CARBON COMPOUNDS. The element carbon is remark able for the number and variety of compounds to which it can give rise in combination with common elements such as hydrogen, oxygen and nitrogen. Organic chemistry is defined as "the chem istry of carbon compounds," and a few of the vast number known (some hundreds of thousands) are described in that article and un der individual headings ; only some of the simplest are described here.
Three oxides of carbon are well defined, viz., the suboxide C302, the monoxide (or carbonic oxide), CO, and the dioxide (or car bonic acid gas), other suboxides, such as mellitic anhydride, have been described. Carbon suboxide is formed by the ac tion of phosphoric oxide on malonic acid or its ethyl ester at 300° C under diminished pressure; the reaction is essentially one of dehydration : = 2 and the oxide has the properties to be expected of a ketene (q.v.), since it is probably OC : C : CO, and of an anhydride of malonic acid. Its composition is confirmed by explosion with oxygen, whereupon it gives the correct amount of carbon dioxide (C302+202=3CO2), and by vapour-density determinations. It boils at 7° C and has a colourless, suffocating vapour. When kept in a sealed tube, it slowly undergoes polymerization (q.v.) to a dark red mass which is soluble in water.
Carbon monoxide and dioxide are produced simultaneously when carbon is burnt in oxygen (T. F. E. Rhead and R. V. Wheeler), the relative proportions depending upon the amount of oxygen and other conditions. They are both concerned in many balanced reactions of great industrial importance ; thus, in metal lurgical processes and producer-gas the equilibrium CO2+C=2C0 is often involved, and under atmospheric pressure this is largely in favour of the monoxide, which constitutes 99% of the gas at C and 94% at 850° C; increased pressure tends to increase the proportion of the dioxide. With steam and hydrogen, as in the "water-gas" equilibrium, both oxides are involved: CO-1-H20 at 400° C the dioxide and hydrogen predominate largely, but above 83o° C the other pair predominates; this equilib rium is theoretically independent of the pressure. A knowledge of these equilibria (for the fundamental principles of which refer ence should be made to the articles CHEMICAL ACTION and CHEM ISTRY : Physical) is necessary in dealing with the production of these two oxides now to be described.
Carbon monoxide is found in volcanic gases and is a constituent (6-12%) of ordinary coal-gas. It was first prepared by J. M. F. Lassone (1776) by heating zinc oxide and carbon, and was con sidered to be identical with hydrogen, but W. Cruikshank (1800) and F. Clement and J. B. Desormes (1801) showed it to contain only carbon and oxygen, and J. Dalton (1803) showed that it had only half as much of the latter as carbon dioxide. It may be pre pared by passing carbon dioxide over red-hot carbon (see above) or red-hot iron; by heating many metallic oxides with carbon; by heating formic or oxalic acid or their salts with sulphuric acid (in the case of oxalic acid an equal volume of dioxide is produced) ; or best, by heating potassium ferrocyanide with a large volume of concentrated sulphuric acid: It is a colourless, odour less gas of density 0.967 (air= 1), aid is not easily liquefied; its critical temperature is 139.5° C and its critical pressure 35.5 atmospheres; its boiling point is 190° C and its melting point 200° C; it is only very slightly soluble in water. In all these and many other physical properties it shows an extremely close resemblance to nitrogen--a matter of interest in view of the elec tronic structure of the two molecules. Carbon monoxide burns with a well-known pale blue flame, giving the dioxide, but its burning is greatly retarded in dry air. It is very poisonous, uniting with the haemoglobin of the blood to give carboxyhaemoglobin, which has a characteristic absorption spectrum capable of detect ing minute traces of the gas. It combines directly with heated alkaline hydroxides (or soda-lime) to give formates (KOH+CO = H.COOK), and with chlorine to give carbonyl chloride (see be low). Under high pressures and with a suitable catalyst, it com bines with hydrogen to give alcohols (see ALCOHOLS ; PRESSURE CHEMISTRY) a reaction of increasing importance. It unites with many metals (e.g., nickel, iron, cobalt, molybdenum and ruthe nium) to give carbonyls (q.v.), that of nickel, being used in the manufacture of the pure metal; potassium carbonyl, at times formed in heating the carbonate with coke, as well as by direct union, is explosive and is of a different type, being a benzene derivative. Carbon monoxide is a powerful reduc ing reagent, and its reduction of iodine pentoxide to iodine is util ized in detecting and estimating minute quantities of the gas in air; it is absorbed by ammoniacal or hydrochloric acid solutions of cuprous chloride (colourless crystals of the resulting compound, may be obtained), and this reaction is used in gas analysis. For a series of interesting syntheses from carbon monoxide and various other gases and vapours in the silent elec tric discharge, see S. M. Losanitsch and M. Z. Jovanitschitsch, Berichte, 3o, p. Carbon dioxide, first discovered by Van Helmont 0577 1644) in the products of combustion and fermentation, was called gas sylvestre, and later J. Black called it "fixed air," owing to its being a constituent of carbonated alkalis. A. L. Lavoisier (1781-1788) demonstrated its nature by burning carbon in oxygen. It is a regular component of the atmosphere (normally 3 volumes per 1 o,000) , and is found in mineral waters, volcanic gases, cer tain grottos near Naples, and in the Poison Valley, Java. It occurs as carbonate in many minerals (see CARBONATES). It may be pre pared by burning carbon in excess of air or oxygen, by heating many carbonates, such as chalk, or bicarbonates or by the action of acids on these: It is formed in the process of respiration, in the combustion of all carboniferous material, and in fermentation, much of the liquid carbon dioxide of commerce being derived from breweries.
