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CHEMISTRY 731 lias no disturbing effect, but apparently even strengthens of fundamental types, and it is therefore justifiable to the affinity of these latter for each other. assume that their formation is governed by comparatively The case of thiophene is even more remarkable. In this simple laws. compound two of the CH groups in benzene are displaced In comparison with that of the closed chain compounds by a single atom of sulphur. Notwithstanding the great the behaviour of open chain compounds is in principle far difference in composition, in general chemical behaviour simpler. In the case of saturated compounds the two compounds are almost indistinguishable, corre- substitution appears to take place directly, but °Pen chaia sponding derivatives manifesting the most remarkable there is no evidence to show the precise manner com~ similarity, extending even to their boiling-points. Thio- in which it is effected • in fact, little more can be P phene is more readily acted upon by halogens than is said than that it occurs the more easily the higher the benzene, being attacked even by iodine, but it shows no molecular weights. Whether or not at the moment of collitendency to combine with them, and yields only substitu- sion of the interacting molecules weak associations are estabtion derivatives. The most striking circumstance in con- lished between them cannot at present be determined : the nexion with thiophene, however, is the complete inertness most that can be said is that it is by no means improbable of the sulphur atom, which shows no tendency to combine, that such is the case, and that the assumption is one which and cannot even be oxidized ; yet it cannot well be present apparently would bring the behaviour of saturated into otherwise than as a dyad. This behaviour appears the harmony with that of unsaturated compounds generally. more remarkable when thiophene is compared with the A point of importance to be noted in this connexion, howcorresponding compounds furfurane and pyrrole, containing ever, is the fact that the occurrence of several carbon respectively oxygen and the imido (NH) group instead of atoms in association usually constitutes a position of weaksulphur, these latter being far more active substances and ness in the molecule. For example, to take the simplest remarkably sensitive to the action of acids. Whether case, propane, CH3.CH2.CH3, is chiefly attacked in the the oxygen and imidogen exercise a special attractive influ median CH2 group, a larger proportion of isopropyl ence, such as has been previously referred to, which sulphur chloride, CHg.CHCl.CHg, than of the isomeric propyl cannot exercise, or whether the difference originates in chloride, CH3.CH2.CH2C1, being formed when the hydroa difference in constitution, cannot at present be decided, carbon is chlorinated. In the case of a hydrocarbon of though the former appears to be the more probable ex the type CHR 3, the hydrogen atom of the CH group is planation. It is noteworthy that in diphenylene sulphide still more sensitive to attack than that in the CH„ group (dibenzothiophene)—a compound which bears the same of the type CH2R'2. Facts such as these serve to suggest relation to thiophene that anthracene bears to benzene that even in what are conventionally regarded as saturated — the sulphur behaves normally in the sense that it is compounds carbon exercises an attractive influence similar oxidizable, like the sulphur in open chain sulphides such as to that pictured above as exercised by oxygen, nitrogen, etc. ethyl sulphide (C2H5)2S. As thiophene in no way exhibits Such a conclusion is in harmony with that already arrived the behaviour of an ethenoid compound, it is ap- at with reference to the influence exercised by saturated propriately represented by the centric formula in hydrocarbon radicles in the case of the homologues of the margin. But it is conceivable that the sul- benzene. phur, although acting only as a dyad, has its extra N / Effects similar to those produced by the association of ' S affinities twisted inwards into the ring: hence several carbon atoms are observed to follow the introduction their inactivity. No such condition need prevail in of negative radicles such as chlorine, bromine, diphenylene sulphide, as there would or OH, the rule being that negative radicles Effect of be no carbon affinities directed inwards tend to accumulate. Thus the primary alcohols radicles. C c in the median ring if the lateral rings of the formula RICH.,(OH), are converted into had a centric structure. This, again, is s aldehydrols, R'.CH(OH)2, by most oxidizing agents • and an example of the manner in which Diphenylene bromethane, CH3.CH2Br, when heated with bromine, yields sulphide. an instructive comparison may be made chiefly ethylidene bromide, CH3.CHBr2. But there are between single and complex cycloids. The difference be- many apparent exceptions to this rule. Thus, although tween a “ strutted ” benzenoid system and a flaccid aldehyde, CH3.COH, is entirely converted into acetic acid, saturated cycloid is well shown in the fact that, although CH3.C02H, when oxidized by bichromate mixture—the it is impossible to obtain an anhydride from metaphthalic aldehydrol being presumably converted into the acid-hydrol, acid, hexhydrometaphthalic acid, like glutaric acid, is with- R'.C(OH)3, which breaks up into acid and water—when out difficulty converted into an anhydride; and the same oxidized by nitric acid it yields glycollic acid, kind of difference appears to exist between thiophene and CH2(0H).C02H, and glyoxal, COH.COH. Similarly, if dibenzothiophene. bromethane be brominated in presence of ferric bromide, it In concluding this section, the question may be raised is converted into ethylene bromide, CH2Br.CH2Br ; in fact, whether the centric structure may still be preserved in experiments made by Y. Meyer and his pupils have shown compounds formed from benzene by satisfying two of the that when bromides or chlorides are brominated in presence centric units of affinity. In the case of the derivatives of of ferric bromide, or chlorides chlorinated by means of dihydrobenzene, C6H8, such as the dihydrophthalic acids, antimony pentachloride, instead of a second atom of halogen there can be little doubt that the compound has lapsed into becoming attached to the one carbon atom, it associates the ethenoid form, as these readily combine not only itself with the contiguous atom. In the case of the more /b' with bromine, but also with the halhydrides. But complex hydrocarbons, by continuation of this treatment, quinone, on the other hand, is a relatively inert the action may be extended from atom to atom along compound, and it is therefore not improbable the chain—hexane, for example, giving a hexbromide in that it retains the centric structure, and may which each carbon atom is associated with a bromine atom. o consequently be so represented—as in the margin. Such departures from the rule, however, but serve to prove Sufficient will have been said to indicate the nature of it. In seeking to explain such variations, it is necessary some of the chief problems presented by cycloid com- to bear in mind that variations in the course of the interpounds. The number of varieties, especially of hetero- change may be conditioned by the use of different agents, cycloids, made known since 1875 is very large, but appa- and that intermediate unperceived changes occur in all rently these may all be referred to a very limited number such cases. A catalyst such as ferric bromide plays an