ORGANIC CHEMISTRY Organic chemistry, in its modern sense, is the chemistry of all carbon compounds. Carbon, occupying a position in the periodic table between the metals and non-metals, is capable of combining with a large number of elements having widely varying chemical properties, and, in addition, has the special property of combining with itself to an unlimited extent. The result is that the carbon compounds are so numerous that a special branch of chemistry is devoted to their study. Usually, however, the simpler compounds, such as the oxides, cyanogen, etc., are dealt with under inorganic chemistry, and organic chemistry is reserved for the extensive treatment of the more complicated compounds, including those existing in living matter.
Whereas inorganic chemistry originally arose from the exami nation of the metals and mineral or earthy materials, the organic side was developed by the study of substances of vegetable and animal origin. The beginnings of organic chemistry were hindered by the alchemists' quest for the philosopher's stone, which should transmute the base metals into gold, and for the elixir of life, a universal medicine to confer immortality, or at least, longevity upon those who drank it. The iatrochemists in the 16th century confined their attention chiefly to the search for medicinal prod ucts, which included a study of the distillation of such materials as heibs and resins. The only general classification of substances, based on their physical and superficial properties, led to the cor relation of organic and inorganic compounds without regard to their chemical characteristics. All organic substances were grouped with the combustibles in the period of the phlogistic theory. .
Development of organic chemistry in the 18th century was confined to improvements in a few practical applications, espe cially in the arts of medicine and dyeing. It received some impetus from the work of Karl Wilhelm Scheele, and later from that of Michel Eugene Chevreul, who showed the general occurrence of glycerine and fatty acids in all oils and fats, both of vegetable and animal origin. Theoretical speculation was revived by Antoine Laurent Lavoisier, who, after explaining the true nature of corn bustion, and devising methods of analysis, concluded that vege table substances in general contained carbon, .hydrogen and oxy gen, whilst animal substances contained, in addition, nitrogen and sometimes phosphorus and sulphur.
It is well said that the mechanism of chemical combination is the ultimate problem of chemistry, and theories regarding it were propounded even prior to the advent of the atomic theory. In 1787, Guyton de Morveau suggested that all compounds consisted of oxygen combined with a base. Lavoisier went further and con sidered inorganic compounds to be oxides of simple substances, and organic compounds, oxides of complexes or radicals. He had observed that some elements were capable of forming more than one oxide, and concluded that organic radicals acted similarly. He thus regarded sugar as a neutral oxide of a hydrocarbon radi cal (a complex consisting of carbon and hydrogen), and oxalic acid as a higher oxide of the same radical, since it was formed from sugar by treatment with the powerful oxidizing agent nitric acid. Lavoisier considered all radicals as bases, but the latter term was later restricted to the oxides of metals.
There was, at this time, a wide-spread belief, held until much later by Jons Jacob Berzelius, Leopold Gmelin and others, that the formation of organic compounds was conditioned by the inter vention of a so-called vital force, a force which could not be imi tated for the purpose of a laboratory synthesis. Berzelius was of the opinion that in the sphere of living nature the elements obeyed laws totally different from those ruling in inanimate material. In fact, he stated (181I) that the composition of the majority of organic compounds appeared to be inconsistent with the require ments of the ordinary laws of simple and multiple proportions. Improved appliances and methods of analysis convinced him (1813-14) that, although they possessed much more complicated compositions than the mineral compounds, they did, in fact, follow the simple laws applicable to the inorganic world. The first real blow to the vitalist theory was the synthesis, by Friedrich Wohler (1828), of urea from ammonium cyanate. Urea, a product of animal metabolism, was thus obtained from a compound hitherto regarded as truly inorganic.
