(1) Claus and Koerner, (2) Dewar, and (3) Ladenburg. None of these, however, explains all the observed reactions of benzene

and its derivatives as well as the structural formula originally proposed by Kekulé.

2. Formation of Benzene Homologues.-In the case of open-chain hydrocarbons, whether of the saturated or unsaturated series, the successive homologues were formed by lengthening the chain or adding on additional carbon atoms with the requisite saturating hydrogen. Thus, following methane we had ethane, propane, butane, etc., and following ethylene we had propylene, butylene, etc. In the case of benzene the closed-chain structure precludes this method of forming an homologous series. But we have following benzene, C.Ha, toluene, C,Hg, xylene, CH107 etc., as far as C12H18. A slight examination of these compounds, as, for example, with oxidizing agents, shows that there are two parts in them of very different stability, a nucleus, CH, or CH, not oxidizable, and one or two side-groups, CH, which have replaced hydrogen atoms of the original C.H., and are easily oxidized to COOH groups. In other words, toluene is methyl-benzene, xylene is dimethyl-benzene, etc., and their formulas may be written C,H,. CH, and C,H,(CH3)2, etc., up to hexamethyl-benzene, C(CH3)g, when this series of homologues stops. It is true we can have ethyl-benzene and diethyl-benzene or methyl-propyl-benzene and similar derivatives, but these are not found in any great number in nature. Nor are their derivatives of the same importance as those of the methylated benzenes.

3. Differences between the Benzene Hydrocarbons and the Open-Chain Hydrocarbons.-(a) We notice first that the action of concentrated nitric acid is quite different. With the closed-chain hydrocarbons a hydrogen atom of the nucleus is readily replaced by the group NO2, as C.H. + NO„.OH = CH. NO2+ H2O, the product being called nitrobenzene. With paraffin hydrocarbons nitric acid has little or no action, even when heated for a time.

(b) With concentrated sulphuric acid the benzene hydrocar

bons give rise to sulphonic acids, as C.H. SO,H.OH = СН(SO ̧Н) + H2O, the product being called benzene-sulphonic acid. Concentrated sulphuric acid has no action on the paraffin hydrocarbons, and with the olefines it forms addition compounds without displacement of hydrogen.

(c) As mentioned before, under the influence of oxidizing agents the homologues of benzene are easily oxidized, yielding the corresponding carboxylic acids. Thus, toluene, CH-CH, is oxidized by dilute nitric acid or by chromic acid to benzoic acid, CH,.COOH. The open-chain hydrocarbons are only acted upon by oxidizing agents with great difficulty.

(d) The hydroxyl derivatives like C,H,.OH are quite different from the simple OH derivatives of the paraffin or other openchain hydrocarbons. The former have more of an acid character, while the latter are basic hydrates. Thus, C,H,OH is phenol or carbolic acid, while C2H. OH is ethyl alcohol, which forms esters or salts with the acids.

4. Isomerism in the Closed-Chain Hydrocarbons.-In the hydrocarbons of the paraffin series it is possible to obtain isomeric mono-substitution derivatives, as, for example, normal propyl chloride, CH,Cl.CH.CH,, and isopropyl chloride, CH,. CHC1.CH,; in the case of benzene, isomeric mono derivatives cannot be obtained. The six hydrogen atoms of the benzene seem to possess an equal value. It is a closed-chain structure, and it matters not at what point in the ring thus formed the single substitution takes place. This fact has been proved experimentally. It is different when two atoms of hydrogen in the benzene molecule are replaced by other atoms or groups. These di-substitution derivatives may exist in three isomeric modifications. We may have three dichlor-benzenes, three dimethylbenzenes, three dinitro-benzenes, etc. This also has been established experimentally, and the limit set at three.

