Sulphuric acid acts, therefore, upon anthracendihydride as an oxidising agent, and reconverts it into anthracen :

C11H12+SH2O1 =С11H10+2H2O+SO2•

12

14

If to a molecule of anthracendihydride, dissolved in disulphide of carbon, a molecule of bromine is added by drops, hydrobromic acid escapes in abundance, and there remains, after evaporation of the disulphide of carbon, a crystalline mass, which is resolved by alcohol into a substance readily soluble in this liquid, and one almost insoluble. The former is undecomposed anthracendihydride, melting at 106°; the latter is bibromanthracen, which melts between 219° and 220°. Anthracen is not obtained from this reaction. The action of 2 mol. bromine upon 1 mol. of anthracendihydride is similar, but the yield of bibromanthracen is greater. It is formed most abundantly by the reaction of 3 mol. bromine and 1 mol. anthracendihydride. The bibromanthracen thus obtained agrees in every respect with that obtained directly from anthracen, as described below.

The formation of bibromanthracen will be readily understood from the above experiments, which show that the action of bromine upon anthracen is most complete when 3 mol. of the former act upon 1 mol. of anthracendihydride.

Two atoms of hydrogen are carried away by two atoms of bromine, whilst simultaneously bromine replaces two more atoms of hydrogen.

C1H10H2+6Br=C11H ̧Br2+4BrH.

2

Anthracendihydride is readily oxidised by chromate of potassa and sulphuric acid, and converted into the characteristic oxidation product of anthracen,-anthraquinon. This agrees in every respect with that to be described below, sublimes in the same yellow needles, and melts at 275°. If anthracendihydride is heated with dilute nitric acid, anthraquinon is obtained along with nitrated products, amongst

which binitroanthraquinon may be recognised by the characteristic compound which it forms with anthracen.

For the oxidation of anthracendihydride we require four atoms of oxygen, which are furnished by the oxidising agent, chromic or nitric acid.

C11H12+40=C11H。(O2)” + 2H2O.

14 12

An atom of oxygen effects the removal of the two added atoms of hydrogen, and the three others complete the conversion into anthraquinon, by substituting the group (0,)" for two atoms of hydrogen.

Anthracendihydride in an alcoholic solution cannot be further reduced by sodium-amalgam. Even if the action is prolonged for days, there are formed only traces of a compound richer in hydrogen, whilst the melting-point sinks for some degrees. Hydriodic acid (melting-point 127°) has no further reducing action upon anthracendihydride at 160°, but at 200-220°, converts it into anthracenhexahydride.

An alcoholic solution of iodine at the boiling-point has no effect.

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If we survey the behaviour of the hydrocarbon just described, C1H12, its complete agreement with the behaviour of the so-called addition-products of the aromatic series cannot be ignored. In all the reactions of anthracendihydride there appears the characteristic phenomenon, that compounds are always formed belonging to the anthracen type, poorer in hydrogen; this agrees with the observations on hydrophthallic acid, hydromellitic and quinic acids, from which there have been hitherto obtained merely substitutionproducts of benzol, but no substances belonging to a type rich in hydrogen. Graebe and Liebermann, therefore, selected for the first reduction-product of anthracen the name anthracendihydride, which agrees with the name benzolhexachloride, CH. Cl. The formation of anthracendihydride must, therefore, be regarded as a transformation of the

double combination of the carbon atoms of anthracen into a single one, and saturation of the bonds thus set free, by two atoms of hydrogen. By the reformation of anthracen and its derivatives from the dihydride, occasioned by the action of various reagents, the more intimate combination of the two atoms of carbon by every two unities of affinity is reproduced.

