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HOOH 0 Cl2 (I) (a) When phenol is oxidized in See also:acid See also:solution by See also:chlorine, tetrachlorquinone is obtained, a See also:compound also obtainable from hydroquinone. By conducting the chlorination in alkaline solution, Reduction A. Hantzsch (Ber., 1889, 22, p. 1238) succeeded in ob- to alkaline taining derivatives of o-diketo-R-hexene, which yield solution. R-pentene and aliphatic compounds on decomposition. When thus chlorinated phenol (I) yields trichlor-o-diketo-R-hexene (2), which may be hydrolysed to an acid (3), which, in turn, suffers rearrangement to trichlor-R-pentene-oxycarboxylic acid (4). See also:Bromine See also:water oxidizes this substance to oxalic acid and tetrabromdichloracetone (5). OH O HOOC OH Cl2o0 HCl2C1 \CO Cl2C[:::C`000HCI,BrC•CO•CBr3+ H H2 HC See also:CH2 HC CH2 CCl H02C•See also:CO2H CH CH cH CH Iv CH VI connected by single bonds to (say) four other atoms, then in a certain unit of See also:time it will collide with each of these atoms in turn. Now suppose two of the attached atoms are replaced by one See also:atom, then this atom must have two valencies directed to the central atom ; and consequently, in the same unit of time, the central atom will collide once with each of the two monovalent atoms and twice with the divalent. Applying this notion to See also:benzene, let us consider the impacts made by the See also:carbon atom (I) which we will assume to be doubly linked to the carbon atom (2) and singly linked to (6), Jr See also:standing for the See also:hydrogen atom. In the first unit of time, the impacts are 2, 6, h, 2; and in the second 6, 2, h, 6. If we represent graphically the impacts in the second unit of time, we perceive that they point to a configuration in which the See also:double linkage is between the carbon atoms I and 6, and the single linkage between i and 2. Therefore, according to See also:Kekule, the double linkages are in a See also:state of continual oscillation, and if his dynamical notion of See also:valency, or a similar See also:hypothesis, be correct, then the difference between the 1.2 and 1.6 di-derivatives rests on the insufficiency of his See also:formula, which represents the configuration during one set of oscillations only. The difference is only apparent, not real. An analogous oscillation prevails in the pyrazol See also:nucleus, for L. Knorr (See also:Ann., 1894, 279, p. 188) has shown that 3- and 5-methylpyrazols are identical. The explanation thus attempted by Kekule was adversely criticized, more especially by A. Ladenburg, who devoted much See also:attention Laden- to the study of the substitution products of benzene, and See also:burg's to the support of his own formula. His views are presented formula. in his pamphlet: Theorie der aromatischen' Verbindungen, 1876. The See also:prism formula also received support from the following data: protocatechuic acid when oxidized by nitrous acid gives carboxytartronic acid, which, on See also:account of its ready de-See also:composition into carbon dioxide and tartronic acid, was considered to be HO•C(COOH)3. This implied that in the benzene complex there was at least one carbon atom linked to three others, thus rendering Kekule's formula impossible and Ladenburg's and Claus' possible. Kekule (Ann., 1883, 221, p. 230), however, reinvestigated this acid; he showed that it was dibasic and not tribasic; that it gave tartaric acid on reduction; and, finally, that it was dioxytartaric acid, HOOC•C(OH)2•C(OH)2•COOH. The formation of this substance readily follows from Kekule's formula, while considerable difficulties are met with when one attempts an explanation based on Ladenburg's See also:representation. Kekule also urged that the formation of trichlorphenomalic acid, shown by him and O. Strecker to be trichloracetoacrylic acid, was more favourably explained by his formula than by Ladenburg's. Other objections to Ladenburg's formula resulted from A. von See also:Baeyer's researches (commenced in 1886) on the reduced See also:phthalic Baeyer's acids. Baeyer pointed out that although benzene deri-Baeyer's vatives were obtainable from hexamethylene compounds, researches- yet it by no means follows that only hexamethylene compounds need result when benzene compounds are reduced. He admitted the possibility of the formulae of Kekule, Claus, See