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HCOCH HCOCH HC CH HC CH CH CH Ocnzenc, Abbrenrt<e.Oxyccnceoe.ebererfeua. ' From the See also:benzene See also:nucleus we can derive other aromatic nuclei, graphically represented by fusing two or more hexagons along See also:common sides. By fusing two nuclei we obtain the See also:formula of See also:naphthalene, C1oH3 ; by fusing three, the See also:hydrocarbons See also:anthracene and phenanthrene, C,4Hlo; by fusing four, chrysene, C13H12, and possibly See also:pyrene, C16H1,,; by fusing five, See also:picene, C22H,4. But it must be here understood that each member of these condensed nuclei need not necessarily be identical in structure; thus the central nuclei in anthracene and phenanthrene differ very considerably from the terminal nuclei (see below, Condensed Nuclei). Other See also:hydrocarbon nuclei generally classed as aromatic in See also:character result from the See also:union of two or more benzene nuclei joined by one or two valencies with polymethylene or oxidized polymethylene rings; instances of such nuclei are See also:indene, hydrindene, See also:fluorene, and See also:fluoranthene. From these nuclei an immense number of derivatives may be obtained, for the See also:hydrogen atoms may be substituted by any of the radicals discussed in the preceding See also:section on the See also:classification of organic compounds. We now proceed to consider the properties, syntheses, decompositions and constitution of the benzene complex. It has already been stated that benzene derivatives may be Dlstiacregarded as formed by the replacement of hydrogen tions atoms by other elements or radicals in exactly the ttphattc same manner as in the aliphatic See also:series. Important and See also:differences, however, are immediately met with aromatic when we consider the methods by which derivatives comare obtained. For example: nitric See also:acid and sulphuric pounds. acid readily react with benzene and its homologues with the See also:production of nitro derivatives and sulphonic acids, while in the aliphatic series these acids exert no substituting See also:action (in the See also:case of the olefines, the latter acid forms an addition product); another distinction is that the benzene complex is more See also:stable towards oxidizing agents. This and other facts connected with the stability of benzenoid compounds are clearly shown when we consider mixed aliphatic-aromatic hydrocarbons, i.e. compounds derived by substituting aliphatic radicals in the benzene nucleus; such a See also:compound is methylbenzene or See also:toluene, See also:C6H5•See also:CH3. This compound is readily oxidized to benzoic acid, C6H5•000H, the aromatic See also:residue being unattacked; nitric and sulphuric acids produce nitro-toluenes, See also:C6H4•CH3•NO2, and toluene sulphonic acids, C6H4•CH3•SO3H; chlorination may result in the formation of derivatives substituted either in the aromatic nucleus or in the See also:side See also:chain; the former substitution occurs most readily, chlor-toluenes, C6H4•CH3•Cl, being formed, while the latter, which needs an See also:elevation in temperature or other See also:auxiliary, yields benzyl chloride, C6H5•CH2C1, and benzal chloride, C6H5•CHC12. In See also:general, the aliphatic residues in such mixed compounds retain the characters of their class, while the aromatic residues retain the properties of benzene. Further differences become apparent when various typical compounds are compared. The introduction of hydroxyl See also:groups Li-See also:Ito the benzene nucleus gives rise to compounds generic-ally named phenols, which, although resembling the aliphatic See also:alcohols in their origin, differ from these substances in their increased chemical activity and acid nature. The phenols more closely resemble the See also:tertiary alcohols, since the hydroxyl See also:group is linked to a See also:carbon See also:atom which is See also:united to other carbon atoms by its remaining three valencies; hence on oxidation they cannot yield the corresponding See also:aldehydes, See also:ketones or acids (see below, Decompositions of the Benzene See also:Ring). The See also:amines also exhibit striking differences: in the aliphatic series these compounds may be directly formed from the alkyl haloids and See also:ammonia, but in the benzene series this reaction is quite impossible unless the haloid atom be weakened by the presence of other substituents, e.g. nitro groups. Moreover, while methyl-amine, dimethylamine, and trimethylamine increase in basicity corresponding to the introduction of successive methyl groups, phenylamine or See also:aniline, diphenylamine, and triphenylamine are in decreasing See also:order of basicity, the salts of diphenylamine being decomposed by See also:water. Mixed aromatic-aliphatic amines, both secondary and tertiary, are also more strongly basic than the pure aromatic amines, and less basic than the true aliphatic compounds; e.g. aniline, C6H5NH2, monomethyl aniline, C6H5•NH•CH3, and dimethyl aniline, C6H5•N(CH3)2, are in increasing order of basicity. These observations may be summarized by saying that the benzene nucleus is more negative in character than the aliphatic residues. See also:Isomerism of Benzene Derivatives.