For deciding on the chemical character of anthracendihydride, it is interesting to compare it with stilben, which contains the same number of atoms of hydrogen and carbon. According to the researches of Limpricht and Schwanert, stilben has the rational formula:

CH-CH-CH-CH,

and behaves in many respects like ethylen. It combines directly with two atoms of bromine, and forms bromide of stilben, corresponding to bromide of ethylen, whilst hydrogen is withdrawn from anthracendihydride by bromine. On oxidation, stilben yields oil of bitter-almonds CH,C=0; and benzoic

acid

CH,COH

0,

\H

its molecule being split up. Anthracendihydride, on the other hand, is converted into anthraquinon. Anthracenhexahydride, C11H.—Anthracenhexahydride is the second reduction-product obtained by the action of hydriodic acid upon anthracen. A tetrahydride does not appear to exist. The substance formerly described by Graebe and Liebermann as a tetrahydride, the analysis of which agreed very well with the formula C1H11, proved on closer examination to be a mixture. It is extremely troublesome to obtain the hexahydride in quantity, as the sealed tubes often explode from the liberation of phosphuretted hydrogen.

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The best material for its preparation is anthracendihydride. This is heated for ten or twelve hours in sealed tubes

to 200° to 220°, along with to part of amorphous phosphorus, and about 5 parts of hydriodic acid (boiling-point 127°). The tubes must be opened with caution, as much phosphuretted hydrogen is always generated. The hydrocarbon obtained is washed with water to remove the hydriodic acid, dissolved in alcohol, and separated by filtration from the residual phosphorus. The mass obtained on evaporation of the alcohol, is pressed between bibulous paper in order to separate the anthracenhexahydride from small quantities of an accompanying liquid body. It is then submitted to distillation, and the portion boiling at 290° is collected separately. Thus is obtained a hydrocarbon, melting at 63°, whose analysis leads to the formula C1H16°

In its physical properties anthracenhexahydride agrees very closely with the dihydride. It crystallises from solvents in leaflets, and is readily soluble in alcohol, ether, and benzol.

If conducted through an ignited tube, it is resolved into anthracen and free hydrogen. It resists the action of nitric acid much better than the dihydride.

Anthracenhexahydride is formed from anthracen, by the conversion of the double combination of three couples of carbon atoms into a single one, thus enabling six atoms of hydrogen to be taken up.

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141

4.

Bromine derivatives of anthracen.-Anderson was the first chemist who studied the action of bromine upon anthracen, and described two compounds, to which he assigned the formulæ, C1HBг, and C1H ̧Bг. The former, which he obtained by the action of an excess of bromine upon anthracen, he considers as an addition-product, six atoms of bromine to one molecule of anthracen. From this he prepared the second compound by the action of an alcoholic solution of potassa, and regarded it as bibromanthracendibromide. Berthelot, who observed subsequently that hydrobromic acid is formed when bromine reacts upon anthracen, suggested that both these bromine compounds

probably contained two atoms hydrogen less than Anderson had assumed in his formulæ. The behaviour of the former bromine-compound with alcoholic potassa is in favour of this view. An addition-product with six atoms of bromine must, like benzolhexachloride, become a tri-substitutionproduct, whilst a compound is formed containing four atoms of bromine. Graebe and Liebermann have, therefore, reexamined the bodies described by Anderson, and have ascertained experimentally, that Anderson's formulæ contain two atoms hydrogen too much. Besides these compounds, the above-mentioned chemists have obtained a bibromanthracen and a tribromanthracen. A monobromanthracen has not yet been produced. From anthracen dissolved in disulphide of carbon, there is formed by the reaction of one molecule on exactly two atoms of bromine, bibromanthracen, a corresponding quantity of anthracen remaining unchanged. By lixiviation with alcohol, the anthracen is removed from the sparingly soluble bromine compound, and may by re-crystallisation be isolated, perfectly free from bromine.

Bibromanthracen, C1H.Br.

As already mentioned, bibromanthracen is formed by the action of bromine upon anthracen, dissolved in disulphide of carbon. It is the only substitution-product formed under these circumstances. Even if more than four atoms of bromine are applied to 1 mol. of anthracen, there is no formation of a compound richer in bromine. For the preparation of bibromanthracen, it is sufficient to diffuse the anthracen in disulphide of carbon, and to add by degrees the calculated amount of bromine. The disulphide of carbon becomes hot, and the bibromanthracen dissolves as it is formed. On cooling it is deposited in fine yellow needles, often above 1 centm. in length, which are purified by recrystallisation from disulphide of carbon, or the homologues of benzol. The portion of bibromanthracen which remains