also:Dewar and Ladenburg, although as to the last di-trimethylene derivatives should be possible reduction products, being formed by severing two of the prism edges; and he attempted to solve the problem by a systematic investigation of the reduced phthalic acids. Ladenburg's prism admits of one mono-substitution derivative and three di-derivatives. Furthermore, it is in accordance with certain See also:simple syntheses of benzene derivatives (e.g. from See also:acetylene and See also:acetone); but according to Baeyer (Ber., 1886, 19, p. 1797) it fails to explain the formation of dioxyterephthalic ester from succinosuccinic ester, unless we make the See also:assumption that the transformation of these substances is attended by a See also:migration of the substituent See also:groups. For succinosuccinic ester, formed by the See also:action of See also:sodium on two molecules of succinic ester, haseitherof the formulae (I) or (II) ; oxidation of the See also:free acid gives dioxyterephthalic acid in which the See also:para-positions must remain substituted as in (I) and (II). By projecting Ladenburg's prism on a See also:plane and numbering the atoms so as to correspond with Kekule's See also:form, viz, that 1.2 and 1.6 should be ortho-positions, 1.3 and 1.5 See also:meta-, and 1.4 para-, and following out the transformation on the Ladenburg formula, then an ortho-dioxyterephthalic acid (IV) should result, a fact denied by experience, and inexplicable unless we assume a wandering of atoms. Kekule's formula (III), on the other See also:hand, is in full agreement (Baeyer). This explanation has been challenged by Ladenburg Co C•OH C•OH (s)OH HyC CH•CO2Et HC 0 CH•CO,Et HC C•co,Et Eto2c.(5) (5)H EtO3C•HC EtOgCC CHq Eto2C•CCH Sto2c.0 (s)OH CO C.OH C•OH (4)H (s sr... xis (Ber., 1886, 19, p. 971; See also:Bee., 1887, 20, p. 62) and by A. K. See also:Miller (J.C.S. Trans., 1887, p. 208). The transformation is not one of the oxidation of a hexamethylene compound to a benzenoid compound, for only two hydrogen atoms are removed. Succinosuccinic ester behaves both as a ketone and as a phenol, thereby exhibiting desmotropy; assuming the ketone formula as indicating the constitution, then in Baeyer's See also:equation we have a migration of a hydrogen atom, whereas to bring Ladenburg's formula into See also:line, an See also:oxygen atom must migrate. The relative merits of the formulae of Kekule, Claus and Dewar were next investigated by means of the reduction products of benzene, it being Baeyer's intention to detect whether double linkages were or were not See also:present in the benzene complex. To follow Baeyer's results we must explain his nomenclature of the reduced benzene derivatives. He See also:numbers the carbon atoms placed at the corners of a hexagon from I to 6, and each See also:side in the same See also:order, so that the carbon atoms i and 2 are connected by the side I, atoms 2 and 3 by the side 2, and so on. A doubly linked pair of atoms is denoted by the sign A with the See also:index corresponding to the side; if there are two pairs of double links, then indices corresponding to both sides are employed. Thus A' denotes a tetrahydro derivative in which the double See also:link occupies the side i ; A1.3, a dihydro derivative, the double links being along the sides I and 3. Another form of See also:isomerism is occasioned by spatial arrangements, many of the reduced See also:terephthalic acids existing in two stereo-isomeric forms. Baeyer explains this by See also:analogy with fumaric and maleic acids: he assumes the reduced benzene See also:ring to See also:lie in a plane; when both carboxyl groups are on the same side of this plane, the acids, in See also:general, resemble maleic acids, these forms he denotes by rcis-cis, or shortly cis-; when the carboxyl groups are on opposite sides, the acids correspond to fumaric acid, these forms are denoted by rcis-trans, or shortly trans-. By reducing terephthalic acid with sodium See also:amalgam, care being taken to neutralize the See also:caustic soda simultaneously formed by passing in carbon dioxide, See also:A2.5 dihydroterephthalic acid is obtained; this results from the splitting of a para-linkage. By boiling with water the W.5 acid is converted into the Di.s dihydroterephthalic acid. This acid is converted into the 01.4 acid by soda, and into the O2 