On heating to higher temperatures these polymerised aldehydes split up again into the simple molecules.

388. The aldehydes, as the compounds intermediate between the primary alcohols and acids, can readily be converted into either.

By treatment with nascent hydrogen-by sufficiently energetic action of sodium amalgam upon a slightly acidulated aqueous solution of aldehyde-two atoms of hydrogen are combined, and the primary alcohol regenerated, from which the aldehyde can be obtained by CmH2m+

oxidation :

CmH2m+1
H

H

+ 2H : = C H
он

The aldehydes are slowly converted into acids by mere exposure to atmospheric oxygen, and this change occurs considerably more quickly with oxidising agents:

The action of argentic oxide is especially characteristic. By adding ammonia to argentic nitrate until the argentic oxide first precipitated is redissolved, a liquid is obtained which, when added to aldehyde at ordinary temperatures, but more quickly on warming, gives a deposit of metallic silver, whilst the ammonic salt of the acid remains in solution:

If the aldehyde is miscible with water the silver separates in great part as a coherent, mirror-like film on the sides of the vessel. If an aldehyde be brought into contact with freshly precipitated argentic oxide, the argentic salt of the acid is formed:

CmH2m+1.CO.H + 3AgOH = CmH2m+1.CO.OAg + Ag2 + 2H2O.

In detecting or purifying aldehydes use is made of their combinations with the alkaline hydric sulphites and with ammonia (see later).

By action of phosphoric chloride the aldehydes and their polymers are converted into the dichlorides of the radicals (§ 379).

Formic Aldehyde, or Methene Oxide, CH2O.

389. Methene oxide, methylic aldehyde, also known (as the aldehyde of formic acid) as formic aldehyde, is only known in the state of vapour and in aqueous solution, which probably contains it as a dihydrate. In absence of water it polymerises with great readiness.

The aqueous solution of formic aldehyde is most readily obtained by oxidation of methylic alcohol. For this purpose some methylic alcohol is placed on the bottom of a tri-tubulated Woulff's bottle. In one of the side necks a glass tube open at both ends is placed so as to dip into the methylic alcohol, whilst the other is connected with a condenser, this with a receiver, and this latter attached air-tight to a powerful aspirator. The condenser is filled with ice-cold water, with which the receiver is also surrounded. When the aspirator has been set working, the middle neck of the Woulff's bottle is closed with a stopper, from which hangs a spiral of platinum wire which has been heated to glowing. A powerful stream of air is now drawn through the open tube, which bubbling through the methylic alcohol becomes charged with its vapour. The hot platinum wire causes the oxidation of the vapour, sufficient heat being thereby evolved to keep the wire at the necessary temperature. The resulting formic aldehyde, together with water and unaltered methylic alcohol, liquefy in the condenser and collect in the receiver.

The distillate has a penetrating smell, reduces solutions of silver salts, and is converted into a yellow oil by potassic hydrate. Formic aldehyde has not yet been obtained from it in a pure state.

If the liquid be evaporated on the water bath the greater part of the aldehyde is evolved. A part, however, remains as a solid mass of paramethyl aldehyde.

390. Paramethyl aldehyde, or trimethene oxide, formerly known as dioxy-methylene, C3H6O3, can be obtained in the following ways, in addition to the one given above :

It is obtained directly from methene diiodide by heating with argentic oxalate. This latter must be mixed with twice its weight of powdered glass in order to moderate the reaction:

3CH2I2+3Ag2CO1 = 6AgI + 3CO2 + 3CO + C3H6O3.

In well-cooled condensers paramethyl aldehyde separates from the stream of gases in the solid state.

On heating methene diiodide with argentic acetate and acetic acid, methene diacetate, boiling at 170°, is formed:

CH2I2+2AgO.C2H,O=2AgI + CH2(OC2H3O)2;

from which, by heating with water at 100° in sealed tubes, paramethyl aldehyde is obtained:

3CH2(O.C2H2O)2 + 3H2O = 6HO.C2H3O + C3H6O3.

It is obtained most readily by heating glycollic acid with concentrated sulphuric acid in a retort to 150°:

CH2.OH

30.0H

CO.OH

+ 3H2SO4 = 3(H2SO,,H2O) + 3CO + C3H6O3;

paramethyl aldehyde condenses on the neck of the retort.

Paramethyl aldehyde is a colourless, indistinctly crystalline body, insoluble in water, alcohol, and ether. Though itself odourless it gives the penetrating odour of formic aldehyde when heated. It

S

. sublimes freely at 100°, melts at 152°, and boils at a slightly higher temperature. The vapour evolved is not paramethyl aldehyde, but the simple formic aldehyde. On cooling it polymerises again. The vapour density is found to be 1.037, which corresponds to the formula CH,O.