At this stage another problem was introduced into organic chemistry, namely, to account for the occurrence of two or more substances with similar percentage compositions, but dissimilar chemical and physical properties. John Dalton (182o) discovered a hydrocarbon in oil-gas having the same percentage composition as ethylene and it was later suggested that it consisted of two ethylene particles (the formulae in brackets represent the correct modern version). This was confirmed by Michael Faraday (1825), who called the gas butylene Justus von Liebig (18 23) announced that his analysis of silver fulminate gave the same results as those ascribed to silver cyanate by Wohler in the previous year. Berzelius (1826-27) followed up Gay-Lussac's suggestion that the difference in properties of these substances with the same percentage composition must be due to differences in the arrangements of the same relative numbers of the same atoms, a suggestion which gained ground with Wohler's urea syn thesis, but it was not until 1832-33 that Berzelius finally accepted this doctrine, which he called isomerism.
Jean Baptiste Andre Dumas and Polydore Boullay (1827) put forward the theory that ether and the simple derivatives of ethy lene all contained the ethylene group, for which Berzelius proposed .the name etherin. Liebig, however, adopted the dualistic view and regarded ether as the oxide of the radical which he called etheryl or ethyl, and alcohol as the hydrate of ethyl. He was wrong, since he attributed to alcohol twice its true molecular weight, but despite this and similar errors, the value of the Ethyl Radical Theory was recognized. In 1832, Wailer and Liebig published their outstanding researches on the radical of benzoic acid, and showed that the radical containing carbon, hydrogen and oxygen (C6H5•CO), which they named ben zoyl, formed the basis of benzaldehyde, benzoic acid, benzoyl chloride, benzoyl bromide, and benzoyl sulphide, benzamide and benzoic ether. Berzelius recognized the importance of this work, although it overthrew his belief that oxygen could play no part in a compound radical. In the following year he reverted to his earlier views and his electrochemical theory. He then regarded benzoyl as an oxide of the radical which he named picramyl, the peroxide being benzoic acid. Many workers were attracted to the task of investigating radicals, and of these, for myl, methyl, amyl and acetyl were soon established. In this con nection should be mentioned the very important work of Robert Wilhelm Bunsen (1839) on cacodyl. He proved that the so-called fuming liquor of Cadet, the product of distillation of potassium acetate with arsenious acid, contained the oxide of an arseniuretted group, which he showed to be a true radical.
In 1837, Dumas admitted the failure of the etherin theory, and, in conjunction with Liebig, outlined what is now known as the Older Theory of Radicals, and defined organic chemistry as the chemistry of compound radicals. In the following year Liebig described a compound radical as the unchanging constituent of a series of compounds, which should be capable of replacement in these by simple substances, and, where it is combined with a single element, the latter should be exchangeable for its equivalent of another element. The radical was supposed to be constituted of atoms held together by forces stronger than those uniting the corn plete group to other atoms. These ideas attained a real significance for chemists, and, as the dualistic theory and the theory of com pound radicals became more definitely useful, they rendered the atomic theory, on which they were founded, essential to chemistry.
Instances had already been recorded of the substitution of electropositive atoms by electronegative ones. Gay-Lussac ob tained cyanogen chloride from hydrocyanic acid, and Faraday, hexachlorethane from ethylene dichloride, in both cases a replace ment of hydrogen by chlorine. Dumas, especially, studied these substitutions, and from his examination of the action of chlorine on alcohol, put forward his Substitution Theory, in which he pos tulated that for every hydrogen atom eliminated, an equivalent amount of chlorine was taken up, without any material alteration in the nature of the resulting product. Dumas' pupil, Auguste Laurent (1836), went further, and compared in detail the original compounds with their substitution products. As a result, he pro pounded his Nucleus Theory, which differed from the original radical theory in that it recognized that the groups formerly termed radicals were subject to change, such as substitution by the inter change of equivalent amounts of their constituents. The discovery of chloracetic acid afforded Dumas the occasion for attacking the radical theory, and for supporting Laurent's view. There was vigorous opposition from Liebig and Berzelius, especially the lat ter, who resorted to fantastic hypotheses to explain the observed facts in order to suit his own theories. Finally he abandoned experiment for pure speculation, and there arose an embittered controversy which lasted for years.