tetrahydro acid by reduction. From this acid the &- dihydro and the h.' tetrahydro acids may be obtained, from both of which the hexahydro acid may be prepared. From these results Baeyer concluded that Claus' formula with three para-linkings cannot possibly be correct, for the A2.5 dihydroterephthalic acid undoubtedly has two See also:ethylene linkages, since it readily takes up two or four atoms of bromine, and is oxidized in warm aqueous solution by alkaline See also:potassium permanganate. But the formation of the A2.5 acid as the first reduction product is not fully consistent with Kekule's See also:symbol, for we should then expect the h.'3 or the Ai_5 acid to be first formed (see also See also:POLYMETHYLENES). The stronger See also:argument against the ethylenoid linkages demanded by Kekule's formula is provided by the remark-able stability towards oxidizing and reducing agents which characterizes all benzenoid compounds. From the fact that reduction products containing either one or two double linkages behave exactly as unsaturated aliphatic compounds, being readily reduced or oxidized, and combining with the halogen elements and haloid acids, it seems probable that in benzenoid compounds the See also:fourth valencies are symmetrically distributed in such a manner as to induce a See also:peculiar stability in the See also:molecule. Such a configuration was proposed in 1887 by H. E. See also:Armstrong (J.C.S. Trans., 1887, p. 258), and shortly afterwards by Baeyer (Ann., 1888, 245, p. 103). In this formula, the so-called " centric formula," the assumption made is that the fourth valencies are simply directed towards the centre of the ring; nothing further is said about the fourth valencies except that they exert a pressure towards the centre. Claus maintained that Baeyer's view was identical with his own, for as in Baeyer's formula, the fourth valencies have a different See also:function from the peripheral valencies, being See also:united at the centre in a form of potential See also:union. It is difficult to determine which configuration most accurately explains the observed facts; Kekule's formula undoubtedly explains the synthetical See also:production of benzenoid compounds most satisfactorily, and W. Marckwald (Ann., 1893, 274, p. 331; 1894, 279, p. 14) has supported this formula from considerations based on the syntheses of the See also:quinoline ring. Further researches by Baeyer, and upon various nitrogenous ring systems by E. See also:Bamberger (a strong supporter of the centric formula), have shown that the nature of the substituent groups influences the See also:distribution of the fourth valencies; therefore it may be concluded that in compounds the benzene nucleus appears to be capable of existence in two tautomeric forms, in the sense that each particular derivative possesses a definite constitution. The benzene nucleus presents a remarkable See also:case, which must be considered in the formulation of any See also:complete theory of valency. From a study of the reduction of compounds containing two ethylenic bonds united by a single See also:bond, termed a "conjugated See also:system," E. Thiele suggested a See also:doctrine of " partial valencies," ev which assumes that in addition to the See also:ordinary valencies, each doubly linked atom has a partial valency, by which the atom first interacts. When applied to benzene, a twofold conjugated system is suggested in which the partial valencies of adjacent atoms neutralize, with the formation of a potential double link. The stability of benzene is ascribed to this conjugation.' Physico-chemical properties have also been See also:drawn upon to decide whether double unions are present in the benzene com- plex; but here the predilections of the observers P.'2ysico- apparently See also:influence the nature of the conclusions to chemical methods. be drawn from such data. It is well known that singly, doubly and trebly linked carbon atoms affect the See also:physical properties of substances, such as the refractive index, specific See also:volume, and the See also:heat of See also:combustion; and by determining these constants for many substances, fairly definite values can be assigned to these groupings. The general question of the relation of the refractive index to constitution has been especially studied by J. W. See also:Bruhl, who concluded that