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CHAPTER XXXVIII.

SOME CARBON COMPOUNDS.

520. Organic Chemistry. -It was formerly believed that the complex substances produced by animals and plants, such as sugar, starch, uric acid, etc., could not be made in the chemical laboratory; hence there arose a distinction between organic" and " " and "inorganic" compounds. Organic compounds, it was thought, needed " vital energy for their formation, while inorganic substances could be made without the intervention of living things. However, the preparation of urea, CO (NH2)2, an animal product, by Wöhler in 1828 and the many organic syntheses made since that time have destroyed the force of this distinction. At the present time the term "Organic Chemistry" is practically equivalent to "Chemistry of the Carbon Compounds.

The carbon compounds exceed in number those of all the other elements together, yet each substance is composed of relatively few elements. Many compounds contain only carbon and hydrogen; a much larger number, carbon, hydrogen, and oxygen. Many important organic substances contain carbon, hydrogen, and nitrogen, while others have oxygen in addition. Phosphorus, sulphur, and the halogens are also present in many organic compounds.

The compounds described in this chapter are classified as (1) hydrocarbons, (2) alcohols, (3) ethers, (4) aldehydes, (5) acids, (6) organic salts, or esters, (7) amines and alkaloids, (8) carbohydrates, and (9) phenol derivatives.

521. Hydrocarbons of Marsh Gas Series. - Substances composed of only hydrogen and carbon are called hydrocarbons. Several have already been mentioned in §§ 291 to 297. Methane (§ 291) is the first member of a large series the marsh gas, or paraffin series in which the formula of each member differs from that of the next member by CH2. Such a series is said to be homologous.

The names and formulas of some members of the paraffin series are given in the first column of the accompanying table. The formulas show that the number of hydrogen atoms in the molecule of each of these hydrocarbons is two more than twice the number of carbon atoms; hence the series may be represented by the general formula CnH2n+2.

To explain the fact that so large a number of marsh gas hydrocarbons exists, chemists have assumed that carbon is tetravalent in these compounds (cf. §§ 133 and 135), and that the carbon atoms are united in chains.

The constitutional formula for marsh gas has been given (cf. § 218). That of ethane is

The highest known member of the marsh gas series has the formula C60H 122. The members from butane to pentadecane. C15H32, are liquids at ordinary temperatures; those of greater molecular weight, solids.

522. Ethylene and Acetylene Series. As we learned in § 294, ethylene can unite with chlorine and bromine directly; it can also take up hydrogen, producing ethane.

The molecule of acetylene can take up twice as much of these substances as the molecule of ethylene.

Because of their ability to unite with hydrogen, etc., to form marsh gas derivatives, both ethylene and acetylene are said to be unsaturated, to distinguish them from methane and its homologues, which are saturated. We explain the unsaturation existing in these compounds by assuming that all the valences of the carbon atoms are not used.

Ethylene is represented by the constitutional formula,

The homologues of ethylene and acetylene contain chains of carbon atoms with both saturated and unsaturated groups. Thus, the constitutional formula of butylene (see table) is

H H H H

523. Closed and Open Chains. The chains of carbon atoms in substances already described are called " open "chains to distinguish them from groups of atoms in rings, which are called "closed" chains. The polymethylene and benzene derivatives named in the table contain closed chains.

524. Benzene Hydrocarbons. In § 297 it was stated that benzene is obtained from coal tar. When the coal tar is distilled, it yields several fractions the lower of which contain benzene and toluene, while higher-boiling portions consist of xylene, naphthalene, anthracene, etc.

Benzene is a colorless, aromatic liquid which boils at 80° C. and does not mix with water. It burns with a smoky flame. Solid benzene melts at 6° C. Benzene is used as a solvent and as a source of benzene derivatives, chiefly nitrobenzene, CH¿NO2 (cf. § 232).

Nitrobenzene is a yellow liquid boiling at 205° C. It has the odor and taste of bitter almonds.

Toluene is methylbenzene, CH,CH3. It boils at 110°.

Napthalene, C10Hs, is a white solid melting at 79° and boiling at 218°. It is used for "moth balls" and in the preparation of dyes. It reacts with bromine according to the equation,