which exist between aliphatic and benzenoid compounds make the transformations of one class into the other especially interesting.
In the first place we may 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 Greek πῦρ, fire, and γεννάω, I produce); the predominance of benzenoid, and related compounds—naphthalene, anthracene, phenanthrene, &c.—in coal-tar is probably to be associated with similar pyrocondensations. Long-continued treatment with halogens may, in some cases, result in the formation of aromatic compounds; thus perchlorbenzene, C6Cl6, frequently appears as a product of exhaustive chlorination, while hexyl iodide, C6H13I, yields perchlor- and perbrom-benzene quite readily.
The trimolecular polymerization of numerous acetylene compounds—substances containing two trebly linked carbon atoms, —C:C—, to form derivatives of benzene is of considerable interest. M. P. E. Berthelot first accomplished the synthesis of benzene in 1870 by leading acetylene, HC⫶CH, through tubes heated to dull redness; at higher temperatures the action becomes reversible, the benzene yielding diphenyl, diphenylbenzene, and acetylene. The condensation of acetylene to benzene is also possible at ordinary temperatures by leading the gas over pyrophoric iron, nickel, cobalt, or spongy platinum (P. Sabatier and J. B. Senderens). The homologues of acetylene condense more readily; thus allylene, CH⫶C·CH3, and crotonylene, CH3·C⫶C·CH3, yield trimethyl- and hexamethyl-benzene under the influence of sulphuric acid. 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⫶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 series of condensations may be grouped together as resulting from the elimination of the elements of water between carbonyl (CO) and methylene (CH2) groups. A historic example is that of the condensation of three molecules of acetone, CH3·CO·CH3, in the presence of sulphuric acid, to s-trimethylbenzene or mesitylene, C6H3(CH3)3, first observed in 1837 by R. 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-isopropyl-methylbenzene or cymene; of the condensation product of methylethylacrolein and acetone, CH3·CH2·CH:C(CH3)·CH:CH·CO·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 di-hydro-m-xylene from methyl heptenone, (CH3)2C:CH·(CH2)2·CO·CH3. Certain a-diketones condense to form benzenoid quinones, two molecules of the diketone taking part in the reaction; thus diacetyl, CH3·CO·CO·CH3, yields p-xyloquinone, C6H2(CH3)2O2 (Ber., 1888, 21, p. 1411), and acetylpropionyl, CH3·CO·CO·C2H5, yields duroquinone, or tetramethylquinone, C6(CH3)4O2. 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, CH3·CO·CH:CH(OH), formed by acting on formic ester with acetone in the presence of sodium ethylate, readily yields [1.3.5]-triacetylbenzene, C6H3(CO·CH3)3; oxymethylene acetic ester or formyl acetic ester or β-oxyacrylic ester, (HO)CH:CH·CO2C2H5, formed by condensing acetic ester with formic ester, and also its dimolecular condensation product, coumalic acid, readily yields esters of [1.3.5]-benzene tricarboxylic acid or trimesic acid (see Ber., 1887, 20, p. 2930).
In 1890, O. Doebner (Ber. 23, p. 2377) investigated the condensation of pyroracemic acid, CH3·CO·COOH, with various aliphatic 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 [1.3.5]-methylisophthalic acid or uvitic acid, C6H3·CH3·(COOH)2, was obtained, with propionic aldehyde [1.3.5]-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 chloroform (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·COOH, when acetoneoxalic ester, CH3·CO·CH2·CO·CO·CO2C2H5, is boiled with baryta (Ber., 1889, 22, p. 3271). Of interest also are H. B. Hill and J. Torray’s observations on nitromalonic aldehyde, NO2·CH(CHO)2, formed by acting on mucobromic acid, probably CHO·CBr:CBr:COOH, with alkaline nitrites; this substance condenses with acetone to give p-nitrophenol, and forms [1.3.5]-trinitrobenzene when its sodium salt is decomposed with an acid.
By passing carbon monoxide over heated potassium J. von Liebig discovered, in 1834, an interesting aromatic 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 light a considerable number of highly interesting derivatives (see Quinones). Another hexa-substituted benzene compound capable of direct synthesis is mellitic acid or benzene carboxylic acid, C6(COOH)6. This substance, first obtained from the mineral honeystone, aluminium mellitate, by M. H. Klaproth in 1799, is obtained when pure carbon (graphite or charcoal) is oxidized by alkaline permanganate, or when carbon forms the positive pole in an electrolytic cell (Ber., 1883, 16, p. 1209). The composition of this substance was determined by A. von Baeyer in 1870, who obtained benzene on distilling the calcium salt with lime.
Hitherto we have generally restricted ourselves to syntheses which result in the production of a true benzene 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 dicarboxylic ester, which readily yields dioxyterephthalic acid and hydroquinpne (F. Herrmann, Ann., 1882, 211, 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. 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 Sidgwick, Ber., 1904, 37, p. 3816). Many diketo compounds suffer condensation between two molecules to form hydrobenzene derivatives, thus α, γ-di-acetoglutaric ester, C2H5O2C(CH3·CO)CH·CH2·CH(CO·CH3)CO2C2H5, yields a methyl-ketohexamethylene, while γ-acetobutyric ester, CH3CO(CH2)2CO2C2H5, is converted into dihydroresorcinol or m-diketohexamethylene by sodium ethylate; this last reaction is reversed by baryta (see Decompositions of Benzene Ring). For other syntheses of hexamethylene derivatives, see Polymethylenes.
Decompositions of the Benzene Ring.—We have previously alluded to the relative stability of the benzene complex; consequently reactions which lead to its disruption are all the more interesting, and have engaged the attention of many chemists. If we accept Kekulé’s formula for the benzene nucleus, then we may expect the 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 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 stable towards oxidizing agents than amino- and oxy-benzenes, aminophenols, quinones, and oxy-carboxylic 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 Kekulé’s observations that nitrous acid oxidizes pyrocatechol or [1.2]-dioxybenzene, and protocatechuic acid Simple oxidation.or [3.4]-dioxybenzoic acid to dioxytartaric acid, (C(OH)2·COOH)2 (Ann., 1883, 221, p. 230); and O. 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 powers. L. Carius showed that potassium chlorate and sulphuric acid oxidized benzene to trichlor-phenomalic acid, a substance afterwards investigated by Chlorination and oxidation.Kekulé and O. Strecker (Ann., 1884, 223, p. 170), and shown to be β-trichloracetoacrylic acid, CCl3·CO·CH:CH·COOH, which with baryta gave chloroform and maleic acid. Potassium chlorate and hydrochloric acid oxidize phenol, salicylic acid (o-oxybenzoic acid), and gallic acid ([2.3.4] trioxybenzoic acid) to trichlorpyroracemic acid (isotrichlorglyceric acid), CCl3·C(OH)2·CO2H, a substance also obtained from trichloracetonitrile, CCl3·CO·CN, by hydrolysis. We may also notice the conversion of picric acid, ([2.4.6]-trinitrophenol) into chloropicrin, CCl3NO2, by bleaching lime (calcium hypochlorite), and into bromopicrin, CBr3NO2, by bromine water.
