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COON Hz See also:NH2 NH, NH2 ()NH2 which exist between aliphatic and benzenoid compounds make the transformations of one class into the other especially interesting. In the first See also:place we may See also:notice a tendency of several aliphatic compounds, e.g. methane, tetrachlormethane, &c., to yield aromatic compounds when subjected to a high temperature, the so-called pyrogenetic reactions (from See also:Greek Trip, See also:fire, and yevvbm, I produce) ; the predominance of benzenoid, and related compounds—naphthalene, See also:anthracene, phenanthrene, &c.—in See also:coal-See also:tar is probably to be associated with similar pyrocondensations. See also:Long-continued treatment with See also:halogens may, in some cases, result in the formation of aromatic compounds; thus perchlorbenzene, C6CI6, frequently appears as a product of exhaustive chlorination, while hexyl iodide, C6H13I, yields perchlor- and perbrom-See also:benzene quite readily. The trimolecular polymerization of numerous See also:acetylene compounds—substances containing two trebly linked See also:carbon atoms, —C : C —, to See also:form derivatives of benzene is of considerable See also:interest. M. P. E. See also:Berthelot first accomplished the See also:synthesis of benzene in 187o by leading acetylene, HC i CH, through tubes heated to dull redness; at higher temperatures the See also:action becomes reversible, the benzene yielding See also:diphenyl, diphenylbenzene, and acetylene. The condensation of acetylene to benzene is also possible at See also:ordinary temperatures by leading the See also:gas over pyrophoric See also:iron, See also:nickel, See also:cobalt, or spongy See also:platinum (P. See also:Sabatier and J. B. Senderens). The homologues of acetylene condense more readily; thus allylene, CH i C•CH3, and crotonylene, CH3•C i C•CH3, yield trimethyl- and hexamethyl-benzene under the See also:influence of sulphuric See also:acid. See also:Toluene or mono-methylbenzene results from the pyrocondensation of a mixture of acetylene and allylene. Substituted acetylenes also exhibit this form of condensation; for instance, bromacetylene, BrC i CH, is readily converted into tribrombenzene, while propiolic acid, HC: C•COOH, under the influence of sunlight, gives benzene tricarboxylic acid. A larger and more important See also:series of condensations may be grouped together as resulting from the elimination of the elements of See also:water between carbonyl (CO) and methylene (See also:CH2) See also:groups. A historic example is that of the condensation of three molecules of See also:acetone, CH3•CO•CH3, in the presence of sulphuric acid, to s-trimethylbenzene or mesitylene, C6Ha(See also:CH3)3, first observed in 1837 by R. See also:Kane; methylethyl ketone and methyl-n-propyl ketone suffer similar condensations to s-triethylbenzene and s-tri-n-propylbenzene respectively. Somewhat similar condensations are: of geranial or citral, (CH3)2CH•CH2•CH:CH•C(CH3):CH•CHO, to p-isopropylmethylbenzene or cymene; of the condensation product of methylethylacrolein and acetone, CH 3.CH2.CH:C(CH3).CH:CH•C0•CH3, to [1. 3. 4]-trimethylbenzene or pseudocumene; and of the condensation product of two molecules of isovaleryl aldehyde with one of acetone, C3H7•CH2•CH:C(C3H7)-CH:CH•CO•CH3, to (1)-methyl-2-4-di-isopropyl benzene. An analogous synthesis is that of dihydro-m-See also:xylene from methyl heptenone, (CH2)2C:CH • (CH2)2•CO•CH3. Certain a-See also:diketones condense to form benzenoid See also:quinones, two molecules of the diketone taking See also:part in the reaction; thus diacetyl, CH3.CO.CO.CH3, yields p-xyloquinone, C6H2(CH3)202 (Ber., 1888, 21, p. 1411), and acetylpropionyl, CH3•CO•CO•C2H5, yields duroquinone, or tetramethylquinone, C6(CH3)402, Oxymethylene compounds, characterized by the grouping >C:CH(OH), also give benzene derivatives by hydrolytic condensation between three molecules; thus oxymethylene acetone, or formyl acetone, CHa• CO• CH :CH (OH), formed by acting on formic ester with acetone in the presence of See also:sodium ethylate, readily yields [1.3.5]-triacetylbenzene, C6H3(CO•CH3)a; oxymethylene acetic ester or formyl acetic ester or #-oxyacrylic ester, (HO)CH:CH•CO2C2H6, formed by condensing acetic ester with formic ester, and also its dimolecular condensation product, coumalic acid, readily yields See also:esters of [1.3.5]-benzene tricarboxylic acid or trimesic acid (see Ber., 1887, 20, P. 2930).
