An Elementary Study of Chemistry, William McPherson [icecream ebook reader txt] 📗
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Saponification. When an ethereal salt is heated with an alkali a reaction expressed by the following equation takes place:
This process is known as saponification, since it is the one which takes place in the manufacture of soaps. The ordinary soaps are made by heating fats with a solution of sodium hydroxide. The reactions involved may be illustrated by the following equation representing the reaction between palmitin and sodium hydroxide:
In accordance with this equation the ethereal salts in the fats are converted into glycerin and the sodium salts of the corresponding acids. The sodium salts are separated and constitute the soaps. These salts are soluble in water. When added to water containing calcium salts the insoluble calcium palmitate and stearate are precipitated. Magnesium salts act in a similar way. It is because of these facts that soap is used up by hard waters.
ETHERSWhen ethyl alcohol is heated to 140° with sulphuric acid the reaction expressed by the following equation takes place:
The resulting compound, (C2H5)2O, is ordinary ether and is the most important member of the class of compounds called ethers. Ordinarily ether is a light, very inflammable liquid boiling at 35°. It is used as a solvent for organic substances and as an anæsthetic in surgical operations.
KETONESThe most common member of this group is acetone (C3H6O), a colorless liquid obtained when wood is heated in the absence of air. It is used in the preparation of other organic compounds, especially chloroform.
ORGANIC BASESThis group includes a number of compounds, all of which contain nitrogen as well as carbon. They are characterized by combining directly with acids to form salts, and in this respect they resemble ammonia. They may, indeed, be regarded as derived from ammonia by displacing a part or all of the hydrogen present in ammonia by hydrocarbon radicals. Among the simplest of these compounds may be mentioned methylamine (CH3NH2) and ethylamine (C2H5NH2). These two compounds are gases and are formed in the distillation of wood and bones. Pyridine (C5H6N) and quinoline (C9H7N) are liquids present in small amounts in coal tar, and also in the liquid obtained by the distillation of bones. Most of the compounds now classified under the general name of alkaloids (which see) also belong to this group.
CARBOHYDRATESThe term "carbohydrate" is applied to a class of compounds which includes the sugars, starch, and allied bodies These compounds contain carbon, hydrogen, and oxygen the last two elements generally being present in the proportion in which they combine to form water. The most important members of this class are the following:
Cane sugar (C12H22O11). This is the well-known substance commonly called sugar. It occurs in many plants especially in the sugar cane and sugar beet. It was formerly obtained almost entirely from the sugar cane, but at present the greatest amount of it comes from the sugar beet. The juice from the cane or beet contains the sugar in solution along with many impurities. These impurities are removed, and the resulting solution is then evaporated until the sugar crystallizes out. The evaporation is conducted in closed vessels from which the air is partially exhausted. In this way the boiling point of the solution is lowered and the charring of the sugar is prevented. It is impossible to remove all the sugar from the solution. In preparing sugar from sugar cane the liquors left after separating as much of it as possible from the juice of the cane constitute ordinary molasses. Maple sugar is made by the evaporation of the sap obtained from a species of the maple tree. Its sweetness is due to the presence of cane sugar, other products present in the maple sap imparting the distinctive flavor.
When a solution of cane sugar is heated with hydrochloric or other dilute mineral acid, two compounds, dextrose and levulose, are formed in accordance with the following equation:
This same change is brought about by the action of an enzyme present in the yeast plant. When yeast is added to a solution of cane sugar fermentation is set up. The cane sugar, however, does not ferment directly: the enzyme in the yeast first transforms the sugar into dextrose and levulose, and these sugars then undergo alcoholic fermentation.
When heated to 160° cane sugar melts; if the temperature is increased to about 215°, a partial decomposition takes place and a brown substance known as caramel forms. This is used largely as a coloring matter.
Milk sugar (C12H22O11). This sugar is present in the milk of all mammals. The average composition of cow's milk is as follows:
When rennin, an enzyme obtained from the stomach of calves, is added to milk, the casein separates and is used in the manufacture of cheese. The remaining liquid contains the milk sugar which separates on evaporation; it resembles cane sugar in appearance but is not so sweet or soluble. The souring of milk is due to the fact that the milk sugar present undergoes lactic fermentation in accordance with the equation
The lactic acid formed causes the separation of the casein, thus giving the well-known appearance of sour milk.
Isomeric compounds. It will be observed that cane sugar and milk sugar have the same formulas. Their difference in properties is due to the different arrangement of the atoms in the molecule. Such compounds are said to be isomeric. Dextrose and levulose are also isomeric.
