The aldehyds are characterized by their remarkable tendency to combine with oxygen, in consequence of which they absorb it by mere exposure to the air, and become acid. Each molecule

of aldehyd absorbs an atom of oxygen, and is converted into the corresponding acid; for example :—

The aldehyds of the form (EH) combine with ammonia, and form with it crystalline compounds which are insoluble in ether. When mixed with a solution of potash, they form with the alkali a brown resinous mass. Many of them when heated with a solution of nitrate of silver, to which a small quantity of ammonia has been added, decompose the salt of silver, and reduce the metal in the form of a mirror-like coating upon the inner surface of the vessel in which they are heated. Another remarkable property of the aldehyds is their power of forming with the acid sulphite of potassium or of sodium crystalline compounds of stable character and sparing solubility. These compounds contain the elements of one atom of acid sulphite of the alkali metal, and one of the aldehyd, with or without water of crystallization, having the general formula (KHSÐ ̧‚¤„H1⁄2„Ð ̧ÑН„Ð).

n 2n

Many essential oils, such as those of bitter almonds, cinnamon, cumin, and spiræa, form similar compounds. These essences are regarded as the aldehyds of a different class of alcohols (1123).

Aniline reacts upon the aldehyds with elimination of water, giving rise to diammonias isomeric with the corresponding compounds derived from the glycols :—

According to Schiff this reaction with aniline is as characteristic of an aldehyd as that with the alkaline bisulphites.

When digested with an amalgam of sodium Wurtz has shown that the aldehyds combine with 2 atoms of nascent hydrogen, and are thus reconverted into their corresponding alcohol.

If the vapour of an aldehyd be transmitted over heated caustic potash, hydrogen is evolved, and the corresponding acid is formed; as for example:

The aldehyds are decomposed by potassium and sodium with evolution of hydrogen, and the formation of a soluble compound with an alkaline reaction, which absorbs oxygen readily; for example :

When hydrocyanic acid is mixed with an aldehyd, and acted upon by hydrochloric acid, a remarkable amido-acid is formed; as for example:

The aldehyds are polymeric with the ethers of the monobasic acid in the series to which they belong, but they yield vapours of but half the density of that of the corresponding compound ether; for example, acetic aldehyd is polymeric with acetic ether; 2 ¤ ̧H ̧Ð=¤ ̧Н ̧¤‚=¤‚н¿‚¤‚Í‚Ð ̧; and valeric aldehyd is poly3 2; meric with amyl valerate: 2 EH100=10H2002=¤ ̧н1‚¤ ̧н ̧Ð ̧ The aldehyds are isomeric with the oxide of the glycol of the same series; for example:

4

8 2

Acetic aldehyd.

Ethylene oxide."

FORMIC AND ACETIC ALDEHYD.

339

They may be prepared synthetically by a reaction first suggested by Piria; viz., by distilling a mixture of equivalent quantities of calcic formiate, with the calcic salt of the acid corresponding to the aldehyd sought; for example:

Calcic formiate.

Calcic valerate.

Calcio carbonate.

Valeric aldehyd.

€a 2 €ᎻᎾ, + €a 2 €.Ꮋ Ꮎ, = 2 Ca€Ꮎ + 2 € HᎾ.

2

2

5 9 2

3

5 10

The constitution of the aldehyds has been variously represented; they are now often conveniently regarded as compounds in some measure analogous to hydrogen itself, acetic aldehyd being

represented by the formula H, in accordance with the

H'

These views will be discussed more fully

when the properties of acetic aldehyd are considered.

The aldehyds are unstable compounds, several of them passing into bodies which are isomeric with them, but which possess properties entirely different from the true and rapidly oxidizable aldehyds.

1. FORMIC ALDEHYD has not hitherto been satisfactorily isolated; but methylal, a body homologous with acetal, is contained amongst the products of the gradual oxidation of wood spirit by means of peroxide of manganese and sulphuric acid.

(1249) 2. ALDEHYD, Acetic Aldehyd (E,HO, or CH2O2). Sp. gr. of liquid at 32°, 08009; of vapour 1532; Rel. wt. 22; Boiling pt. 68° (20° C.) Kopp.-Acetic aldehyd was the earliest discovered member of the group of aldehyds, and owing to its connexion with the process of acetification it is by far the most important of them.

