Page:1902 Encyclopædia Britannica - Volume 26 - AUS-CHI.pdf/785

 CHEMISTRY 729 the direct action of iodine, the latter change is a these elements exercise their functions by studying phenol particularly striking proof of the transference of a radicle from one position to another within the molecule. That and aniline. It is too often forgotten that phenol is a derivative of oxygen, and aniline a derivative of nitrogen, such transferences are but the end result of a series of and that these compounds are not merely to be regarded changes more or less of the character of those pictured as derivatives of benzene; in point of fact, it is only when above is more than probable. they are considered from the former point of view that In the case of phenol and aniline, whatever the agent them benzene function can be properly understood, and used in effecting substitution, the entering radicle as a that it becomes apparent how false an estimate is usually rule always takes up either the para- or the ortho-position orme. of this function. In the formation of benzenoid relatively to the OH or NH2 group; in fact, when this is in cases in which the attack proceeds from not the case, and a meta-derivative is formed, it is probably aderivatives, radicle in position 1, the para - derivative is almost because—as will be explained later—the compound no invariably the chief if not the sole product, and so much longer acts as an OH or ]MH2 derivative. In other cases is this the case that attention has been called, over and in which para- and ortho-derivatives are exclusively formed over again, to the need of giving expression to the fact in the influence exercised by the radicle present in the e formula of benzene. The centric symbol, more than mono-derivative may obviously be regarded as of the any other, may be said to indicate and almost to require same order as that exercised by either oxygen or nitrogen, such a result. For if the view be taken that, as explained and it is a logical extension of this argument to assume above, addition precedes substitution, whatever change that the process by which the higher derivative is formed place m region 1, it is- to be supposed that the is. similar in character, whether the mono-derivative con- takes opposite centric valency will be the one most affected : tain OH, NH2, Cl, CH3, or any other radicle which and if the effect be—as it most probably is—to release it conditions the entry of the second group into the para- either partially or entirely, region 4 will, as a rule, prevail and ortho-positions only. If this conclusion be accepted, over the other regions as a centre of activity into which it follows that CH,> and other hydrocarbon radicles must the entering group will be determined. From this point be regarded as possessed of considerable attractive power, of view the formation of ortho-derivatives is to be regarded as the homologues of benzene are so much more readily as a consequence of the immediate contiguity of regions 1 acted on than benzene itself. and 2. But contiguity constitutes a disadvantage as well Certain radicles, notably acid radicles such as NO.,, as an advantage, as it renders the possibility of reversal SO3H and C02H, condition the formation of meta-di- greater, and it is only in cases in which reversal occurs derivatives, mo no-derivatives of benzene containing these with comparative difficulty that the ortho-compound is radicles yielding but a small proportion of the ortho- and formed m any considerable proportion. Thus the fact para-compounds. Thus nitrobenzene is almost entirely that phenol yields only a small proportion of the orthoconverted into metabromonitrobenzene when brominated, bromo-, but a considerable proportion of the ortho-chloroalthough when bromobenzene is nitrated the chief pro- denvative, may be ascribed to the circumstance that when duct is parabromonitrobenzene, no meta-compound being once introduced chlorine is both more firmly held and less formed. Agents generally act on all mono-derivatives liable to be displaced by a reversal of the interchange. which afford meta-di-derivatives less readily than on Although similar to benzene in general behaviour the benzene • hence it may be inferred that radicles such as hydrocarbons formed by the “fusion” of two or more N02 have little if any power of attracting “acid” agents benzene all exhibit distinctive properties j „. such as the halogens, nitric and sulphuric acids, &c. the studyrings of these is of special interest and cycloid It may further be supposed that not only do they not importance in considering the manner in which hydroattract, but that they actually protect the ortho- and properties are correlated with structure. Naph- carbons. para - positions against attack, exercising an inhibiting thalene, anthracene and phenanthrene may be taken as power similar to, but even stronger than, that which as previously pointed out—the oxygen and nitrogen of examples. . Naphthalene, C10H8, is proved beyond question to conphenol and aniline appear to exert when associated sist of two benzene rings with one pair of carbon atoms in with acid radicles. Consequently, if mono-derivatives common. It is also certain that the two rings are alike, containing acid radicles are acted on by agents which and that the compound as a whole is symmetrically conare sufficiently powerful to act on the hydrocarbon ring, they behave much as benzene behaves, but the position stituted double hexagon symbol, and for all practical purposes this attacked is that which is least exposed to the influence symbol is sufficient. As in the case of benzene, the of the acid radicle—that is to say, the meta-position. determination of the inner structure of the molecule is a Proof of the correctness of this argument is afforded by matter of great difficulty. It is noteworthy, in the first the fact, that if an agent be used which is attracted by place,^ that naphthalene is much more sensitive to attack the acid radicle, substitution no longer takes place in than is benzene ; as no distortion of either ring is involved the meta-position • thus when nitrobenzene is heated with in the fusion ” of two rings, this fact is not in harmony powdered caustic soda it is converted into orthonitro- with v. Baeyer’s mechanical hypothesis. If the carbon atoms phenol. On the other hand, if the basic or acid- are represented as alternately singly and doubly linked, as attracting power of aniline be sufficiently lowered or in Kekule’s formula of benzene, there would be no fewer neutralized, as for instance by combination with sulphuric than five pairs of doubly - linked carbon atoms in the acid or by benzoylation, meta-derivatives are formed in no molecule. The behaviour of naphthalene cannot, however, inconsiderable proportion on nitration and sulphonation. be said to be that which a compound so constituted would It will be obvious that the issues raised by the study of be likely to exhibit, although it is distinctly more ethenoid the formation of substitution derivatives from benzene in character than that of benzene. To apply the centric Value of are many and complicated, and that, in order to hypothesis to naphthalene, it is necessary to assume that centric Ipohmd the facts, it is necessary to take many one of the affinities of each of the two central carbon formula. considerations into account. In studying phenol atoms common to the two rings acts into both rings, as and aniline, the peculiarities of oxygen and nitroin the symbol, which is an assumption involving gen must be reckoned with as much as those of benzene • pictured a somewhat wide departure from all ordinary views as on the other hand, much is learnt of the manner in which to the manner in which affinity acts. Nevertheless the S. II. _ 92
 * consequently naphthalene is represented by the