That paramethyl aldehyde is correctly represented by the formula C3H6O3 can only be inferred from its analogy with parathio-methyl aldehyde, into which it is readily converted by hydric sulphide, and which has been shown to possess the formula C3H6S3.

If paramethyl aldehyde be heated with much water to 130°-150° in sealed tubes it dissolves, and the liquid then possesses the properties of a solution of formic aldehyde. In presence of water, therefore, the change into the polymeric modification does not occur, probably in consequence of the solution containing not CH2O, but the dihydrate CH2(OH)2.

Baric hydrate solution, on boiling, colours paramethyl aldehyde yellow, and the liquid then contains some baric formate.

By careful addition of argentic oxide the aqueous solution of formic aldehyde is converted into formic acid:

.CH,O + 2AgOH=Ag + CHO.OH + H,O.

Acetic Aldehyde, or Ethylidene Oxide, CH4O.

391. Acetic aldehyde, ethylic aldehyde, or simply aldehyde, is the first oxidation product of ethylic alcohol:

and is also formed in smaller quantity in the oxidation of many other bodies, such as albumen, sugars, fermentation lactic acid, &c.

From fermentation lactic acid it is also obtained, together with other products, by dry distillation:

in larger quantity, together with formic acid, by heating with moderately concentrated sulphuric acid to 150° in sealed tubes :

Much aldehyde is also obtained by dry distillation of an intimate mixture of calcic acetate and formate:

392. To prepare aldehyde two parts of 80 % alcohol are heated with three parts of manganic dioxide and a mixture of three parts of sulphuric acid and two parts of water, and the vapours condensed by strong cooling. The crude distillate, which contains several other bodies, such as acetal, ethylic acetate, water, and unaltered alcohol, is, in order to purify the aldehyde, mixed with ether, and the liquid saturated with dry ammonia gas. Aldehyde ammonia, insoluble in ether, is thereby separated:

CH .CHO + NH=CH,.CH(OH).NH,,

in crystals, whilst all impurities remain in solution. The crystals, after washing with ether and drying over sulphuric acid, are dissolved in a little water, mixed with dilute sulphuric acid, and distilled, when aldehyde and some water pass over :

2CH,.CH(OH).NH, + H,SO =(NH,),SO4 + 2CH,.CHO.

The distillate is allowed to stand some time over fused calcic chloride and then rectified, the pure aldehyde being collected in receivers cooled with iced water.

Instead of manganic dioxide, potassic dichromate, in pieces about the size of peas, is generally used as the oxidising agent. Fifteen parts of this is treated with a mixture of ten parts of alcohol and twenty parts of sulphuric acid, which has been previously diluted with three times its volume of water.

The reaction, which speedily begins, is so violent that the liquid enters into ebullition. As the accompanying bodies are more difficultly volatile than aldehyde, it is usual to separate them from the latter by partial condensation before condensing it by strong cooling.

For this purpose the reaction vessel A (fig. 20), whose volume must be four times that of the mixed ingredients, is connected with the lower end of a glass worm B which is surrounded with water of 60°. In this the main portion of the accompanying vapours are condensed and returned into the retort a. The aldehyde vapours are conducted from the upper end of the worm tube into two cylinders c, c, containing anhydrous ether, and surrounded by a mixture of ice and salt, in which the aldehyde liquefies. At the end of the reaction, which is completed by slight heating of the reaction vessel by hot water, all the aldehyde is found mixed with ether in the two cylinders. Dry ammonia gas is now passed into the cylinders, and the crystalline aldehyde ammonia treated as already described to obtain pure aldehyde.

393. Acetic aldehyde is a mobile liquid boiling at 21°, of sp. gr. 801 at 0°. In the air it evaporates with great celerity and large absorption of heat. The odour is peculiar, and when strongly diluted not unpleasant. The vapour, when breathed in large quantity, produces a feeling of tightness in the chest.

The vapour density is 1.52.

Acetic aldehyde mixes in every proportion with water, alcohol, and ether, in the two first cases with considerable evolution of heat.

Aqueous aldehyde becomes rapidly acid on exposure to air, being converted by that, as by all other oxidising agents, into acetic acid. It precipitates metallic silver from ammoniacal silver solutions, ammonic acetate being formed:

CH .CHO + 2NH,AgOH=Ag + NH3 + H,O + CH.CO.0.NH

By treatment of the slightly acidulated aqueous solution with sodium amalgam aldehyde is reconverted into ethylic alcohol :

but some other products are formed at the same time, especially B-butylene glycol.

Phosphoric chloride converts acetic aldehyde into ethylidene dichloride (§ 381), and alkalies form from it a yellow resin (aldehyde resin) of still uncertain composition.

By long contact with aqueous hydrochloric acid aldehyde is converted into aldol; by heating with a little zincic chloride two molecules of aldehyde condense, with elimination of a molecule of water, into crotonic aldehyde.