Charles Frederic Gerhardt also rejected the theory of unchange able radicals and put forward his Theory of Residues, by which the substitution process consisted in the union of two residues to form a unitary whole. He considered radicals to be merely the residues of molecules which took no part in a particular chemical reaction, but which appeared as independent portions of those molecules, only when displaced in a reaction by combination be tween the reacting portions of molecules. Such residues then joined together to form a copulated or conjugated compound. He thus represented Liebig and Wohler's reactions with the benzoyl compounds as double decompositions.
Charles Adolphe Wurtz (1849) described the amines or substi tuted ammonias, previously predicted by Liebig. August Wilhelm von Hofmann continued the investigation, and established their recognition as ammonia in which one or more hydrogen atoms had been replaced by alcohol or hydrocarbon radicals, thus formulating the ammonia type. Alexander William Williamson (185o) demon strated in a similar manner that the alcohols and ethers could be referred to the water type. In 1851, Adolph Wilhelm Hermann Kolbe put forward his New Theory of Radicals, but the main theo retical conclusions were better correlated by Gerhardt (1853) in his Theory of Simple Types, in which he regarded all compounds as derived from hydrogen, hydrogen chloride, water, and ammonia types. Thus the hydrogen type included the hydrocarbons, alde hydes and ketones, the hydrogen chloride type, all halogen deriva tives, the water type, alcohols, ethers, monobasic acids, acid anhy drides and analogous sulphur compounds, and the ammonia type, amines, acid amides, and the analogous phosphorus and arsenic compounds. Gerhardt's services to chemistry can never be ques tioned.
The recognition of the polybasicity of some acids, following the researches of Thomas Graham, led Williamson to suggest that dibasic acids could be referred to a double water type, and the multi-basic acids to multiple water types. These views were extended by William Odling, and adopted by Gerhardt. A further generalization was effected by August Kekule (18S7), who rejected the hydrogen chloride type as being a variant only of the hydrogen type, and introduced instead the methane type. He elaborated Gerhardt's suggestion of mixed types, compounds derived from more than one fundamental type, and in so doing, laid the foundation of the doctrine of valency.
Kolbe and Frankland had recognized the quadrivalency of car bon, but this was first expressly stated by Kekule (1858). Kekule went further, and suggested the manner in which two or more of these quadrivalent carbon atoms could unite with one another, mutually satisfying some of their saturation capacities. In the same year Archibald Scott Couper, quite independently, developed a similar system of atomic linkage. From this arose the idea of atomic carbon chains, either open, as in the fatty, or closed, as in the aromatic and cyclic series. This gave precision to the earlier observation of Jacob Heinrich Wilhelm Schiel (1842) that a very simple relation existed between the then known alcohols (general formula and to Dumas' demonstration in the same year of the existence of a similar relation between the several members of the fatty acids (CnH2.+t •COON) . Gerhardt was the first to use this knowledge of related series of chemical compounds to predict the possibility of hitherto unknown mem bers. These series of compounds, called homologous series, are now known to possess the same characteristic groups at the ends of carbon chains of variable length. Thus the first members of the simple fatty acid series are : acetic acid, CH,COOH ; propionic acid, butyric acid, etc.
The doctrine of valency permitted the graphic representation of the molecule. These representations were at first written in one plane only, but a number of cases were then discovered which did not appear to be cap able of explanation by such structural formulae. Ludwig Carius (1864) suggested the term physical isomerism to describe these anomalies, but Johannes Wislicenus (1873) introduced the term geometrical isomerism and showed that it was possible to account for the phenomenon if such mole cules were represented in three dimensions. This Structure Theory of the carbon compounds, founded by Kekule and Couper, was further developed by the work of Jacobus Henricus Van't Hoff and Joseph Achille Le Bel on stereochemistry or chemistry in space. By its aid the molecule is represented as a collection of atoms connected together by chemical bonds or valencies in such a man ner that the part played by each is adequately represented. After 187o the determination of the constitution of complex molecules became one of the chief aims of chemistry, to be brought to a logical conclusion by subsequent synthesis.