Dextrose (grape sugar, glucose) (C6H12O6). This sugar is present in many fruits and is commonly called grape sugar because of its presence in grape juice. It can be obtained by heating cane sugar with dilute acids, as explained above; also by heating starch with dilute acids, the change being as follows:
Pure dextrose is a white crystalline solid, readily soluble in water, and is not so sweet as cane sugar. In the presence of yeast it undergoes alcoholic fermentation. It is prepared from starch in large quantities, and being less expensive than cane sugar, is used as a substitute for it in the manufacture of jellies, jams, molasses, candy, and other sweets. The product commonly sold under the name of glucose contains about 45% of dextrose.
Levulose (fruit sugar)(C6H12O6). This sugar is a white solid which occurs along with dextrose in fruits and honey. It undergoes alcoholic fermentation in the presence of yeast.
Cellulose (C6H10O5). This forms the basis of all woody fibers. Cotton and linen are nearly pure cellulose. It is insoluble in water, alcohol, and dilute acids. Sulphuric acid slowly converts it into dextrose. Nitric acid forms nitrates similar to nitroglycerin in composition and explosive properties. These nitrates are variously known as nitrocellulose, pyroxylin, and gun cotton. When exploded they yield only colorless gases; hence they are used especially in the manufacture of smokeless gunpowder. Collodion is a solution of nitrocellulose in a mixture of alcohol and ether. Celluloid is a mixture of nitrocellulose and camphor. Paper consists mainly of cellulose, the finer grades being made from linen and cotton rags, and the cheaper grades from straw and wood.
Starch (C6H10O5). This is by far the most abundant carbohydrate found in nature, being present especially in seeds and tubers. In the United States it is obtained chiefly from corn, nearly 80% of which is starch. In Europe it is obtained principally from the potato. It consists of minute granules and is practically insoluble in cold water. These granules differ somewhat in appearance, according to the source of the starch, so that it is often possible to determine from what plant the starch was obtained. When heated with water the granules burst and the starch partially dissolves. Dilute acids, as well as certain enzymes, convert it into dextrose or similar sugars. When seeds germinate the starch present is converted into soluble sugars, which are used as food for the growing plant.
Chemical changes in bread making. The average composition of wheat flour is as follows:
In making bread the flour is mixed with water and yeast, and the resulting dough set aside in a warm place for a few hours. The yeast first converts a portion of the starch into dextrose or a similar sugar, which then undergoes alcoholic fermentation. The carbon dioxide formed escapes through the dough, making it light and porous. The yeast plant thrives best at about 30°; hence the necessity for having the dough in a warm place. If the temperature rises above 50°, the vitality of the yeast is destroyed and fermentation ceases. In baking the bread, the heat expels the alcohol and also expands the bubbles of carbon dioxide caught in the dough, thus increasing its lightness.
SOME DERIVATIVES OF BENZENEAttention has been called to the complex nature of coal tar. Among the compounds present are the hydrocarbons, benzene, toluene, naphthalene, and anthracene. These compounds are not only useful in themselves but serve for the preparation of many other important compounds known under the general name of coal-tar products.
Nitrobenzene (oil of myrbane) (C6H5NO2). When benzene is treated with nitric acid a reaction takes place which is expressed by the following equation:
The product C6H5NO2 is called nitrobenzene. It is a slightly yellowish poisonous liquid, with a characteristic odor. Its main use is in the manufacture of aniline.
Aniline (C6H5NH2). When nitrobenzene is heated with iron and hydrochloric acid the hydrogen evolved by the action of the iron upon the acid reduces the nitrobenzene in accordance with the following equation:
The resulting compound is known as aniline, a liquid boiling at 182°. When first prepared it is colorless, but darkens on standing. Large quantities of it are used in the manufacture of the aniline or coal-tar dyes, which include many important compounds.
Carbolic acid (C6H5OH). This compound, sometimes known as phenol, occurs in coal tar, and is also prepared from benzene. It forms colorless crystals which are very soluble in water. It is strongly corrosive and very poisonous.
Naphthalene and anthracene. These are hydrocarbons occurring along with benzene in coal tar. They are white solids, insoluble in water. The well-known moth balls are made of naphthalene. Large quantities of naphthalene are used in the preparation of indigo, a dye formerly obtained from the indigo plant, but now largely prepared by laboratory methods. Similarly anthracene is used in the preparation of the dye alizarin, which was formerly obtained from the madder root.
THE ALKALOIDSThis term is applied to a group of compounds found in many plants and trees. They all contain nitrogen, and most of them are characterized by their power to combine with acids to form salts. This property is indicated by the name alkaloids, which signifies alkali-like. The salts are soluble in water, and on this account are more largely used than the free alkaloids, which are insoluble
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