  Organic Chemistry. About a century ago, when the science of Chemistry was still in its infancy, several substances were known which could then only be obtained from animals or plants. The composition of these substances was not understood, and they were not classified; moreover, since none of them had ever been prepared artificially, it was supposed that it was impossible to do this—the reason given was that “vital force” was necessary for their production. In time, however, some of the most typical animal and vegetable products were prepared in the laboratory, and the belief in vital force disappeared. In later times it was proved that substances like sugar, starch, urea, indigo, and a great many more, all contain the element carbon. At the present time, more than 100,000 compounds of this element are known; and since they resemble one another, and at the same time differ in several important respects from the compounds of other elements, it is both natural and convenient that they should be classed together and studied separately. This branch of Chemistry is called organic. It must not, however, be supposed that all organic compounds are necessarily produced by some living organism. A great many are, but there are many more which are purely synthetic products. Inorganic Chemistry includes all the other elements and their derivatives. The element carbon, and also some of its simpler compounds, such as carbon monoxide, carbon dioxide, carbonic acid, and carbonates, are more appropriately placed in the inorganic section. The acids which have been considered up to this point are all inorganic acids, and those which follow are organic. Sulphuric, nitric, and hydrochloric acids are often distinguished as the mineral acids in contradistinction to oxalic, citric, tartaric, and some others which were first obtained from unripe fruits and therefore called vegetable acids. Organic acids have all the general properties of the class, but they are much weaker than the mineral acids mentioned above. This is shown by their solvent action on metals, oxides, and carbonates, which is in all cases slight. Vinegar is the trade name for what is essentially a dilute solution of acetic acid which has been made by the acetous fermentation of saccharine fluids containing weak alcohol. In addition to acetic acid, vinegar contains minute quantities of a large number of compounds. Some of these help to produce that agreeable flavour and aroma which distinguishes vinegar from diluted acetic acid. The nature and quantity of the flavouring constituents depend mainly upon the nature of the alcoholic solution used. Since the acetic acid in vinegar is always produced by fermentation, all processes for the manufacture of vinegar are essentially arrangements for promoting the vigorous growth and development of Mycoderma aceti, the organism which produces the vinegar ferment. Like all other plants, Mycoderma aceti will flourish only under certain favourable conditions. In the first place, it requires nourishment, and therefore certain nitrogen compounds and salts must be present in the alcoholic solution. These are contained in wines, beer, cider, and malt liquors, but not in spirits of wine, which is pure alcohol distilled from liquids which have undergone vinous fermentation. If the plant is placed in Alcohol is changed to acetic acid by the process of oxidation, and therefore, in making vinegar, arrangements have to be made to bring together weak alcohol and air in the presence of the plant. The ferment which is secreted by the plant then causes an acceleration of the reaction. There is a considerable amount of similarity between fermentation and contact action. In this connection, it is interesting to note that the conversion of alcohol into acetic acid can also be brought about by exposing a mixture of alcohol vapour and air to the action of platinum black; in fact, there is one process for making vinegar in this way. French Vinegar. New wine, especially that which contains a low percentage of alcohol, is liable to many kinds of “sickness.” It may turn bitter, it may turn sour, or it may undergo what is called lactic fermentation. In either case, it becomes unsaleable as a beverage. Wine which has turned sour is the best material for making vinegar, and when this is done by the French or slow process, a product with a very fine bouquet is obtained. The methods adopted are very simple. Formerly, the wine was poured into barrels leaving the top portion empty, and providing for a current of air over the surface. The barrels were often set up in rows in the open air in an enclosure which was then known as a “vinegar field.” The process of souring which had already begun went on naturally, and in the course of a few months, nearly the whole of the alcohol was converted into acetic acid. The process now in use in some of the French factories is somewhat similar. Large casks holding about 100 gallons are set up in a room, and provision is made for keeping the temperature uniform. Each cask is first acidulated by allowing strong vinegar to stand in it until the vinegar plant has developed on the surface. The casks are then filled up very gradually by adding a few gallons of wine every eight or ten days. When the cask is full, a fraction of the contents is drawn off and replaced by wine. The process then becomes continuous, until it is necessary to clean out the generator and start again. In recent times, the manufacture of wine vinegar has been carried out on more scientific principles. The vinegar plant is actually cultivated and examined microscopically before being used, in order to make sure of the absence of moulds and bacteria, which set up other fermentations, producing substances which affect adversely the taste and aroma of the finished product. The cultivated ferment is then added to the wine in shallow vessels and the process is carried on as described above. Malt Vinegar. A dilute solution of alcohol which is made from malt by