benzene contained 3 double linkages; whereas, in 1901, Pellini (Gazetta, 31, i. p. I) calculated that 9 single linkages were present. A similar See also:contradiction apparently exists with regard to the specific volume, for while benzene has a specific volume correspinding to Claus' formula, See also:toluene, or methylbenzene, rather points to Kekule's. The heat of combustion, as first determined by See also:Julius See also:Thomsen, agreed rather better with the presence of nine single unions. His See also:work was repeated on a finer See also:scale by M. P. E. See also:Berthelot of See also:Paris, and F. C. A. Stohmann of See also:Leipzig; and the new data and the conclusions to be drawn from them. formed the subject of much discussion, Bruhl endeavouring to show how they supported Kekule's formula, while Thomsen maintained that they demanded the benzene union to have a different heat of combustion from the acetylene union. Thomsen then investigated heats of combustion of various benzenoid See also:hydrocarbons—benzene, See also:naphthalene, See also:anthracene, phenanthrene, &c.—in the crystallized state. It was found that the results were capable of expression by the empirical relation CaH2b= 104.3b+49.o9m+Io5'47n, where CaH2b denotes the formula of the See also:hydrocarbon, m the number of single carbon linkings and n the number of double linkings, m and n being calculated on the Kekule formulae. But, at the same time, the constants in the above relation are not identical with those in the corresponding relation empirically deduced from observations on fatty hydrocarbons; and we are therefore led to conclude that a benzene union is considerably more See also:stable than an ethylene union. Mention may be made of the absorption spectrum of benzene. According to W. N. See also:Hartley (J.C.S., 1905, 87, p. 1822), there are six bands in the ultra-See also:violet, while E. C. C. Baly and J. N. Collie (J.C.S., 1905, 87, p. 1332; 1906, 89, p. 524) See also:record seven. These bands are due to molecular oscillations; Hartley suggests the carbon atoms to be rotating and forming alternately single and double linkages, the formation of three double links giving three bands, and of three single links another three; Baly and Collie, on the other hand, suggest the making and breaking of links between adjacent atoms, pointing out that there are seven combinations of one, two and three pairs of carbon atoms in the benzene molecule. Stereo-chemical Configurations.—Simultaneously with the discussions of Kekule, Ladenburg, Claus, Baeyer and others as to the merits of various plane formulae of the benzene complex, there were published many suggestions with regard to the arrangement of the atoms in space, all of which attempted to explain the number of isomers and the equivalence of the hydrogen atoms. The development of stereo-isomerism a.t the hands of ' See also:Victor See also:Meyer and G. Heyl (Ber., 1895, 28, p. 2776) attempted a solution from the following data. It is well known that di-orthosubstituted benzoic acids are esterified with difficulty. Two acids corresponding to the formula of Kekule and Claus are triphenyl acrylic acid, (CsHs)2C: C(COOH)•See also:C6H5, and triphenyl acetic acid, (C6H5)3C.COOH. Experiments showed that the second acid was much more difficult to esterify than the first, pointing to the conclusion that Claus' formula for benzene was more probable than Kekule's. J. See also:Wislicenus, Le See also:Bel and See also:van 't Hoff has resulted in the introduction of another See also:condition which formulae for the benzene complex must satisfy, viz. that the hydrogen atoms must all lie in one plane. The See also:proof of this statement rests on the fact that if the hydrogen atoms were not co-planar, then substitution derivatives (the substituting groups not containing See also:asymmetric carbon atoms) should exist in enantiomorphic forms, differing in crystal form and in their action on polarized See also:light; such See also:optical See also:antipodes have, however, not yet been separated. Ladenburg's prism formula would give two enantiomorphic ortho-di-substitution derivatives; while forms in which the hydrogen atoms are placed at the corners of a See also:regular See also:octahedron would yield enantiomorphic tri-substitution derivatives. The octahedral formula discussed by Julius Thomsen (See also:Ben., 