Preparation.-Aldehyd may be obtained by the gradual oxidation of alcohol in various ways: it is formed, for example, when the vapour of alcohol mixed with air is transmitted through a porcelain tube heated to low redness, or when alcohol is acted upon by dilute nitric or chromic acid: owing to the action of nitric acid upon the elements of alcohol, it is produced during the preparation of the fulminates of silver and mercury, and it is always present in nitrous ether: it may also be procured by the dry distillation of lactic acid, or of cupric lactate; dilute alcohol also furnishes it when acted on by chlorine. Another remarkable mode in which aldehyd may be obtained has been pointed out by Natanson: he finds that when diethylenium dichloride (1383) is

mixed with nitrate of silver and a little sulphuric acid, a copious evolution of nitrogen occurs, and aldehyd is formed abundantly :Aldehyd.

Diethylenium dichloride.

(¤ ̧H ̧)”1⁄2H ̧Ñ ̧Cl2+2 AgNÐ ̧=2 ¤¿H ̧¤+2 N2+2 H ̧Ð+2 AgCl. Diethylenium is itself derived from Dutch liquid by the action of ammonia upon it; whence it would appear that a close relationship exists between olefiant gas and aldehyd.

None of these reactions are employed for the preparation of aldehyd, since there are more convenient methods. One of these consists in placing 3 parts of powdered potassic dichromate (K ̧¤r ̧Ð,) in a retort, and allowing a mixture of 4 parts of sulphuric acid diluted with 3 times its volume of water to flow into the retort, containing 2 parts of alcohol, cooled by immersion in a freezing mixture; distillation commences without the aid of heat; the vapours must be condensed in a proper refrigeratory.

Aldehyd is however usually procured by Liebig's method of distilling in a capacious retort a mixture of 6 parts of sulphuric acid, 4 of alcohol (sp. gr. o'850), 4 of water, and 6 of finely powdered black oxide of manganese. In this process the oxide of manganese is decomposed, manganous sulphate is formed, whilst the second atom of the oxygen of the manganic dioxide combines with the hydrogen of part of the alcohol, and aldehyd is formed and passes over. The product being very volatile must be condensed in vessels cooled with ice, and the process must be stopped when the distillate becomes acid. Since, however, it is in a very dilute and impure condition, it is to be rectified from an equal weight of calcic chloride, in order to free it from alcohol and water. This operation is repeated twice, or even three times. The distilled liquid still retains alcohol, besides a small quantity of acetic ether, and a liquid termed acetal (1251). For the purpose of obtaining it free from these impurities it is mingled with twice its bulk of anhydrous ether, and saturated with dry ammoniacal gas, while the liquid is artificially cooled: prismatic needies of snowy whiteness are thus formed; these crystals consist of a compound of aldehyd with ammonia (H,N‚¤‚H ̧¤) which is insoluble in ether though very soluble in water; they may be dried in the open air; they fuse at a heat a little below 212°, and at 212° may be sublimed unchanged. If heated in the open air they take fire, and burn with flame. After a time, even when kept in closed vessels, the crystals become brown, and emit an odour like that of burned feathers. To obtain pure aldehyd from them, 2 parts

2 4

ACETIC ALDEHYD-PRODUCTS OF ITS DECOMPOSITION.

341

of the crystals dissolved in 2 of water, are distilled with 3 of oil of vitriol diluted with 4 parts of water; the product is received in vessels surrounded with ice, and is finally rectified from calcic chloride.

Properties.-Aldehyd is a volatile inflammable liquid, with a peculiar pungent irritating odour, which has a faint resemblance to that of apples. It may be mixed in all proportions with alcohol, ether, and water. It has no acid reaction upon litmus paper, but it can scarcely be exposed to the air without absorbing oxygen, and it then forms an acid compound. Aldehyd yields with ammonia the peculiar crystalline body already described, but when similarly treated with caustic potash it gives a brown resinous mass: this is one of its most characteristic reactions. Another striking one is its power of reducing the salts of silver, and of yielding, when boiled with their solutions, a mirror-like coating of silver upon the sides of the tube in which the experiment is made: the solution should be rendered feebly alkaline by the addition of a few drops of a weak solution of ammonia. In this experiment a portion of the aldehyd is converted into an acid, at the expense of the oxygen derived from a portion of the salt of silver which is thus reduced to the metallic state, and the newly formed acid enters into combination with a portion of the excess of alkali.

Chlorine displaces a portion of the hydrogen of the aldehyd, converting it into chloral (1176) :—

Aldehyd, when transmitted over a mixture of lime and caustic potash in a heated tube, first becomes brown, and is then decomposed with disengagement of hydrogen, leaving a white mass composed of potassic acetate :

Potassium when placed in contact with aldehyd disengages hydrogen, and forms a soluble compound (E,H,OK) which possesses an alkaline reaction, and absorbs oxygen readily. This compound contains the elements of aldehyd, in which one atom of potassium has displaced one of hydrogen.

According to Lieben (Liebig's Annal. cvi. 336), if aldehyd be saturated with dry hydrochloric acid, while kept cold by im