The existence of tertiary (and higher) sub stitution products of benzene makes it possible to identify the ortho-, meta- and para- di-sub stitution compounds, so that the proper designa tion can be attached to each of them. The di bromobenzenes afford a good example of the way in which this is accomplished. The three essentially different compounds obtained by sub stituting bromine for two of the hydrogen atoms in benzene are (according to the notation just given) Br: Br=1: 2, Br: 1: 3, and Br: Br = 1 these being ortho-dibromobenzene, meta-dibromobenzene and pa ra-dibromoben zene, respectively. Now if an atom of hydrogen in the first of these be replaced by another atom of bromine, it is evident that the new bromine atom may have the position 3, 4, 5 or 6; but the compounds in which the bromine occupies the positions 1 : 2 : 3 and 1 : 2 : 6 must be regarded as identical, as will be seen by constructing the diagram; and, similarly, those in which it occu pies the positions and 1:2:5 must be considered identical. Hence the further intro duction of bromine into ortho-dibromobenzene can give rise only to the two distinct tri-bromo benzenes 1 :2 :3 and 1 :2 :4. If the remain ing dibromobenzenes be examined in the same way, it will be found that meta-dibromoben zene can yield (upon further bromination) the three distmct tn-bromobenzenes 1: 2: 3, 1: 3: 4 and 1: 3: 3. Finally, it will be found that para-dibromobenzene can yield only one tri bromobenzene; namely, 1: 2: 4: The identifi cation of a di-substitution bromobenzene as or tho-, meta- or pars- is therefore seen to be equivalent to determining how many different tn-bromobenzenes the given di-bromobenzene can yield. This problem has been fully worked out in the case here taken as an illustration, and it has been shown that of the three known di-bromobenzenes, the ortho- compound is the one boiling at 435° F. the meta- compound is
the one boiling at 427 F., and the para- com pound is the one melting at F. The mode of identification here discussed in detail for bromine substitution products can be applied in other cases also, but the labor involved in the operation is so great that it is usually easier to ascertain the proper prefix for a new di-substi tution compound by noting which of the bromo substitution products must be used as a starting point, in the synthesis of the proposed com pound. There is usually but little difference in the boiling points of ortho-, meta- and para compounds, but the para- compounds have the highest melting points. . The benzene ring of ortho- compounds is liable to be broken up by oxidation, while in the other. two classes the ring usually persists. The following general law appears to hold true of di-substitution aro matic compounds: When a radical is introduced into a benzene ring in which one hydrogen atom has already been replaced by a radical, the second radical will take a position °meta° to the first one, provided the first was COOH, SO:1-1, NO2, or (probably) CN, CHO or CO.CH,. In most other cases the second rad ical will mainly take the °pare position, though some of the uortho° compound is almost in variably produced at the same time. The aro matic bodies include many acids, the simpler of which may be conveniently classified according to the number of molecules of carboxyl (COOH) that they contain, and according to the number of hydrogen atoms that have been displaced in the original benzene ring. The simpler and more familiar aromatic acids mostly contain one carboxyl group, and are therefore said to be umono-carboxylic.' In the mono-carboxylic group, benzoic acid, GMCO OH, is mono-hydricp ; salicylic acid, C51-150H.