—Although See also:Kekule founded his famous benzene formula in 1865 on the assumptions that the six hydrogen atoms in benzene are See also:equivalent and that the See also:molecule is symmetrical, i.e. that two pairs of hydrogen atoms are symmetrically situated with reference to any specified hydrogen atom, the See also:absolute demonstration of the validity of these assumptions was first given by A. Ladenburg in 1874 (see Ber., 1874, 7, p. 1684; 1875, 8, p. 1666; Theorie der aromatischen Verbindungen, 1876). These results may be graphically represented as follows: numbering the hydrogen atoms in cyclical order from 1 to 6, then the first thesis demands that whichever atom is substituted the same compound results, while the second thesis points out that the pairs 2 and 6, and 3 and 5 are symmetrical with respect to 1, or in other words, the di-substitution derivatives 1.2 and 1.6, and also 1.3 and 1.5 are identical. Therefore three di-derivatives are possible, viz. 1.2 or 1.6, named ortho- (o), 1.3 or 1.5, named See also:meta- (m), and 1.4, named See also:para- compounds (p). In the same way it may be shown that three tri-substitution, three tetra-substitution, one penta-substitution, and one hexa-substitution derivative are possible. Of the tri-substitution derivatives, 1.2.3.-compounds are known as " adjacent " or " vicinal " (v), the 1.2.4 as " asymmetrical " (as), the 1.3.5 as " symmetrical " (s); of the tetra-substitution derivatives, 1.2.3.4-compounds are known as " adjacent," 1.2.3.5 as " asymmetrical," and 1.2.4.5 as " symmetrical." Di-derivatives x x o to p v as s v as s Here we have assumed the substituent groups to be alike; when they are unlike, a greater number of isomers is possible. Thus in the tri-substitution derivatives six isomers, and no more, are possible when two of the substituents are alike; for instance, six diaminobenzoic acids, CsH3(See also:NH2)2COOH, are known; when all are unlike ten isomers are possible; thus, ten oxytoluic acids, C6H3-CH3.OH•COOH, are known. In the case of tetra-substituted compounds, See also:thirty isomers are possible when all the groups are different. The preceding considerations render it comparatively easy to follow the reasoning on which the experimental verification of the above statements is based. The See also:proof is divided into two Equfva- tenceof parts: (1) that four hydrogen atoms are equal, and (2) four hydro. that two pairs of hydrogen atoms are symmetrical with gen atoms. reference to a specified hydrogen atom. In the first thesis, phenol or oxybenzene,See also:C6H 6.OH, inwhich wewill assumethe hydroxyl group to occupy position 1, is converted into brombenzene, which is then converted into benzoic acid, C6H5.000H. From this substance, an oxybenzoic acid (meta-), C6H4.OH•COOH, may be prepared ; and the two other known oxybenzoic acids (ortho- and para-) may be converted into benzoic acid. These three acids yield on See also:heating phenol, identical with the substance started with, and since in the three oxybenzoic acids the hydroxyl groups must occupy positions other than 1, it follows that four hydrogen atoms are equal in value. R. See also:Hubner and A. See also:Petermann (See also:Ann., 1869, 149, p. 129) provided the proof of the equivalence of the atoms 2 and 6 with respect Symmetry to 1. From meta-brombenzoicacid twonitrobrombenzoic ofpa/rsof acids are obtained on See also:direct nitration; elimination of the hydrogen See also:bromine atom and the reduction of the nitro to an amino atoms. group in these two acids results in the formation of the same ortho-aminobenzoic acid. Hence the positions occupied by the nitro groups in the two different nitrobrombenzoic acids must be symmetrical with respect to the carboxyl group. In 1879, Hubner (Ann., 195, p. 4) proved the equivalence of the second pair, viz. 3 and 5, by starting out with ortho-aminobenzoic acid, previously obtained by two different methods. This substance readily yields ortho-oxybenzoic acid or salicylic acid, which on nitration yields two mononitro-oxybenzoic acids. By eliminating the hydroxy groups in these acids the same nitrobenzoic acid is obtained, which yields on reduction an aminobenzoic acid different from the starting-out acid. Therefore there must be another pair of hydrogen atoms, other than 2 and 6, which are symmetrical with respect to 1. The symmetry of the second pair was also established in 1878 by E. Wroblewsky (Ann., 192, p. 196). See also:Orientation of Substituent Groups.