On mixing a solution of biuret with argentic nitrate, and then carefully adding alkali, a colourless but easily blackened precipitate of diargento-biuret, C2ÓN,H,Ag2, is obtained. Anhydrous biuret melts at 190°, and at a slightly higher temperature decomposes into cyanuric acid and ammonia: 3C2O2N2H5=3NH3 + 2C ̧Ñ2H303. Heated to 100° in an atmosphere of hydrochloric acid, it yields a saline compound of the formula 2C2O2N,H,,HC1; at higher temperatures it gives guanidine hydrochloride, and carbonic anhydride : C2O2N3H5 + HCl = CO2 + CN2H5.HCl, together with sal ammoniac and a difficultly soluble compound, which has been termed urea-cyanurate : This latter is probably a body of analogous composition to biuret, and may be viewed as tetruret : NH,.CO.NH.CO.NH, (biuret). NH,.CO.NH.CO.NH.CO.NH.CO.NH, (tetruret). The relations of biuret to the so-called allophanic ethers will be spoken of later. 138. Corresponding to carboxyl is the radical CS, thiocarbonyl, which has, however, as yet never been obtained uncombined like carbonic oxide; it has only to be mentioned here in connection with its amide derivatives, which in their behaviour closely resemble the carboxylamide compounds. NH2 139. Sulpho-carbamic acid, CH,NS2 = CS ammonic sul SH pho-carbamate, is formed (like the corresponding oxy-derivative) by action of ammonia on carbonic disulphide, best when alcoholic solutions of both are employed: + 2NH3 = -NH, S.NH From this salt, which crystallises in yellow prisms, sulpho-carbamic acid is obtained, by action of hydrochloric acid, as a reddish-coloured oily liquid, which solidifies at ordinary temperatures to a crystalline mass, and after a time decomposes spontaneously into sulpho-cyanic acid and hydric sulphide : By heating with excess of ammonia, ammonic sulpho-carbamate is converted into ammonic sulpho-cyanate: S CEN + 2NH2 = + (NH4)2S. S.NH 140. Sulphurea, or sulpho-carbonyl diamide, CSN,H4. Whilst ammonic cyanate changes at ordinary temperatures into urea, ammonic sulpho-cyanate only undergoes the same change at higher temperatures: CEN NH2 In order to prepare sulphurea, dry ammonic sulpho-cyanate is heated slowly to 170°, and kept at that temperature for several hours; it is then cooled to 100°, and dissolved in an equal weight of water at 80°, filtered and cooled to crystallisation. The crystals are freed from the mother liquor, which contains unaltered ammonic sulpho-cyanate, and purified by repeated recrystallisation from very little hot water. Sulphurea crystallises in prisms, dissolves readily in water and alcohol, but little in ether, and fuses at 149°. Like urea, it yields with nitric acid a difficultly soluble nitrate: -NH2 Argentic and mercuric derivatives are also known, which decompose readily with formation of metallic sulphides. Similarly also on boiling its aqueous solution with argentic or mercuric oxides, the respective metallic sulphides are precipitated, whilst dicyandiamide remains in solution. By heating with water it is reconverted into ammonic sulpho-cyanate. Hydro-halogen derivatives of sulphurea are formed easily, together with carbonic disulphide, by simultaneous action of nascent hydrogen and hydro-haloids upon persulpho-dicyanic acid: ETHANES OR PARAFFINS. HYDROCARBONS OF THE SERIES CnH2n+2. 141. The homologous series of paraffins embraces those hydrocarbons in which the maximum possible number of hydrogen atoms is attached to the carbon nucleus (§ 40.) Its first member is marsh gas or methane, CH4, from which the higher members of the series can be obtained by substitution of its hydrogen by the hydrocarbon radicals of the series CnH2n+1: CH1 H+ CnH2n+1= CH3. CnH2n+1 = C(n+1) H2(n+1)+2 CH-2H+2 CnH2n+1= CH2. (CnH2n+1)2= C(2n+1) H2(2n+1)+2 CH-3H+3 CnH2n+1=CH (CnH2n+1)3 = C(3n+1) H2(3n+1)+2 CH-4H+4 CnH2n+1 = C (CnH2n+1)4 C(4n+1) H2(4n+1)+2 = The series is therefore also known as the marsh gas series. The first three members (CH4, C2H6, and CзH,) each exist in only a single modification; those richer in carbon show isomers, the possible number