Page:The New International Encyclopædia 1st ed. v. 04.djvu/239

* CARBON COMPOUNDS. 199 CARBON COMPOUNDS. isomerism of derivatives of benzene and the choice of the pailieuhir foruiula corresponding to each of a given set of isomeric compounds. Kekulo's formuUi is based principally on two facts: (1) When one hydrogen atom of benzene is replaced by one atom of some other clement, only one i)roduet is obtained; thus, only one inoiio-cliloro-benzene. ColI^Cl, can be prepared. (2» When t«o hydrogen atoms of benzene are replaced by two atoms of some other element, three ditlercnt products may be obtained; thus, three dilVereut dichloro-benzenes, CoHjCU, can be prepared. C'orresj)onding to the tirst of these facts, the six hydrogen atunis occupy identical positions in Kekule's formula, so that only one formula can be obtained by substituting, say, a chlorine atom for a hydrogen atom : ft)r the fol- lowini; formulas are evidentlv identical: CI CH HC CH H H HC 'C1 ^^'s H CH HC CH H H HC^ CH H HC CH HC CCl H ccAh CIC CH HC, CH ^G^ V HC I II HC CH CI H H The second of the facts just incntioned may be found to be expressed Ijy Kekule's fornuila, by examining all the imaginable arrangements that could be made in the fornuila 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: CI CI CI x5-C ^^ ,^^ HC(6)*i)CCl HC^)(i>(2)CH HC(6)''>C2)CH HC(3),.,(3iCH ^c/ HC<5),.,(3)CC1 ^c/ HC|<,)^l'jH H H CI 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 ; t!ie position (I, 4) of the third formula is called the para- position. Any otlier imaginable position wimld really be identical with one of these three; thus the position (2, 3) 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 fornuilas are called, respectively, ortho-dichloro-benzene, meta-dichlo- ro-benzene, and jiara-diehloro-benzene, and may be denoted as follows: o-CoH,CL, m-CoH.CU, and ])C„H,C1,; or as follows: C„H,CL (1, 2), CJI.CI, (1, 3), and C„H,C1, (1, 4). Bui then how do we know w-hich of these com- pounds is ortho, which is mcta, and which is para? In other words, being given three different compounds ami three dill'erent fonnulas corre- sponding to them, how do we decide which for- mula is to be assigned to which compound? Several different methods are employed in decid- ing this important question — important he- 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 Kiirner. It is based on the following considerations. When an ortho- di-substitution product like ortlio-dichloro-ben- zene is transformed into ii tri-substitution pro- duct by the introduction of some grou|i, say, the nitro-group NO., tiiu, and only two, dill'erent compounds may be obtained according to the formula of benzene, viz. : CI CI a HC cci HC CCl f HO ccn „l I aud„l « from I 1 } m CNO, h6 CH [ HC CHJ H NO, u A inefa-di-substitution product would simi- larly yield three, and only three, tri-substitution products, as follows: CI CI CI // // HC CNO, HC CH HC I II I II and I II IC CCl NOjC CCl ^" (11 HC CCl Wc/ H HC NO, H from CI //^•^ HC I HC ^ ^C^ H CH 1 Finally, a para-di-substitution product would yield one, and only one, tri-substitution product, viz.: CI >C. HC CH CI /^ HC CNO., r I Ii " from I II |. HC CH I HC {;h J ^c/ ^7/ CI CI In order, therefore, to determine whether a given disubstitution product is ortho, meta. or para, the given compound may be transformed into a tri-substitution product. If it is then found that the tri-substitution product is capable of existing in two isomeric modifications, the con- clusion is that the particular di-substitution product from which it is derived corresponds to the orifto-formula. Similarly, if the number of isomeric tri-substitution [)roducts is three, the corresponding di-substitution product is seen to be necessarily a mefa-comjiound. And if only one tri-substitution product can possibly be ob- tained from a given di-substitution product, the latter is concluded to be a para-(,'om|)ound. Ohoanic Reaction.s. Now, in reality it would be a tremendous undertaking to go through a demonstration of the kind just described, in the case of every newly discovered compound. In many cases years would have to be spent in de- termining the constitution of a single compound. Chemists therefore make use of an additional working principle — a generalization made from