of sodium on a mixture of epichlorhydrin and methyl iodide, C$_3$H$_5$OCl＋CH$_3$I＋2Na＝C$_3$H$_4$O＋NaI＋NaCl＋CH$_4$. It is a colourless liquid, with a very pungent smell, and attacks the mucous membrane very rapidly. It boils at 52·4° C. and is soluble in water. It oxidizes readily: exposure to air giving acrylic acid, nitric acid giving oxalic acid, bichromate of potash and sulphuric acid giving carbon dioxide and formic acid. It combines with bromine to form a dibromide, from which E. Fischer, by the action of baryta water, obtained the synthetic sugars α- and β-acrose (Berichte, 1889, 22, p. 360). Metacrolein, (C$_3$H$_4$O)$_3$, is a polymer of acrolein. By passing acrolein vapour into ammonia, acrolein ammonia, C$_6$H$_9$NO, is obtained. It is a reddish amorphous mass, insoluble in alcohol, and when distilled yields picoline (methyl pyridine) (A. Baeyer, Ann., 1870, 155, p. 283). Citronellal, rhodinal and geranial are also unsaturated aldehydes (see Terpenes).
The aromatic aldehydes resemble the aliphatic aldehydes in most respects, but in certain reactions they exhibit an entirely different behaviour. They do not polymerize, and in the presence of caustic alkalies do not resinify, but oxidize to alcohols and acids (see Benzaldehyde for Cannizzaro’s reaction). When heated with alcoholic potassium cyanide they are converted into benzoins (q.v.). Vanillin does not give the Cannizzaro reaction, but with alcoholic potash forms vanillic acid, HOOC(1)·C$_6$H$_3$·OCH$_3$(3)·OH(4), and vanilloin. With ammonia, benzaldehyde does not form an aldehyde ammonia, but condenses to hydrobenzamide, (C$_6$H$_5$CH)$_3$N$_2$, with elimination of water. Cumic aldehyde (cuminol), (CH$_3$)$_2$CH(1)C$_6$H$_4$·CHO(4), is found in Roman caraway oil and in oil of the water hemlock. It is a liquid, boiling at 235° C., and has a specific gravity of 0·973. On distillation with zinc dust it forms cymene (1·4 methyl isopropyl benzene).
Salicylic aldehyde (ortho-hydroxybenzaldehyde), HO(1)·C$_6$H$_4$·CHO(2), an aromatic oxyaldehyde, is a colourless liquid of boiling point 196° C. and specific gravity 1·172 (15°). It is found in the volatile oils of Spiraea, and can be obtained by the oxidation of the glucoside salicin, (C$_{13}$H$_{18}$O$_7$), which is found in willow bark. It is usually prepared by the so-called “Reimer” reaction (Ber, 1876, 9, p. 1268), in which chloroform acts on phenol in the presence of a caustic alkali,
C$_6$H$_5$OH＋CHCl$_3$＋4KHO＝3KCl＋3H$_2$O＋KO·C$_6$H$_4$·CHO,
some para-oxybenaldehyde being formed at the same time. It ​is volatile (para-oxybenzaldehyde is not) and gives a violet coloration with ferric chloride. For dioxybenzaldehydes and their derivatives see Piperonal and Vanillin.
Cinnamic aldehyde (β-phenyl acrolein), C$_6$H$_5$·CH:CH·CHO, an unsaturated aromatic aldehyde, is the chief constituent of cinnamon oil. It is prepared by oxidizing cinnamyl alcohol, or by the action of sodium ethylate on a mixture of benzaldehyde and acetaldehyde. It is a colourless aromatic-smelling oily liquid, which boils at 247° C. and readily oxidizes on exposure.
By condensation of aldehydes with pyruvic acid and naphthylamines, the α-alkyl-naphthoquinoline-γ-carboxylic acids are produced; the same reaction takes place with the aromatic amines generally (O. Doebner, Ann. 1894, 281, p. 1),