74 METAMORPHOSES BY OXIDATION. extent than in the wood from which it was formed. Such actions take place generally under the combined influence of moisture and of a temperature of about 59° (15° C.) or upwards. If the substance be perfectly dry, or if the temperature fall to the freezing point, oxidation is usually completely arrested. Contact with a body which is itself actively undergoing slow oxidation will frequently induce this oxidizing action in substances which have a strong attraction for oxygen, but which do not, when in a pure state, themselves undergo oxidation. Alcohol, for example, does not undergo oxidation when allowed to evaporate in air; but if it be mixed with a little must or some substance susceptible of oxidation, the alcohol, when brought into contact with atmospheric air, becomes oxidized, and furnishes aldehyd and acetic acid (1250). Sometimes the steps of the process may be pretty clearly traced; for example, a solution of grape sugar in an alcoholic solution of potash does not absorb oxygen when exposed to the air; but the addition of a minute quantity of sulphindylic acid (1514) effects the oxidation of the sugar; probably, as Traube suggests, by the absorption of oxygen on the part of the sulphindylic acid, and the transfer of oxygen to the sugar; the sulphindylic acid absorbing fresh oxygen and again parting with it to fresh portions of sugar, acting in fact as a carrier of oxygen, just as nitric oxide does to sulphurous acid, during the manufacture of oil of vitriol (412). Another analogous experiment of Traube's consists in heating a solution of cupric oxide containing a large quantity of hydrochlorate of ammonia with grape sugar; no reduction takes place even when the liquid is boiled; but if a small quantity of reduced and colourless sulphindylic acid be added, the blue colour of the indigo reappears, owing to the absorption of oxygen by the indigo acid from the cupric oxide; the acid then gives up its oxygen to the sugar, after which it again takes up more oxygen from a fresh quantity of cupric oxide; so that in a short time the whole of the sugar is oxidized, and the cupric oxide reduced to cupreous oxide, whilst the sulphindylic acid is left apparently unaltered. The sulphindylic acid is in fact much more easily reduced than cupric oxide, and yet it has the power of easily absorbing oxygen from this oxide. An action similar to this of the sulphindylic acid in favouring oxidation in the sugar is supposed to exist in many other cases; as for example in the ordinary process of vinegarmaking from sugar and water, where the presence of certain oxidizable matters is found by experience to be necessary. These slow processes of combustion are intimately connected METAMORPHOSES BY OXIDATION. 75 with the development of ozone. In certain cases the formation of ozone is so abundant during such actions, that they furnish some of the best means of obtaining this singular body. Ozone, for instance, is always formed during the slow oxidation of phosphorus in air; and it appears also to be produced in the oxidation of oil of turpentine and of ether under particular circumstances, as when a hot platinum wire is suspended in a mixture of the vapour of ether and atmospheric air. The ozone thus developed has in its turn a powerful influence in promoting the oxidation of the organic body, though of course it cannot be supposed that the presence of ozone is essential to the production and development of such oxidations, for they constantly occur where no ozone can be believed to exist; whilst in other cases the production of ozone appears to be a consequence rather than a cause of the process. The importance of studying the action of such cases of oxidation must be obvious, when it is borne in mind that it is always by the agency of oxygen that the various complex substances contained in the animal body are gradually resolved into carbonic acid, water, and ammonia; in one or other of which forms the organic products return again to the domain of inorganic chemistry. (1071) 2. Metamorphoses by Reduction.-From the foregoing observations it is manifest that processes of oxidation are essentially analytic and destructive in their nature. The synthetic processes, or processes of reduction, are, however, not less important. They are in perpetual operation in the growing plant, and less frequently occur in the animal; they may also in a limited degree be imitated in the laboratory. These reducing operations, however, are less completely under the control of the chemist than those of oxidation, but they are gradually becoming better understood. Nascent hydrogen may act in four ways. 1. It may combine directly with an organic compound; when, for instance, sodium amalgam is made to act upon inverted cane sugar (1086), mannite is produced: A similar effect is sometimes produced by the action of sulphuretted hydrogen, as when alloxan is by its means converted into alloxantin : 76 METAMORPHOSES BY REDUCTION AND SUBSTITUTION. And in a somewhat similar manner blue indigo, under the combined action of ferrous oxide and alkalies, becomes converted into white indigo, whilst ferric oxide is formed : 2. Sometimes simple removal of oxygen may take place, as in the deoxidation which occurs during the process of fermentation; as when calcic malate is