of which increases with the carbon contents of each member, and is equal to the number of possibilities of structure in the nucleus, only nucleus-isomerism (§ 46) being possible. The hydrogen atoms of the paraffins can be replaced in different amounts, partly directly, as by the halogens, partly indirectly by other elements or radicals. There thereby results an enormous number of organic bodies which are known as the marsh gas derivatives, and which in their totality are known as fatty compounds. The unattacked remaining hydrocarbon residues of increasing valency: CnH2n+1; CnH2n; CnH2n-1; CnH2n-2; CnH2n-3; CnH2n-4, &c., remain as radicals, and are named after their simplest oxides, which are the oxygen substitution-derivatives of the paraffins. 142. By substitution of only a single hydrogen atom in a paraffin, a residue of the formula CnH2n+1 remains, which exists in combination with the substituting element or compound radical. Ithis latter hydroxyl there results the hydrate of the residue, which is termed a monacid alcohol. The residue itself is termed an alcohol radical or alkyl. 143. If substitution of two hydrogen atoms occurs in a paraffin, it can be either at two carbon atoms-one at each-or at one single primary or secondarily combined carbon atom of the nucleus. In the first case there results, if hydroxyl be the substituting radical, the so-called diacid alcohols, or glycols: On the other hand, if the substitution of two hydrogen atoms be at a single carbon atom, there results, if this is a. A terminal carbon atom, the compounds of the aldehyde radicals, so-named from their oxides the aldehydes : CnH2n+1 H or C(n+1)H2n+1)0 or CnH2n0, if n+1 = n b. If, on the contrary, the substitution be at a secondarily combined, intermediate carbon atom, the residue CnH2n is termed a ketone radical, as the oxides are termed ketones : CnH2n+1 من C2Hm+1 = C(2n+1)H2(2n+1)O or CnH2nO, if 2n+1 = n' 144. When three hydrogen atoms in a paraffin are replaced by other elements the complication is greatly increased. On a nucleus containing at least three carbon atoms, each hydrogen atom may be substituted on a different carbon atom; the resulting residue is the radical of the trivalent alcohols or glycerines, that being the name by which their hydrates are distinguished: If the substitution takes place at two different carbon atomsi.e. at one a single, at a second a double substitution there must result oxides which combine the characters of alcohols with those of alde Triple substitution can only take place at a single carbon atom, if the latter be at the end of a chain; the simplest oxides of the residues have then the characters of acids: CH3 он 145. The more hydrogen atoms there are replaced in a paraffin the more numerous must be the resulting products. But as, with exception of marsh gas, the series never contains carbon atoms united with more than three hydrogen atoms, there results, from a greater substitution than three, only combinations of the simpler categories, and there are obtained every time: 1. By replacement of only one hydrogen atom at a carbon atom, by OH, an alcohol, or alcohol derivative, if other elements or compound radicals are the replacing bodies. The number of these varieties of substitution that can take place in a paraffin expresses the valency of the alcohol radical, or the acidity' of the alcohol. It results from this that an x acid alcohol must contain at least x carbon atoms in its nucleus. 2. By simultaneous replacement of two hydrogen atoms united to one and the same carbon atom, there are obtained: a. If the replacement be at a terminal carbon atom: aldehydes or aldehyde derivatives. b. If at an intermediate carbon atom: ketones or ketone derivatives. 3. Organic acids (in the true sense) or their derivatives are formed when the three hydrogen atoms attached to a terminal carbon atom are simultaneously replaced. As ketone alcohols and aldehyde alcohols are known, so similarly H there are alcohol acids, i.e. CH2.OH , aldehyde acids, CO, and ke CO.OH |