BENZENE, an aromatic hydrocarbon dis covered by Faraday in 1825 and called by him "bicarburet of hydrogen') It has the chemical formula C2H2, and is the fundamental substance from which the extensive series of "aromatic compounds" is obtained. In 1845, Hofmann proved its existence in coal tar and that substance now constitutes its most im portant cotnmercial source. In the manufac ture of benzene, coal tar is distilled at a tem perature not exceeding 300° F., and the dis tillate is treated with caustic soda to remove phenols and subsequently with sulphuric acid to remove basic substances. It is then re distilled, the temperature (at least in the tip per chart of the still) being kept as low as 212° F., in order to prevent toluene from passing over: In order to effect a still further puri fication, the benzene so obtained may be cooled by a freezing mixture of ke and salt. The true benzene solidifies when thus treated and the fluid impurities that it contains may be expelled by pressure or by the aid of a centrifugal dner. Pure benzene is a colorless liquid, strongly refractive, boiling at about 177° F.„ and freezing at 40° F. It does not mix with water, but mixes readily with alco hol, acetone, glacial acetic acid, chloroform and ether. It crystallizes in the trimetric sys tem when solidified by cold and dissolves iodine phosphorus, sulphur, oils, resins, fats and alkaloids. It expands by about 0.00075 of its own bulk, per degree increase in its temperature on the Fahrenheit scale. Its specific gravity is about 0.88 at 15° and its specific heat is 0.40. For the chemical consti tution of benzene, see AROMATIC COMPOUNDS.
Benzene forms two general classes of com pounds, lcnown respectively as (addition" and "substitution* products. In forming an (addi tion° compound, benzene merely takes up atoms or molecules of some other substance, without parting with any of its own atoms; the new substance being simply wadded( to the benzene. Benzene hertabromid, C2H213r2, is a good example of a benzene addition com pound. It is formed by dropping bromine into boiling benzene, in direct sunlight; the hexa bromide crystallizing out upon cooling. The "substitution( compounds of benzene are far more numerous and important than the (ad dition( compounds, however. They are formed by replacing one or more of the typi cal hydrogen atoms in the benzene by an equal number of other atoms or monad radicals. The general theory of benzene substitutions is given under Aromatic Compounds; but a few of the more important examples of such sub stitution products may be given here. The radical C4112 (which is not capable of inde pendent existence) is called (phenyl,' and is often represented by the symbol Ph. The mono-substitution compounds of benzene, in which one atom of the hydrogen in the orig inal benzene has been replaced by a radical (or by an atom different from hydrogen), may then be regarded as addition compounds of the radical phenyl. Thus •monochlorben
zene,i C2H2.C1, may also be regarded as chloride of phenyl and its formula may be written PhCl. Benzene itself may even be regarded as hybrid of phenyl, its formula being written C21-12.H, or PhH. Carbolk acid (or (phenol') is hydrate of phenyl, its formula being PhOH, the radical OH being here substituted for one atom of the hydrogen in the original .benzene. Nitrobenzene, PhNO2, is formed from bcnzene (PhH) by the action of nitric acid, in accord ance with the equation PhH HMO.= PhNO, H20.
It is used in the arts for the manufacture of aniline (q.v.). Aniline itself is an amide of phenyl, obtained by replacing an atom of H in ammonia (NH,) by phenyl, or by replacing an atom of hydrogen in benzene by the radical NH. The formula of aniline may be written PhNH,, and aniline may be called earnido benzene,° or °phenylamine.° (See AMIDE; AMINE). Methyl-benzene, CaLCH,, in which one of the original hydrogen atoms of the benzene is replaced by the radical CH, (Imethy1°) is also an important benzene sub stitution compound and is known to chemists as toluene. That portion of the original benzene which remains intact, after a sub stitution, is called the °benzene residue.° In a mono-substitution compound of benzene, fur ther substitutions may be made by replacing one or more of the hydrogen atoms in the •benzene residue') by univalent radicals, and sec ondary, tertiary, and higher substitution com pounds may be thus formed. The classifica tion of the secondary suktitutioll compounds is given under Aromatic Compotnids. For the classification of higher compounds, special treatises on organic chemistry must be con sulted. It may he mentioned, however, that if A, B, C and D are monad radicals, there arc no less than 30 distinct substances pos sible, which shall all have the same general formula GH.ABCD. This fact illustrates the exceeding complexity of the general the ory of benzene substitutions compounds. The full theory is even more complex than this example indicates, however, for it often hap pens that the hydrogen in a substituted radical can be replaced by another radical, as well as the hydrogen of the °benzene residue.° Thus in methylbenzene (or toluene), C.H. CH,, the radical OH may be substituted for one of the hydrogen atoms. If the hydrogen so displaced occurs in the °benzene residue.° the resulting compound will be (OH).CIL, a substance which (since it is a di-substitution compound) can exist in three isomeric forms. If, on the other hand, the OH takes the place of one of the hydrogen atoms of the methyl° radical, the resulting compound will be alcohol,° (OH).