1886, 19, p. 2944) consists of the six carbon atoms placed at the corners of a regular octahedron, and connected together by the full lines as shown in (I); a plane See also:projection gives a hexagon with diagonals (II). Reduction to hexamethylene compounds necessitates the disruption of three of the edges of the octahedron, the See also:diagonal linkings remaining intact, or, in the plane projection, three peripheral linkages, the hexamethylene ring assuming the form (III): I II III In 1888 J. E. See also:Marsh published a See also:paper (Phil. Mag. [V.], 26, p. 426) in which he discussed various stereo-chemical representations of the benzene nucleus. (The stereo-See also:chemistry of carbon compounds has led to the spatial representation of a carbon atom as being situated at the centre of a See also:tetrahedron, the four valencies being directed towards the apices; see above, and ISOMERISM.) A form based on Kekule's formula consists in taking three pairs of tetrahedra, each pair having a side in See also:common, and joining them up along the sides of a regular hexagon by means of their apices. This form, afterwards supported by Carl Graebe (Ber., 1902, 35,p. 526; see also Marsh's reply, Journ. Chem. See also:Soc. Trans., 1902, p. 961) shows the proximity of the ortho-positions, but fails to explain the identity of 1.2 and 1.6 compounds. Arrangements connected with Claus' formula are obtained by placing six tetrahedra on the six triangles formed by the diagonals of a plane hexagon. The form in which the tetrahedra are all on one side, afterwards discussed by J. Loschmidt (Monats., 189o, Is, p. 28), would not give stereo-isomers; and the arrangement of placing the tetrahedra on alternate sides, a form afterwards See also:developed by W. Vaubel (Journ. Pr. Chem., 1894 [2], 49, p. 308), has the See also:advantage of bringing the meta-positions on one side, and the ortho- and para- on opposite sides, thus exhibiting the' similarity actually observed between these See also:series of compounds. Marsh also devised a form closely resembling that of Thomsen, inasmuch as the carbon atoms occupied the angles of a regular octahedron, and the diagonal linkages differed in nature from the peripheral, but differeng from Thomsen's since rupture of the diagonal and not peripheral bonds accompanied the reduction to hexamethylene. We may also See also:notice the See also:model devised by H. Sachse (Ber., 1888, 21, 2530; Zeit. See also:Pie phys. Chem., II, p. 214; 23, p. 2062). Two parallel triangular faces are removed from a cardboard model of a regular octahedron, and on the remaining six faces tetrahedra are then placed; the hydrogen atoms are at the free angles. This configuration is, according to Sachse, more stable than any other form; no oscillation is possible, the molecule being only able to move as a whole. In 1897, J. N. Collie (Journ. Chem. Soc. Trans., p. 1013) considered in detail an octahedral form, and showed how by means of certain simple rotations of his system the formulae of Kekule and Claus could be obtained as projections. An entirely new See also:device, suggested by B. See also:Konig (Chem. Zeit., 1905, 29, p. 30), assumed the six carbon atoms to occupy six of the corners of a See also:cube, each carbon atom being linked to a hydrogen atom and by single bonds to two neighbouring carbon atoms, the remaining valencies being directed to the unoccupied corners of the cube, three to each, where they are supposed to satisfy each other. Condensed Nuclei. Restricting ourselves to compounds resulting from the See also:fusion of benzene rings, we have first to consider naphthalene, C1oH5, which consists of two benzene rings having a pair of carbon atoms in common. The next members are the isomers anthracene and phenanthrene, CIAIJIO, formed from three benzene nuclei. Here we shall only discuss the structure of these compounds in the light of the See also:modern benzene theories; reference should be made 2 to the articles NAPHTHALENE, ANTHRACENE and PHENANTHRENE for syntheses, decompositions, &c. Naphthalene.—Of the earlier suggestions for the constitution of naphthalene we notice the formulae of Wreden (1) and (2), Berthelot and Balls (3), R. A. C. E. Erlenmeyer (4) and Adolf Claus (5). Additional information and CommentsThere are no comments yet for this article.
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