allowed to ferment in contact with decayed cheese; succinic, acetic, and carbonic acids being the result the formula of malic acid differing from that of succinic acid by containing 1 atom more of oxygen, malic acid being He, and succinic acid, EH... 6 6 3. It may remove oxygen, or one of the halogens, whilst combining with the compound, as when benzoic acid is reduced to benzoic alcohol (Hermann). And, 4. It may remove oxygen, chlorine, or one of the halogens, whilst an equivalent amount of hydrogen takes its place, occasioning what has been called inverse substitution (1074). The action of hydriodic acid upon organic compounds is remarkable. When a basic substance like ammonia or ethylia is presented to it, direct combination ensues, as might have been expected; but hydriodic acid does not usually produce compounds into which iodine is introduced by substitution or double decomposition, though if heated with an iodized compound it often furnishes a substance in which hydrogen is substituted for iodine, whilst iodine is set at liberty, as for instance :— In other cases hydriodic acid acts as a reducing agent, as for example when lactic acid is by its means converted into propionic (1284, 1310): Lactic Acid. Propionic Acid. 3 or when erythrite is converted into ẞtetryl-iodide by its means (1243) : METAMORPHOSES BY SUBSTITUTION. 77 (1072) 3. Metamorphoses by Substitution.-The method to be adopted in the formation of new compounds by displacement or substitution, constitutes an important object of chemical inquiry, for the prosecution of such researches often gives the most satisfactory proof of the molecular grouping of the body under examination, and points out most distinctly its relations to other compounds. The ordinary method of preparing insoluble inorganic compounds by double decomposition, is simply a case of the substitution of one metal or one salt radicle for another: when solutions such as those of potassic carbonate, and baric nitrate are mingled, the barium is exchanged for potassium, and reciprocally potassium for barium: K ̧¤Ð ̧+Ba2NO, becoming Ba€Ð ̧+2KNOŽ. This species of substitution is also a familiar operation amongst the compounds of organic chemistry; for the chemist has learned to displace* various elementary substances by compound groups, as when by the mutual action of hydrocyanic acid and mercuric oxide, mercuric cyanide and water are produced; 2HCy+Hg=HgCy2+H2O: the hydrogen and the mercury being substituted the one for the other; whilst 2 atoms of the monad group Cy (=EN), takes the place of the dyad oxygen in the oxide of mercury. Substitutions of groups of greater complication than these are also readily effected, and it is even possible in many instances to produce true organic compounds from inorganic elements (1075). 3 Few instances of substitution are more remarkable than those by which ammonia may be converted into a complex organic base. When ethyl bromide, for example, is mixed with a solution of ammonia in alcohol, placed in a sealed tube and heated for a few hours to 212°, a reaction occurs, in consequence of which, as the mixture cools, crystals of hydrobromate of ethylia (Є,H,N,HBr) are produced; and if this compound be distilled with caustic potash, potassic bromide, water, and a new base, ethylia (Є,H,N), will be formed. 2 Now, the molecule of ammonia may be represented as consisting of H HN; H and the molecule of ethylia in like manner 2 HN; The French term remplacement is usually but inaccurately rendered replacement; the true meaning of the latter word is putting back into its place and not displacement or substitution, which conveys the meaning of the French word more correctly. 78 SUBSTITUTION OF CHLORINE AND ITS ANALOGUES FOR HYDROGEN. or as ammonia in which I atom of hydrogen has been displaced by H., or its equivalent of ethyl. The substitution will be understood from the following equations : Ethyl Bromide. Hydrobromate of Etbylia, €2H,Br+H2N=Є,H,,H,N,HBr; and Hydrobromate of Ethylia. Ethylia. ЄH ̧‚H2N‚HBr+KH‡ = ¤‚μ¿‚H2N+KBr+H ̧Ð. 2 11 Again, if ethyl bromide be heated similarly with this new base, a second atom of hydrogen may be displaced, and a hydrobromate of a second more complicated base, diethylia, (E,H,,N) will be obtained; and from this hydrobromate the pure base is liberated by distillation with caustic potash. In diethylia two atoms of hydrogen have been displaced by ethyl. For example: And again, if ethyl bromide be made to act upon diethylia, the third atom of hydrogen is displaced, and triethylia (H1N) in combination with hydrobromic acid is produced; and thus a complex ammonia,H, N, is built up step by step, by the suc cessive removal of hydrogen and the substitution of a corresponding number of equivalents of ethyl. Many cases of the same nature will be exhibited when the artificial formation of organic bases comes under the notice of the reader (1360). (1073) Action of Chlorine and the Halogens upon Organic Compounds-Substitution for Hydrogen.-The action of chlorine and bromine upon the compounds of organic chemistry resembles that of oxygen, but that of bromine is weaker than that of chlorine. 1. They may simply remove hydrogen, as in the conversion of alcohol into aldehyd : 2. They may enter into direct combination, as when olefiant gas is converted into ethylene dibromide :-- |