In 189o, O. Doebner (Ber. 23, p. 2377) investigated the condensation of pyroracemic acid, CH3•CO•000H, with various aliphatic See also:aldehydes, and obtained from two molecules of the acid and one of the aldehyde in the presence of baryta water alkylic isophthalic acids: with acetaldehyde [I.3.5]-methylisophthalic acid or uvitic acid, C6H3•CH3•(000H)2, was obtained, with propionic aldehyde [1.3.51-ethylisophthalic acid, and with butyric aldehyde the corresponding propylisophthalic acid. We may here mention the synthesis of oxyuvitic ester (5-methyl-4-oxy-1-3-benzene dicarboxylic ester) by the condensation of two molecules of sodium acetoacetic ester with one of See also:chloroform (See also:Ann., 1883, 222, p. 249). Of other syntheses of true benzene derivatives, mention may be made of the formation of orcinol or [3.5]-dioxytoluene from dehydracetic acid; and the formation of esters of oxytoluic acid (5-methyl-3-oxy-benzoic acid), C6H3•CH3.OH•000H,when acetoneoxalic ester, CH3•CO•CH2•CO•CO•CO2C2H5, is boiled with baryta (Ber., 1889,01
22, p. 3271). Of interest also are H. B. See also: Torray's observations on nitromalonic aldehyde, NO2.CH(CHO)2,formed by acting on mucobromic acid, probably CHO•CBr:CBr:000H, with alkaline nitrites; this substance condenses with acetone to give te-nitrophenol, and forms [I.3.51-trinitrobenzene when its sodium See also:salt is decomposed with an acid. By passing carbon monoxide over heated See also:potassium J. von See also:Liebig discovered, in 1834, an interesting aromatic See also:compound, potassium carbon monoxide or potassium hexaoxybenzene, the nature of which was satisfactorily cleared up by R. Nietzki and T. Benckiser (Ber. 18, p. 499) in 1885, who showed that it yielded hexaoxybenzene, C6(OH)6, when acted upon with dilute hydrochloric acid; further investigation of this compound brought to See also:light a consider-able number of highly interesting derivatives (see QUINONES). Another hexa-substituted benzene compound capable of See also:direct synthesis is mellitic acid or benzene carboxylic acid, C6(COOH)6. This substance, first obtained from the See also:mineral honeystone, See also:aluminium mellitate, by M. H. See also:Klaproth in 1799, is obtained when pure carbon (See also:graphite or See also:charcoal) is oxidized by alkaline permanganate, or when carbon forms the See also:positive See also:pole in an electrolytic See also:cell (Ber., 1883, 16, p. 1209). The See also:composition of this substance was deter-See also:mined by A. von See also:Baeyer in 187o, who obtained benzene on distilling the See also:calcium salt with See also:lime. Hitherto we have generally restricted ourselves to syntheses which result in the See also:production of a true benzene See also:ring; but there are many reactions by which reduced benzene rings are synthesized, and from the compounds so obtained true benzenoid compounds may be prepared. Of such syntheses we may notice : the condensation of sodium malonic ester to phloroglucin tricarboxylic ester, a substance which gives phloroglucin or trioxybenzene when fused with alkalis, and behaves both as a triketohexamethylene tricarboxylic ester and as a trioxybenzene tricarboxylic ester; the condensation of succinic ester, (CH2•CO2C2H5)2, under the influence of sodium to succinosuccinic ester, a diketohexamethylene di-carboxylic ester, which readily yields dioxyterephthalic acid and hydroquinone (F. Herrmann, Ann., 1882, 2I1, p. 306; also see below, Configuration of the Benzene Complex) ; the condensation of acetone dicarboxylic ester with malonic ester to form triketohexamethylene dicarboxylic ester (E. See also:Rimini, Gazz. Chem., 1896, 26, (2), p. 374) ; the condensation of acetone-di-propionic acid under the influence of boiling water to a diketohexamethylene propionic acid (von Pechmann and See also:Sidgwick, Ber., 1904, 37, p. 3816). Many diketo compounds suffer condensation between two molecules to form hydrobenzene derivatives; thus a,y-di-acetoglutaric ester, C2H602C(CH3•CO)CH•CH2•CH(CO•CH3)CO2C2H5, yields a methylketohexamethylene,whiley-acetobutyric ester, See also:CH3CO(CH2)2CO2C2H6, is converted into dihydroresorcinol or m-diketohexamethylene by sodium ethylate; this last reaction is reversed by baryta (see De-compositions of Benzene Ring). For other syntheses of hexamethylene derivatives, see See also:POLYMETHYLENES. Decompositions of the Benzene Ring.