BENZENE AND ITS DERIvATIvEs. One of the most important of the compounds of carbon is ben zene (q.v.), a liquid hydrocarbon found in coal tar. Thousands of substances are derived (ruin benzene, thousands find extensive application in the arts. (See Con-TAR CoLoas.) An exact knowledge of the constitution of benzene is there fore highly desirable for theoretical as well as for inunediate practical reasons. and so chemists have now for many years diligently searched for a graphical formula that might be in perfect cor respondence with all the chemical properties of benzene. A perfect formula has not as yet been devised. Nevertheless, four different formulas, each of which exhibits well sonic of the proper ties of benzene, have been proposed and are com monly referred to, respectively, as 'Kekirle?s formula,' 'the prism formula,' the diagonal for mula,' and 'the centric formula.' The last of these is now supposed to be the best, but the first is still commonly used by chemists and therefore requires explanation in the present sketch. It is as follows: Unlike the formulas already eonsiderod in this article, the formula of benzene has its carbon atoms arranged, not in the form of a chain, but in a ring, with alternating single and double bonds between the atoms. The complete demon stration of this fornmla formed one of the most beautiful chapters of organic chemistry. Noth ing astonishes the young student more than to find that in this demonstration an atom of bro mine, substituted for one of the hydrogen atoms of benzene. is made to travel round the 1111)1ccule, exchanging places successively with each of the hydrogen atoms, and that this migration of the bromine atom is demonstrated beyond the slight est possibility- of doubt. The denionstratiim can not be repeated here, and till account of it must be sought for hi one of the larger works on organ le (or consult la rekwal d's lecture on the theory of benzene published in Ahrens's Sam nanny chemischer and cheinisch I echnischer Fort rayc, Stuttgart, 1897). What requires explanation in the present article is time isomerism of derivatives of benzene and the choice of the particular formula corresponding to each of a given set of isomeric compounds.
KekulCs formula is based principally on two facts: (1) When one hydrogen atom of benzene is replaced by one atom of some other element, only one product is obtained: thus, only one mono-ehloro-benzene, 12,11,C1. can be prepared.
(2) When two hydrogen atoms of benzene are replaced by two atoms of some other element, three different products may be obtained; thus. three different di-ehloro-benzenes. can be prepared. Corresponding to the tirst of these facts, the six hydrogen atoms occupy identical positions in 1:ekuli•s 14 mnula, so that only one formula can he obtained by substituting. say. a chlorine atom for a atom: for the fol lowing formulas are evidently identical: ing this important question— important be cause it presents itself in the case of thousands of valuable compounds. One of these methods, characterized by great simplicity, certainty, and elegance. was discovered by Korner. It is based on the following considerations. When an ortho di-substitution product like ortho-dichloro-ben •ene is transformed into a tri-substitution pro duct by the introduction of some group, say, the nitro-group N( tiro, and only two, different compounds may be obtained according to the formula of benzene, viz.: A meta-di-substitution product would simi larly yield three, and only three, tri-substitution products, as follows: The second of the facts just mentioned may be found to be expressed by Keku16's formula, by examining all the imaginable arrangements that could be made in the formula by substi tuting two- atoms. say, of chlorine for two of its hydrogen atoms. It is thus easy to find that only three different formulas can be constructed. as follows: Finally, a para-di-substitution product would yield one, and only one, tri-substitution product, viz.: The position )1, 2) of the chlorine atoms in the first of these formulas is called the ortho position : the position (1, 3) in the second formula is called the meta-position: the position (1, 1) of the third formula is called the para position. .Any other imaginable position would really be identical with one of these three: thus the position (2, 31 is obviously the same as the position (1. 2) the position (1, 5) is the same as the position (1, 3), etc. The three compounds corresponding to our three formulas are called, respectively, ortho-dichloro-benzene, meta-Weida ro-benzene, and para-dichloro-benzene, and may be denoted as follows: and p-C,11,C1„; or as follows: (1, 2), C„lI,C1, (1, 3), and C,H,CI, (1, 4).