341 342 CHAPTER XVII. ISOMERISM AND STRUCTURAL FORMULAE. In the last chapter we had an instance of isomerism; namely, the existence of three different compounds all having the molecular composition expressed by the formula C ̧H12. 5 The prominent fact of isomerism is, that two or more compounds sometimes exist having identical compositions, and identical specific gravities in the state of gases, and yet exhibiting different properties. The statement of this fact in the language of the molecular and atomic theory is, that two or more gaseous molecules may exist composed of the same number of the same atoms, and yet differing from each other in their properties. An instance of isomerism is furnished by the existence of two compounds having the composition CHO. One of these is ethylic alcohol, the other is methylic ether. Ethylic alcohol interacts with potassium or sodium thus, C2H ̧O + K = C2H ̧KO+H; methylic ether and potassium (or sodium) do not interact. Phosphorus pentachloride interacts with both isomerides; the interactions are these : 6 = 2 5 6 3 (1) alcohol; CHO+ PCI, C,H,Cl + POCI, + HCl. The alcohol is easily oxidised, first to aldehyde C2HO, then to acetic acid CHO,; the ether is not easily oxidised. Ethylic alcohol is a colourless, volatile, liquid, boiling at 7803; methylic ether is a colourless gas which may be condensed by cold to a liquid boiling at -21°. Another instance of isomerism is furnished by the existence of four hydrocarbons having the molecular composition CH, These bodies are all liquids, boiling at 134, 136°—137o, 10° 137°—137°.5, and 140°-141o, respectively. That which boils at 134° is easily oxidised to an acid having the composition CHO,. The other three are oxidised to three different acids all having the composition CHO.. The conditions under 6 which these three hydrocarbons are oxidised vary somewhat. 8 10 The four hydrocarbons C ̧H1, are evidently very similar in 343 their chemical properties; the two compounds CHO are less closely related to each other. Compounds which have the same composition and the same molecular weight, but which shew differences in their chemical properties so decided as to require them to be placed in different classes, are sometimes said to be metameric. Metamerism is included in the wider term isomerism. The molecular and atomic theory endeavours to explain 344 isomerism by saying that the properties of molecules depend, among other conditions, on the arrangement of their parts. Is this assertion justified by facts? Before we can profitably attempt an answer to this question we must understand what is meant by the phrase 'arrangement of the parts of a molecule '. In Chap. XIII. par. 247 a very brief account was given of the 345 use of the expression 'equivalent weights of two alkalis'. We must now look more fully at the notion of chemical equivalency. 88.8 parts by weight of potash (KOH), 63.5 parts by weight of soda (NaOH), and 38.1 parts by weight of lithia (LiOH), severally neutralise 100 parts by weight of nitric acid. These masses, 88.8, 63.5, 38·1, of the three alkalis are therefore equivalent as regards power of neutralising a specified mass of nitric acid, inasmuch as these masses are of equal value in exchange. 100 parts by weight of sulphuric acid are neutralised by 114.3 parts by weight of potash, or by 81.6 parts by weight of soda, or by 49 parts by weight of lithia. These numbers, 114.3, 81.6, and 49, represent masses of the three alkalis which are equivalent as regards power of neutralising a specified mass of sulphuric acid. Now the ratio 88.8 63.5 38.1 is the same as the ratio 114.3 81.6: 49. If the masses of these three alkalis required to neutralise 100 parts by weight of each of several acids are determined, it is found that these masses always bear the same ratio to one another. Hence it is possible to assign values to these three alkalis representing those masses of them which are equivalent as regards power of neutralising one and the same 346 347 348 mass of any specified acid. Similarly, it is possible to assign values to the acids which shall represent those masses of them which severally neutralise one and the same mass of any specified alkali. In determining the equivalent weights of the alkalis it is customary to take one reacting weight (or we may say one molecule) of hydrochloric acid as the unit mass of standard acid. The reacting weight of hydrochloric acid (HCl) is 36.5: one reacting weight of this acid is neutralised by (in round numbers) 56 parts by weight of potash, 40 of soda, and 24 of lithia, respectively. The mass of sulphuric acid which is neutralised by each of these masses of potash, soda, or lithia is 49; the mass of chloric acid is 84.5; the mass of orthophosphoric acid is 32-6; the mass of metaphosphoric acid is 80; the mass of pyrophosphoric acid is 44-5; &c. &c. The numbers 36.5, 49, 84.5, 32.6, 80, 44.5 represent masses of the acids mentioned which are equivalent as regards power of neutralising 56 parts by weight of potash, or 40 parts of soda, or 24 of lithia. The notion of equivalency may be extended to the elements. If we determine the masses of a series of metals which severally combine with 16 parts by weight of oxygen, we shall have determined the equivalent weights of these metals as regards this particular reaction.. Or we might cause a number of metals to interact with hydrochloric acid, and determine