It is a colourless gas, possessing a faint pungent smell and a slightly acid taste. It does not burn and does not ordinarily sup port combustion, but burning magnesium or alkali metals will con tinue to burn in it, giving oxides and liberating carbon. It is 1.529 times heavier than air, and only requires a pressure of 35 atmos pheres to liquefy it at o° C. Its critical temperature and pressure are 31° C and 72.9 atmos. ; the liquid freezes at 56° C under 5 atmos., and the solid sublimes at 79° C under atmos pheric pressure. The solid "snow" is used in the treatment of warts and other skin troubles. At o° C I volume of water dissolves 179 volumes of the gas (0.33%), and it is twice as soluble in alcohol. The physical properties of carbon dioxide display a re markable resemblance to those of nitrous oxide (compare carbon monoxide and nitrogen, above). The aqueous solution is very feebly acidic, owing to the presence of a small proportion of car bonic acid, solutions of caustic alkalis absorb the gas readily to give carbonates (q.v.) and ultimately bicarbonates. With lime- or baryta-water the gas gives a white precipitate of the cor responding carbonate, and these reactions are used in the detection and estimation of the gas. Carbon dioxide and ammonia gases unite to give ammonium carbamate, (see AMMONIA). If passed over molten sodium, carbon dioxide converts it to sodium oxalate, At high temperatures, carbon dioxide dissociates slightly: 2 CO2±2 ; under atmospheric pressure only o 1 % is thus decomposed at 1,300° C, but about 40% at 3,000° C. Plants absorb carbon dioxide from the air and utilize the carbon in "photosynthesis" in virtue of the light-energy transmitted by the chlorophyll. Industrially, carbon dioxide is used in the manufac ture of soda by the Solvay process (see ALKALI INDUSTRY), in the sugar industry, in the manufacture of mineral waters and in mak ing artificial ice.
Carbonyl chloride (phosgene), is produced by the direct union of carbon monoxide and chlorine in sunlight, but in practice charcoal is used as a catalyst ; it may also be prepared by heating carbon tetrachloride with fuming sulphuric acid: It is a colourless, poisonous gas with an unpleasant, pungent smell, and it decomposes rapidly with water: it was used on a large scale in the World War, and has a characteristic action on the heart. It has a vapour den sity of 3.46 (air= I) and may be condensed to a liquid (specific gravity 1.43 at o° C) which boils at 8° C, and to a solid of melting point 127° C ; it is readily soluble in toluene and is often stored in this way; it is decomposed by heat, the equilibrium COC12 being equally distributed at C and entirely to the right at 800° C under ordinary pressure. It reacts with alcohols to form chlorocarbonic (or chloroformic) esters or carbonates, according to which of the reactants is in excess, and with ammonia to give urea (q.v.). It is used commercially in the manufacture of Michler's ketone (a starting point for certain dyes) and benzo phenone (q.v.). Carbonyl bromide is also known.
Carbon tetrachloride, is best prepared by passing chlorine into carbon disulphide containing a little iodine or antimony penta chloride as a "carrier"; it boils at 76° C, and has a specific gravity of 1.595 at o° C. Its chief uses are as a fire-extinguisher, as a solvent and in dry-cleaning; its use as a hair-wash is dangerous. The tetrabromide and tetraiodide are also known. The tetrafluor ide is the only one of the four that can be produced by direct combination of the elements ; the lighter forms of charcoal inflame spontaneously in fluorine, but the denser forms have to be heated somewhat; it boils at 15° C under atmospheric pressure.
Carbon oxysulphide, COS, can be produced by direct union of the monoxide with sulphur, or from the interaction of sulphur trioxide and carbon disulphide at 100° C ; it is better produced by decomposing ammonium thiocarbonate by hydrochloric acid; the only impurities are carbon dioxide and hydrogen sulphide, which are easily removed (A. Stock). The colourless and odour less gas can be liquefied at 47° C; it burns with a blue flame, is decomposed by heat, and dissolves in water to give a solution which slowly decomposes to carbon dioxide and sulphuretted hy drogen ; dissolution in caustic alkalis gives sulphides and carbon ates: (A. D. M.)