fermentation with yeast contains the nutritive substances necessary for the growth of the vinegar plant, and can therefore be used as a starting-point for the manufacture of vinegar. Sprouted barley or malt is mixed with oats, barley, rice, or other starch-containing material. The mixture is mashed with warm water and then fermented with yeast, giving what is called “raw spirit.” This is converted into vinegar by the “quick” process. The vinegar generator (Fig. 10) is a large barrel divided into three compartments by two perforated partitions. The lower disc is fixed about one-third of Fig. 10. QUICK VINEGAR PROCESS Fig. 10. QUICK VINEGAR PROCESS The weak spirit is delivered into the upper portion of the barrel and is distributed in very small drops by the threads; it then passes slowly over the vinegar plant, to which the air also has free access. When it reaches the bottom, it overflows into a reservoir and is again passed through the generator; this is repeated until the product contains the desired amount of acetic acid. The principle of the quick vinegar process is the same Both wine vinegar and malt vinegar when freshly prepared have a stupefying and unpleasant odour. Before the product is ready for the market, it has to be matured in barrels. During this process, a small quantity of alcohol which still remains in the vinegar combines slowly with some of the acetic acid, producing acetic ester, a substance which has a pleasant fruity odour. The colour of wine vinegar is natural, but vinegar which is produced by the quick process is colourless or only faintly coloured. Since the public has a preference for vinegar which is brown in colour, the product of the quick process is coloured artificially, either by adding caramel or by preparing the weak spirit from malt which has been slightly charred in drying. Industrial Acetic Acid. The solutions of acetic acid dealt with above would be too dilute for any industrial purpose; moreover, the acid can be obtained much more cheaply by the distillation of wood. When wood is subjected to a high temperature, it is converted into charcoal and, at the same time, an inflammable gas, an acid liquid, and tar are given off, and can be collected in suitable vessels. The following table, on page 73, gives the relative amounts of the various substances obtained from wood by dry distillation. The quantities are those derived from one cord, that is, 125 cu. ft.
The aqueous liquid that distils over contains methyl alcohol (wood spirit), acetone, and acetic acid. The crude mixture is known as pyroligneous acid. This is neutralized with milk of lime or soda ash, which converts acetic acid into calcium or sodium acetate, but has no action on the methyl alcohol and acetone which are also present. The mixture is then distilled, when methyl alcohol, acetone, and water pass over into the distillate, leaving the acetate in the retort. To obtain the free acid from the acetate, the latter is well dried and then distilled with concentrated sulphuric acid. Acetic acid, being the more volatile of the two acids, distils over, and is nearly pure. The method of removing the last traces of water depends upon the fact that acetic acid solidifies at 17° C. The acid, which is nearly, but not quite, free from water, is cooled until a portion solidifies. The part which still remains liquid is poured away, and the process is repeated until a residue is obtained which solidifies as a whole. This is glacial acetic acid, so called because it is a mass of glistening plates which look like newly-formed ice. The AcetatesAluminium Acetate, made by dissolving alumina in acetic acid, is the “red liquor” which is used as a mordant in dyeing. It is a colourless liquid, but is called “red liquor” because it is used with dyes which give a red colour. Ferrous Acetate, made in a similar way from scrap iron and acetic acid, is the “black liquor” used in dyeing. Verdigris, or basic copper acetate, is a valuable pigment. It is made by interposing cloths soaked in vinegar between plates of copper. After the action has been allowed to go on for a long time, the plates are washed with water and the verdigris is scraped off. The finest verdigris is made in France in the wine-producing district around Montpellier. Here, instead of cloths soaked in vinegar, the solid residue from the wine presses is spread in layers between the copper plates. The product made in this way is called vert de Montpellier. Fig. 11. DUTCH PROCESS FOR WHITE LEAD Fig. 11. DUTCH PROCESS FOR WHITE LEAD Verdigris, like all the copper compounds, is extremely poisonous. It is very liable to be formed on the surface of copper utensils used for cooking purposes. Lead Acetate, or sugar of lead, is used in large quantities in the colour industry for making various The slow action which acetic acid vapour has upon the metal lead finds a very interesting application in what is known as the Dutch process for the manufacture of white lead Future Supply of Acetic Acid. When all the operations involved in the production of acetic acid from wood, from the felling of the tree to the final separation of the glacial substance, are taken into consideration, it will be readily understood how it is that this acid has never been cheap when compared with other acids used on an equally large scale. In addition to this, the competition for wood for paper-making and for the very numerous cellulose industries is rapidly increasing. It is, therefore, not surprising to learn that chemists have turned their attention towards the discovery of newer and cheaper methods of making acetic acid. Such a process seems to have been worked out in If this process should prove to be successful, it will form the starting-point of a new and important industry, for, apart from the large amount of acetic acid which is used in commerce, there is the production of the very important solvent known as acetone, which can be made from acetic acid by a very simple operation. Tartaric Acid. Grape