the mass of each metal which thus produced 1 gram of hydrogen; these masses would represent equivalent weights of the metals as regards this particular reaction. When therefore we speak of the equivalent weight of an element or compound there is always implied a comparison of the specified substance with some other substance as regards power of performing a definite chemical operation. Equivalent weights represent quantities of elements or compounds which can be exchanged in some specified chemical process. The expression equivalent weight of an element is frequently used somewhat loosely. In order to determine equivalent weights, elements are generally compared as regards their combination with oxygen, and 8 parts by weight is usually chosen as the standard mass of oxygen. Hence the equivalent weight of an element generally means the mass of it which combines with 8 parts by weight of oxygen. In the cases of elements which do not combine with oxygen, hydrogen is generally chosen as the standard element, and the equivalent weight is taken to be that mass of the element which combines with 1 part by weight of hydrogen, or sometimes (especially in the case of metals) that mass of the element which interacts with a dilute acid to produce 1 part by weight of hydrogen. Let us apply these various definitions of equivalent weight to the metal tin. Experiment proves; (1) that 59 parts by weight of tin combine with 8 parts by weight of oxygen to produce stannous oxide; (2) that 29.5 parts by weight of the same element combine with 8 parts by weight of oxygen to produce stannic oxide; (3) that 59 parts by weight of tin interact with hydrochloric acid to produce 1 part by weight of hydrogen, stannous chloride being formed at the same time. We therefore get two values for the equivalent weight of 349 tin. But had we accurately defined the meaning to be given to the term equivalent weight we should have got over this difficulty. Let tin and lead be compared as regards the formation of oxides having similar properties and similar compositions. Each metal forms a protoxide MO, and a dioxide MO; the oxides MO are fairly similar chemically, and so are the oxides MO. Experiment shews that 59 parts by weight of tin are equivalent to 103.5 parts by weight of lead as regards power of combining with 8 parts by weight of oxygen to produce oxides belonging to the type MO; and that 29.5 parts by weight of tin are equivalent to 51.75 parts by weight of lead as regards power of combining with 8 parts by weight of oxygen to produce oxides of the type MO, The notion of the equivalency of elements is fairly simple; 350 but it is often very difficult to apply it accurately. The difficulty consists in finding a standard chemical change wherein a specified mass of one element may be exchanged for a specified mass of another without altering the essential character of the reaction. The conception of equivalency has been extended to the 351 elementary atoms. Let the standard action be ability to combine with one, and only one, atom of hydrogen to produce a gaseous molecule; let all atoms which do this be classed together as equivalent. Then the atoms of hydrogen, chlorine, bromine, iodine, and probably fluorine*, are equivalent; the evidence is the existence of the gaseous molecules H., HCl, HBr, HI, [and HF]*; and * There is still some doubt whether the molecule of gaseous hydrogen fluoride is HF or H,F2. the non-existence of gaseous molecules composed of one atom of chlorine, bromine, &c. and more than a single atom of hydrogen. 352 The atoms of hydrogen, chlorine, bromine, iodine and fluorine are placed together in one class, and are called monovalent atoms. 353 It is evident that in asserting these atoms to be equivalent, we have relaxed the strict meaning of the term equivalency. An atom of hydrogen, or an atom of chlorine, or an atom of bromine, &c. combines with only one atom of hydrogen to produce a gaseous molecule; in this respect the atoms are of equal value in exchange. In defining the meaning of the terms divalent, trivalent, &c. atoms, we must a little further relax the definition of equivalency. We assume that an atom which combines with 2 atoms, and not more than 2 atoms, of chlorine, &c. is equivalent to an atom which combines with not more than 2 atoms of hydrogen, &c. By doing this we arrive at the definition of divalent atoms as atoms which combine with not more than two monovalent atoms to form gaseous molecules. 354 Applying these definitions of monovalent, divalent, &c. atoms to all the elements compounds of which with hydrogen, chlorine, bromine, iodine, or fluorine, have been gasified, we arrive at the following classification of atoms. Standard Monovalent atoms; H, F, Cl, Br, I. I. Monovalent atoms; i.e. atoms which combine with one standard monovalent atom to form gaseous molecules .K, Tl, Hg. II. III. Trivalent atoms; i.e. atoms which combine with three standard monovalent atoms to form gaseous molecules... ...B, N, P, As, Sb, Bi, In. IV. Tetravalent atoms; i.e. atoms which combine with four standard monovalent atoms to form gaseous molecules ......C, Si, Ti, Ge, Zr, V, Sn, Th, U. V. Pentavalent atoms; i.e. atoms which combine with five standard monovalent atoms to form gaseous molecules ......P, Nb, Ta, Mo, W. VI. Hexavalent atoms; i.e. atoms which combine with six standard monovalent atoms to form gaseous molecules......W. |