If we now look at the benzene molecule as figured in Kekulé's theory, we see the explanation of this fact. Taking the structu

the top going to the right, we have for the purpose of representation each one indicated. Now, if the replacement take place at

(1) and (2), or (2) and (3), or (3) and (4), or (4) and (5), or (5) and (6), or (1) and (6), the resulting di-substitution compound is one and the same substance. While we figure this hexagon structure for convenience of explanation of the observed characters of benzene, we do not for one moment pretend that it has a fixed position in space with a north and a south corner, or with two eastern and two western carbon atoms. But in the cases just mentioned the substitution has taken place in connection with adjacent or directly connected carbon atoms. To distinguish them all such di-substitution compounds are called ortho compounds, as ortho-dichlor-benzene. If the replacement take place at the carbon atoms marked (1) and (3), or (2) and (4), or (3) and (5), or (4) and (6), or (5) and (1), or (6) and (2), the resulting disubstitution compound is one and the same substance. In these cases the substitution has taken place in connection with carbon atoms separated by one intervening carbon atom. Such compounds are called meta compounds, as meta-dimethyl-benzene. If the replacement take place at the carbon atoms marked (1) and (4), or (2) and (5), or (3) and (6), the resulting di-substitution compound is one and the same substance. In these cases the substitution has taken place in connection with carbon atoms separated by two intervening carbon atoms. Such compounds are called para compounds, as para-dinitro-benzene. Frequently these designations are indicated by the initials only, as o-oxybenzoic acid, m-xylene, and p-phenol-sulphonic acid.

When three hydrogen atoms of the benzene molecule are replaced the case becomes more difficult. We may have three contingencies to consider here: (1) all three substituting atoms or groups are alike; then three isomers may exist; (2) of the substituting elements two are alike and the third is different; in this case six isomers may form; (3) if all three substituting elements are different, ten isomers may form.

When more than three substitutions take place, the number of isomers becomes very great.

In distinguishing between the different tri- and tetra-substitution derivatives, at times the designations v (standing for vicinus, neighboring), s (standing for symmetrical), and a (standing for

substitution derivative which would be marked v, as v-trinitro

designated by the s and a respectively, as s-trimethyl-benzene and a-trichlor-benzene.

These cases of isomerism, it will be noticed, are all concerned with the position in the nucleus assumed by the replacing atom or group. We may also have side-group isomerism, as normal propyl-benzene and isopropyl-benzene.

More important than this last, however, is the case where a substituent enters the benzene nucleus in one case and the sidegroup in another case, giving us the so-called "mixed isomerism." Thus, CHC1. CH ̧, monochlor-toluene, and CHÂCH2Cl, benzyl chloride, or C.H(CH),, xylene, and C.H. CH, ethylbenzene, are isomeric.

The determination of the nature of a di-substitution derivative of benzene, whether ortho, meta, or para, is to be accomplished by the treatment with reagents, whereby the nature of the sidegroup may be changed and the product studied as to its properties. Ortho derivatives through a series of such changes will remain ortho derivatives, and so with the meta and the para compounds. By such a series of transformations it becomes possible correctly to identify the nature of the original compound.

CHAPTER VI.

AROMATIC COMPOUNDS CONTAINING ONE NUCLEUS.

IN explaining the theories held as to the fundamental differences between the aromatic compounds and the open-chain hydrocarbons and their derivatives, we have touched only upon the struct ure of benzene, taking it as the type of closed-chain compounds, and, in fact, the starting-point from which they are derived. We will see, however, later, that two or more of these benzene nuclei may unite, either by simple linking without condensation, or by condensing together to form a compound nucleus, obviously related to the simpler benzene molecule, but built up by its doubling or trebling itself. Thus, diphenyl, diphenyl-methane, triphenyl-methane, and indigo all represent aromatic compounds with more than one benzene nucleus in which the parts are linked together without condensation of the nuclei. On the other hand, naphthalene, anthracene, phenanthrene, quinoline, and acridine all represent molecules formed by the condensation of benzene nuclei.

We shall first confine our attention to those aromatic compounds in which a single benzene nucleus appears as the basis of the molecule.

I. HYDROCARBONS.

1. Saturated Hydrocarbons. We have here to deal with benzene and its homologues. They occur to some extent in the free state in nature, being found in Galician and Hanoverian petroleum, and even in small amount in Pennsylvania petroleum. They are obtained, however, most abundantly as the product of the destructive distillation of bituminous coal, and hence are contained in coal-tar. This is a very complex mixture, and more than forty distinct compounds of the aromatic class have been identified in it. When roughly fractioned from the tar-stills it yields three main fractions: (1) The light oil, sp. gr. o.9, boiling point up to 150°, which contains mainly benzene and its homologues, with some naphthalene; (2) middle oil, sp. gr. up to 1.01, boiling point 150° to 210°, which contains especially naphthalene, carbolic and cresylic acids, and quinoline bases; and (3) heavy oil, sp. gr.