HOOH 0  Cl2 (I) (a) When phenol is oxidized in

acid solution by chlorine, tetrachlorquinone is obtained, a 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) . Bromine 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 CH2 HC CH2 CCl H02C•CO2H CH CH cH CH Iv CH VI connected by single bonds to (say) four other atoms, then in a certain unit of time it will collide with each of these atoms in turn . Now suppose two of the attached atoms are replaced by one 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 benzene, let us consider the impacts made by the carbon atom (I) which we will assume to be doubly linked to the carbon atom (2) and singly linked to (6), Jr standing for the 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 double linkage is between the carbon atoms I and 6, and the single linkage between i and 2 .

Therefore, according to

Kekule, the double linkages are in a state of continual oscillation, and if his dynamical notion of valency, or a similar hypothesis, be correct, then the difference between the 1.2 and 1.6 di-derivatives rests on the insufficiency of his 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 nucleus, for L . Knorr (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 attention Laden- to the study of the substitution products of benzene, and burg's to the support of his own formula . His views are presented formula. in his pamphlet: Theorie der aromatischen' Verbindungen, 1876 . The prism formula also received support from the following data: protocatechuic acid when oxidized by nitrous acid gives carboxytartronic acid, which, on account of its ready de-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

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 Baeyer's researches (commenced in 1886) on the reduced 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, 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 simple syntheses of benzene derivatives (e.g. from acetylene and 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 assumption that the transformation of these substances is attended by a migration of the substituent groups . For succinosuccinic ester, formed by the action of sodium on two molecules of succinic ester, haseitherof the formulae (I) or (II) ; oxidation of the free acid gives dioxyterephthalic acid in which the para-positions must remain substituted as in (I) and (II) . By projecting Ladenburg's prism on a plane and numbering the atoms so as to correspond with Kekule's form, viz, that 1.2 and 1.6 should be ortho-positions, 1.3 and 1.5 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 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;

Bee., 1887, 20, p . 62) and by A . K . 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 equation we have a migration of a hydrogen atom, whereas to bring Ladenburg's formula into line, an 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 present in the benzene complex . To follow Baeyer's results we must explain his nomenclature of the reduced benzene derivatives . He numbers the carbon atoms placed at the corners of a hexagon from I to 6, and each side in the same 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

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 link occupies the side i ; A1.3, a dihydro derivative, the double links being along the sides I and 3 . Another form of isomerism is occasioned by spatial arrangements, many of the reduced terephthalic acids existing in two stereo-isomeric forms . Baeyer explains this by analogy with fumaric and maleic acids: he assumes the reduced benzene ring to lie in a plane; when both carboxyl groups are on the same side of this plane, the acids, in 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 amalgam, care being taken to neutralize the caustic soda simultaneously formed by passing in carbon dioxide, 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 ethylene linkages, since it readily takes up two or four atoms of bromine, and is oxidized in warm aqueous solution by alkaline potassium permanganate . But the formation of the A2.5 acid as the first reduction product is not fully consistent with Kekule's symbol, for we should then expect the h.'3 or the Ai_5 acid to be first formed (see also POLYMETHYLENES) . The stronger 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 fourth valencies are symmetrically distributed in such a manner as to induce a peculiar stability in the molecule .

Such a configuration was proposed in 1887 by H . E .

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 function from the peripheral valencies, being united at the centre in a form of potential union . It is difficult to determine which configuration most accurately explains the observed facts; Kekule's formula undoubtedly explains the synthetical 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 quinoline ring .

Further researches by Baeyer, and upon various nitrogenous ring systems by E .

Bamberger (a strong supporter of the centric formula), have shown that the nature of the substituent groups influences the 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 case, which must be considered in the formulation of any complete theory of valency . From a study of the reduction of compounds containing two ethylenic bonds united by a single bond, termed a "conjugated system," E . Thiele suggested a doctrine of " partial valencies," ev which assumes that in addition to the 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 drawn upon to decide whether double unions are present in the benzene com- plex; but here the predilections of the observers P.'2ysico- apparently 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 physical properties of substances, such as the refractive index, specific volume, and the heat of 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 . 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

contradiction apparently exists with regard to the specific volume, for while benzene has a specific volume correspinding to Claus' formula, toluene, or methylbenzene, rather points to Kekule's . The heat of combustion, as first determined by Julius Thomsen, agreed rather better with the presence of nine single unions . His work was repeated on a finer scale by M . P . E . Berthelot of Paris, and F . C . A . Stohmann of 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 hydrocarbons—benzene, naphthalene, 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 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 stable than an ethylene union .

Mention may be made of the absorption spectrum of benzene . According to W . N .

Hartley (J.C.S., 1905, 87, p . 1822), there are six bands in the ultra-violet, while E . C . C . Baly and J . N . Collie (J.C.S., 1905, 87, p . 1332; 1906, 89, p . 524) 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 '

Victor 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)•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 . Wislicenus, Le Bel and van 't Hoff has resulted in the introduction of another condition which formulae for the benzene complex must satisfy, viz. that the hydrogen atoms must all lie in one plane . The 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 asymmetric carbon atoms) should exist in enantiomorphic forms, differing in crystal form and in their action on polarized light; such optical 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 regular octahedron would yield enantiomorphic tri-substitution derivatives .

The octahedral formula discussed by Julius Thomsen (

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 projection gives a hexagon with diagonals (II) . Reduction to hexamethylene compounds necessitates the disruption of three of the edges of the octahedron, the 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 . Marsh published a paper (Phil . Mag . [V.], 26, p . 426) in which he discussed various stereo-chemical representations of the benzene nucleus . (The stereo-chemistry of carbon compounds has led to the spatial representation of a carbon atom as being situated at the centre of a 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 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 .

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 developed by W . Vaubel (Journ . Pr . Chem., 1894 [2], 49, p . 308), has the 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 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

notice the model devised by H . Sachse (Ber., 1888, 21, 2530; Zeit . 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

device, suggested by B . Konig (Chem . Zeit., 1905, 29, p . 30), assumed the six carbon atoms to occupy six of the corners of a 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 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 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) .