—The determination of the relative positions of the substituents in a benzene derivative constitutes an important See also:factor in the general investigation of such compounds. Confining our See also:attention, for the See also:present, to di-substitution products we see that there are three distinct series of compounds to be considered. Generally if any group be replaced by another group, then the second group enters the nucleus in the position occupied by the displaced group; thismeans that if we can definitely orientate three di-derivatives of benzene, then any other compound, which can be obtained from or converted into one of our typical derivatives, may be definitely orientated. Intermolecular transformations—migrations of substituent groups from one carbon atom to another—are of fairly common occurrence among oxy compounds at elevated temperatures. Thus See also:potassium ortho-oxybenzoate is converted into the See also:salt of para-oxybenzoic acid at 2200; the three bromphenols, and also the brombenzenesulphonic acids, yield m-dioxybenzene or See also:resorcin when fused with potash. It is necessary, therefore, to avoid reactions involving such inter-molecular migrations when determining the orientation of aromatic compounds. Such a series of typical compounds are the benzene dicarboxylic acids (See also:phthalic acids), C6H4(000H)2. C. Graebe (Ann., 1869, 149, p. 22) orientated the ortho-compound or phthalic acid from its formation from naphthalene on oxidation; the meta-compound or isophthalic acid is orientated by its production from mesitylene, shown by A. Ladenburg (Ann., 1875, 179, p. 163) to be symmetrical trimethyl benzene; See also:terephthalic acid, the remaining isomer, must therefore be the para-compound. P. Griess (Ber., 1872, 5, p. 192; 1874, 7, p. 1223) orientated the three diaminobenzenes or phenylene diamines by considering their preparation by the elimination of the carboxyl group in the six diaminobenzoic acids. The diaminobenzene resulting from two of these acids is the ortho-compound; from three, the meta-; and from one the para-; this is explained by the following See also:scheme: NH2 NH2 NH, NH, Nil, NH, NH, NH2 NH2 NH Hooc NH2 See also:COON COON , O' NH2 0 N H2 W. Korner (Gazz. Chem. Ital., 4, p. 305) in 1874 orientated the three dibrombenzenes in a somewhat similar manner. Starting with the three isomeric compounds, he found that one gave two tribrombenzenes, another gave three, while the third gave only one. A scheme such as the preceding one shows that the first dibrombenzene must be the ortho-compound, the second the meta-, and the third the para-derivative. Further See also:research in this direction was made by D. E. Noetling (Ber., 1885, 18, p. 2657), who investigated the nitro-, amino-, and oxy-xylenes in their relations to the three xylenes or dimethyl benzenes.
The orientation of higher substitution derivatives is determined by considering the di- and tri-substitution compounds into which they can be transformed.
Substitution of the Benzene Ring.—As a general See also:rule, homologues and mono-derivatives of benzene react more readily with substituting agents than the See also:parent hydrocarbon; for example, phenol is converted into tribromphenol by the action of bromine water, and into the nitrophenols by dilute nitric acid; similar activity characterizes aniline. Not only does the substituent group modify the readiness with which the derivative is attacked, but also the nature of the product. Starting with a mono-derivative, we have seen that a substituent group may enter in either of three positions to See also:form an ortho-, meta-, or paracompound. Experience has shown that such mono-derivatives as nitro compounds, sulphonic acids, carboxylic acids, aldehydes, and ketones yield as a general rule chiefly the meta-compounds, and this is See also:independent of the nature of the second group introduced; on the other See also:hand, benzene haloids, amino-, homologous-, and hydroxy-benzenes yield principally a mixture of the ortho- and para-compounds. These facts are embodied in the " Rule of Crum See also: See also:Gibson " (Jour. Chem. See also:Soc. 61, p. 367): If the hydrogen compound of the substituent already in the benzene nucleus can be directly oxidized to the corresponding hydroxyl compound, then meta-derivatives predominate on further substitution, if not, then ortho- and para-.derivatives. By further substitution of ortho- and para-diderivatives, in general the same tri-derivative [1.2.4] is formed (Ann., 1878, 192, p. 219); meta-compounds yield [1.3.4] and [1.2.3] tri-derivatives, except in such cases as when both substituent groups are strongly acid, e.g. m-dinitrobenzene, then [1.3.5]-derivatives are obtained. Additional information and CommentsThere are no comments yet for this article.
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