—We have previously alluded to the relative stability of the benzene complex; consequently reactions which See also:lead to its disruption are all the more interesting, and have engaged the See also:attention of many chemists. If we accept See also:Kekule's See also:formula for the benzene See also:nucleus, then we may expect the See also:double linkages to be opened up partially, either by oxidation or reduction, with the formation of di-, tetra-, or hexa-hydro derivatives, or entirely, with the production of open See also:chain compounds. Generally rupture occurs at more than one point; and rarely are the six carbon atoms of the complex regained as an open chain. Certain compounds withstand ring decomposition much more strongly than others; for instance, benzene and its homologues, carboxylic acids, and nitro compounds are much more See also:stable towards oxidizing agents than amino- and oxy-benzenes, aminophenols, quinones, and oxycarboxylic acids. Strong oxidation breaks the benzene complex into such compounds as carbon dioxide, oxalic acid, formic acid, &c. ; such decompositions are of little interest. More important are Kekule's See also:Simple observations that nitrous acid oxidizes pyrocatechol or oxidation. [1.2]-dioxybenzene, and protocatechuic acid or [3.4]-dioxybenzoic acid to dioxytartaric acid, (C(OH)2.COOH)2 (Ann., 1883, 221, p. 230) ; and 0. Doebner's preparation of mesotartaric acid, the internally compensated tartaric acid, (CH(OH).COOH)2, by oxidizing phenol with dilute potassium permanganate (Ber., 1891, 24, P. 1753)• For many years it had been known that a mixture of potassium chlorate and hydrochloric or sulphuric acids possessed strong oxidizing See also:powers. L. Carius showed that potassium Chiorinachlorate and sulphuric acid oxidized benzene to trichlor- tion and phenomalic acid, a substance afterwards investigated by oxidation. Kekule and 0. Strecker (Ann., 1884, 223, p. 170), and shown to be 13-trichloracetoacrylic acid, CC13•CO•CH:CH•000H. which with baryta gave chloroform and maleic acid. Potassium chlorate and hydrochloric acid oxidize phenol, salicylic acid (o-oxybenzoic acid), and gallic acid ([2.3.41 trioxybenzoic acid) to trichlorpyroracemic acid (isotrichlorglyceric acid), CC13•C(OH)2•CO2H, a substance also obtained from trichloracetonitrile, CCI3•CO•CN,.b See also:hydrolysis. We may also notice the See also:conversion of picric acid. [2.4.61-trinitrophenol) into chloropicrin, CC13NO2, by See also:bleaching lime (calcium hypochlorite), and into bromopicrin, CBr3NO2, by See also:bromine water. The action of See also:chlorine upon di- and tri-oxybenzenes has been carefully investigated by Th. Zincke; and his researches have led to the See also:discovery of many chlorinated oxidation products which admit of decomposition into cyclic compounds containing fewer carbon atoms than characterize the benzene ring, and in turn yielding open-chain or aliphatic compounds. In See also:general, the rupture occurs between a keto See also:group (CO) and a keto-chloride group (CC12), into which two adjacent carbon atoms of the ring are converted by the oxidizing and substituting action of chlorine. Decompositions of this nature were first discovered in the See also:naphthalene series, where it was found that derivatives of See also:indene (and of hydrindene and indone) and also of benzene resulted; Zincke then extended his methods to the disintegration of the oxybenzenes and obtained analogous results, R-pentene and aliphatic derivatives being formed (R-symbolizing a ringed nucleus). When treated with chlorine, pyrocatechol (1.2 or ortho-dioxybenzene) (i) yields a tetrachlor ortho-quinone, which suffers further chlorination to hexachlor-o-diketo-R-hexene (2). This substance is transformed into hexachlor-R-pentene oxycarboxylic acid (3) when digested with water; and chromic acid oxidizes this substance to hexachlor-R-pentene (4). The ring of this compound is ruptured by See also:caustic soda with the formation of perchlorvinyl acrylic acid (5), which gives on reduction ethidine propionic acid (6), a compound containing five of the carbon atoms originally in the benzene ring (see Zincke, Ber., 1894, 27, p. 3364) (the carbon atoms are omitted in some of the formulae). Additional information and CommentsThere are no comments yet for this article.
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