The ringed structure of benzene, C 6 H 61 was first suggested in 1865 by August Kekule, who represented the molecule by six CH groups placed at the six angles of a regular hexagon, the sides of which denoted the valencies saturated by adjacent carbon atoms, the fourth valencies of each carbon atom being represented as saturated along alternate sides.
Long-continued treatment with halogens may, in some cases, result in the formation of aromatic compounds; thus perchlorbenzene, C 6 C1 6, frequently appears as a product of exhaustive chlorination, while hexyl iodide, C 6 H 13 I, yields perchlorand perbrom-benzene quite readily.
Various solvents, such as benzene, alcohol and chloroform, will dissolve out the pigment, leaving the plastid colorless.
It may be synthetically prepared by fusing potassium benzene sulphonate with caustic alkalis (A.
Wurtz); by the action of nitrous acid on aniline; by passing oxygen into boiling benzene containing aluminium chloride (C. Friedel and J.
Phys., 1888 (6) 14, p. 435); by heating phenol carboxylic acids with baryta; and, in small quantities by the oxidation of benzene with hydrogen peroxide or nascent ozone (A.
It has a characteristic smell, and a biting taste; it is poisonous, and acts as a powerful antiseptic. It dissolves in water, 15 parts of water dissolving about one part of phenol at 16-17° C., but it is miscible in all proportions at about 70° C.; it is volatile in steam, and is readily soluble in alcohol, ether, benzene, carbon bisulphide, chloroform and glacial acetic acid.
When phenol is passed through a red-hot tube a complex decomposition takes place, resulting in the formation of benzene, toluene, naphthalene, &c. (J.
A thiophenol, C 6 H 5 SH, is known, and is prepared by the action of phosphorus pentasulphide on phenol, or by distilling a mixture of sodium benzene sulphonate and potassium sulphydrate.
The picrate, which is easily soluble in benzene, crystallizes in long red needles melting at 222°.
For example, ethylene, C2H4 j is formed with absorption of 16200 cal., acetylene, C 2 H 2, with absorption of 59100 cal., and liquid benzene, C 6 H 6, with absorption of 9100 cal.
Paraffins are found in all crude oils, and olefines in varying proportions in the majority, while acetylene has been found in Baku oil; members of the benzene group and its derivatives, notably benzene and toluene, occur in all petroleums. Naphthenes are the chief components of some oils, as already indicated, and occur in varying quantities in many others.
It is almost insoluble in water, but mixes in all proportions with absolute alcohol, ether, benzene and various oils.
Exposure to sunlight converts it into trimesic acid (benzene-1.3.5-tricarboxylic acid).
Kekule was the forerunner of his celebrated benzene theory in particular, and of the universal application of structural formulae to the representation of the most complex organic compounds equally lucidly as the representation of the simplest salts.
Thus, he interpreted the interaction of benzene and nitric acid as C6H61-HN03 = C 6 H 5 NO 2 +H 2 0, the "residues" of benzene being C 6 H 5 and H, and of nitric acid HO and N02.
Carbocyclic rings will next be treated, benzene and its allies in some detail; and finally the heterocyclic nuclei.
Polymethylenes can give only secondary and tertiary alcohols, benzene only tertiary; these latter compounds are known as phenols.
This formula, notwithstanding many attempts at both disproving and modifying it, has well stood the test of time; the subject has been the basis of constant discussion, many variations have been proposed, but the original conception of Kekule remains quite as convenient as any of the newer forms, especially when considering the syntheses and decompositions of the benzene complex.
The following diagrams illustrate these statements: - C ` H C OH HC /CH HC CH HC,/CH 'N/ HC CH CH CH From the benzene nucleus we can derive other aromatic nuclei, graphically represented by fusing two or more hexagons along common sides.
Other hydrocarbon nuclei generally classed as aromatic in character result from the union of two or more benzene nuclei joined by one or two valencies with polymethylene or oxidized polymethylene rings; instances of such nuclei are indene, hydrindene, fluorene, and fluoranthene.
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 regarded as formed by the replacement of hydrogen atoms by other elements or radicals in exactly the same manner as in the aliphatic series.
For example: nitric acid and sulphuric acid readily react with benzene and its homologues with the production of nitro derivatives and sulphonic acids, while in the aliphatic series these acids exert no substituting action (in the case of the olefines, the latter acid forms an addition product); another distinction is that the benzene complex is more stable towards oxidizing agents.
Compounds derived by substituting aliphatic radicals in the benzene nucleus; such a compound is methylbenzene or toluene, C 6 H 5 CH 3.
The introduction of hydroxyl groups into the benzene nucleus gives rise to compounds generically named phenols, which, although resembling the aliphatic alcohols in their origin, differ from these substances in their increased chemical activity and acid nature.
The amines also exhibit striking differences: in the aliphatic series these compounds may be directly formed from the alkyl haloids and 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.
These observations may be summarized by saying that the benzene nucleus is more negative in character than the aliphatic residues.
Although Kekule founded his famous benzene formula in 1865 on the assumptions that the six hydrogen atoms in benzene are equivalent and that the molecule is symmetrical, i.e.
Orientation of Substituent Groups.-The determination of the relative positions of the substituents in a benzene derivative constitutes an important factor in the general investigation of such compounds.
Generally if any group be replaced by another group, then the second group enters the nucleus in the position occupied by the displaced group; this means 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.
Such a series of typical compounds are the benzene dicarboxylic acids (phthalic acids), C 6 H 4 (000H) 2.
Ladenburg (Ann., 1875, 179, p. 163) to be symmetrical trimethyl benzene; terephthalic acid, the remaining isomer, must therefore be the para-compound.
Substitution of the Benzene Ring.-As a general rule, homologues and mono-derivatives of benzene react more readily with substituting agents than the 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.
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 independent of the nature of the second group introduced; on the other hand, benzene haloids, amino-, homologous-, and hydroxy-benzenes yield principally a mixture of the orthoand para-compounds.
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 orthoand paraderivatives.
Syntheses of the Benzene Ring.-The characteristic distinctions NH NH, r.-NH, [[Cooh Cooh, _ + Nh2 Nh, H., Cooh]] x x x Tri.
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.
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.
The homologues of acetylene condense more readily; thus allylene, CH: C CH 3, and crotonylene, CH 3.0: C CH 3, yield trimethyland hexamethyl-benzene under the influence of sulphuric acid.
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.