juice contains a large quantity of potassium hydrogen tartrate dissolved in it; when the liquid is fermented and alcohol is formed, this salt crystallizes out because it is not soluble in alcohol. After the new wine has been poured off, the salt is found as a brownish crystalline residue adhering to the sides of the vat. Also the salt goes on crystallizing after the wine is put into barrels, and forms an incrustation on the sides. This is called the lees or sediment of wine. In commerce, the substance is known as argol (sometimes spelt argal), and also tartar of wine. Crude argol is purified by dissolving it in water and destroying the colour by boiling with animal charcoal. When the clear liquid obtained from this mixture by filtration is evaporated, a white crystalline substance separates out. This is potassium hydrogen tartrate or cream of tartar. Tartaric acid is obtained from cream of tartar. The salt is dissolved in water and nearly neutralized with milk of lime. Insoluble calcium tartrate is precipitated, and potassium tartrate remains in solution. A further quantity of calcium tartrate is obtained by adding The general properties of tartaric acid are well known. It is soluble in water, giving a solution which has a pleasantly acid taste. Citric Acid. The sharp flavour of many unripe fruits is due to the presence of citric acid; the juice of lemons contains 5-6 per cent. of the acid. The free acid is obtained in a manner precisely similar in principle to that described for tartaric acid. Oxalic Acid. Oxalic acid and its salts, the oxalates, are very widely distributed in the vegetable kingdom. These compounds are present in wood sorrel (Oxalis acetosella), in rhubarb, in dock, and in many other plants. The acid is made on a large scale by mixing pine sawdust to a stiff paste with a solution containing caustic soda and potash. The paste is spread out on iron plates and heated, care being taken not to heat the mixture to the point at which it chars. The mass is then allowed to cool, and is mixed with a small quantity of water to dissolve out the excess of alkali. This is recovered and used again. Sodium oxalate, which is the main product of the reaction described above, is dissolved in water and treated with milk of lime, whereby insoluble calcium oxalate is obtained, which is subsequently decomposed with sulphuric acid, yielding oxalic acid. Potassium hydrogen oxalate is sometimes called salts of sorrel, and potassium quadroxalate, salts of lemon. The most familiar use of the latter substance is in the removal of ink stains. Oxalic acid and its salts are poisonous. The free Formic Acid (L. formica, an ant) is found both in the vegetable and in the animal kingdom. If the leaf of a stinging nettle is examined with a microscope, it is seen to be covered with long pointed hairs having a gland at the base. This gland contains formic acid. When the nettle is touched lightly, the fine point of the hair punctures the skin, and a subcutaneous injection of formic acid is made, which quickly raises a blister. The inconvenience which arises from the stings of bees and wasps, also from the fluid ejected by ants when irritated, is due to formic acid. The remedy in each case is the same; the acid must be neutralized as quickly as possible with mild alkali, such as washing soda. Formic acid was first made by distilling an infusion of red ants. It is now made from glycerine and oxalic acid. The Fatty Acids. Animal fats and vegetable oils are similarly constituted bodies. They are composed mainly of three chemical compounds known as stearin, palmitin, and oleÏn. Of these, stearin and palmitin are solids at ordinary temperatures, while oleÏn is a liquid. Hard fats like those of mutton and beef are composed mainly of stearin; fats of medium hardness contain stearin, palmitin, and some oleÏn; while oils such as cod-liver oil and olive oil are nearly pure oleÏn. Stearin, palmitin, and oleÏn are analogous in composition to salts. Their proximate constituents are glycerine and certain organic acids, stearic, palmitic, and oleÏc respectively. In order to obtain the fat free from tissue which it contains in its natural state, it is tied up in a muslin bag and heated in boiling water. The fat is squeezed out through the meshes of the fabric and floats on the All fats and vegetable oils can be resolved into their two constituents, the acid and the glycerine. This can be brought about by heating the fat with water to about 200° C. This operation must be carried out in a vessel capable of withstanding pressure and closed with a safety valve; otherwise, the requisite temperature could not be obtained. After this treatment, there is left in the vessel an oily layer which solidifies on cooling and an aqueous layer which contains the glycerine. The solidified oily layer is the fatty acid. In the case of mutton or beef tallow, it would be mainly a mixture of stearic and palmitic acids. This mixture is used to make “stearin” candles. The acids themselves are wax-like solids without any distinctive taste. Stearic acid melts at 69° C. and palmitic at 62° C. They have no perceptible action on the colour of litmus, neither have they any solvent action on metals or carbonates. We should not recognize these substances as acids at all were it not for the fact that they combine with alkalis, forming salts. The salts of the fatty acids are called soaps. To make soap, the fat is boiled with caustic alkali or caustic lye, as it is more often called. This breaks the fat up primarily into the acid and glycerine; but in this case, instead of obtaining the acid as the final product as we did above by heating with water under pressure, we get the sodium or potassium salt of the acid according to the alkali used. When caustic soda is used, the product is a hard soap; when caustic potash is used, it is a soft soap. The treatment of fats in this way with caustic alkalis is called “saponification.” |
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