place we should get the molecule H-C-Cl- Cl — C — H; but this molecule cannot exist (by hypothesis) because the chlorine atom is monovalent; hence the substitution of two atoms of chlorine for a single atom of oxygen in the molecule represented by symbol II. must result in the production of two H molecules each represented by the symbol H-C- Cl. H We therefore conclude that the molecule of ethylic alcohol H H is represented by the symbol H-C—C—O— H, and the This conclusion is borne out by a further study of the properties of the two isomerides. Thus, sodium rapidly interacts with ethylic alcohol to produce C,H,NaO+H; but sodium and methylic ether do not interact. The formula given for ethylic alcohol represents one, and only one, of the 6 hydrogen atoms as directly interacting with an atom of oxygen; we should therefore expect that sodium would replace either 5 atoms, or one atom, of hydrogen from the molecule of ethylic alcohol. As sodium replaces only a single atom of hydrogen we conclude that the atom replaced is that which is represented in the formula as directly interacting with an atom of oxygen. But if this is so, we should conclude that none of the hydrogen atoms in the molecule of methylic oxide would be replaceable by sodium. Another reaction which favours this conclusion regarding the interaction of sodium 2 with ethylic alcohol is that which occurs between sodium and water. This change is formulated thus H2O + Na = NaOH + H. Now the only possible way of representing the molecule H2O" is H-O-H. The interaction between this molecule and atom of sodium must be represented thus, H-O-H+Na = Na-O- H+H; an the atom of hydrogen which is replaced by an atom of sodium Let us take another example of the application of the 363 conception of atomic valency, that is that each atom in a gaseous molecule can directly interact with a limited number of other atoms. A certain hydrocarbon has the molecular composition CH,. Can more than one compound exist having this composition? In other words, can we represent the arrangement of the atoms CIVH in more than one way? As the atom of carbon is tetravalent, and that of hydrogen is monovalent, we must represent the two atoms of carbon in the gaseous molecule CH, as directly interacting; the only possible way of doing this is to write the formula thus, 6 Therefore the hypothesis of valency, when applied to the compound C.H, asserts that one and only one compound having this molecular composition can exist. As a matter of fact only one compound C2H, does exist. 6 There is another hydrocarbon C.H.; what are the ways 364 in which 2 tetra- and 4 mono-valent atoms can be arranged? Each carbon atom in the molecule CH, must interact with another carbon atom; we may write the formula as Formula (1) represents each carbon atom in the molecule CH, as directly interacting with another carbon atom and 2 hydrogen atoms, i.e. as directly interacting with 3 other atoms. Formula (2) represents one of the carbon atoms as directly interacting with 2 other atoms, and the other carbon atom as directly interacting with 4 other atoms. In par. 360 we defined the valency of an atom to be the number expressing the maximum number of other atoms between which and the given atom there is direct interaction in any gaseous molecule. In accordance with this definition, we may say that formula (1) represents each atom of carbon as trivalent in the molecule CH, and formula (2) represents one atom of carbon as divalent, and one atom of carbon as tetravalent, in the molecule C2H. As it is impossible to represent both atoms of carbon as tetravalent, i.e. as directly interacting with 4 other atoms, in the molecule C,H,, it is evident that although the maximum valency of a carbon atom is 4, yet the actual valency of an atom of this element in a specified molecule may be less than 4. 4 The student should particularly notice that the statement, an atom of carbon may act in a specified molecule as a trivalent or divalent atom, only holds good when we attach to the term valency of an atom the meaning given in par. 360. The hypothesis of valency then points to the possible existence of two isomerides C2H. But only one compound CH is known to exist. Which of the two formulae given above shall we assign to this compound? The compound in question is called ethylene. Ethylene readily combines with chlorine to form the dichloride C,HCl. If the formula of 4 ethylene is C C, the formula of the dichloride is almost H-C-C-H, the formula of the dichloride is almost H for the molecule C,HCl,; the formation of Cl - C-C-Cl is much more likely than the formation of H-C- C-H H H Cl H from C-C; because, as the maximum valency of a H H carbon atom is 4, and as each carbon atom in the molecule H C H C is represented as trivalent, it is only necessary H H for each carbon atom to interact directly with one of the chlorine atoms brought into contact with the molecule CH rearrangement of the interactions of the atoms of carbon and hydrogen must occur. The only safe rule to adopt in studying the applications of valency to isomerism is, that no rearrangement of the interactions of atoms must be assumed to take place unless the facts absolutely require it. Two compounds having the molecular composition CHCI, exist: one is formed by the direct addition of chlorine to ethylene, it is called ethylene chloride; the other is produced by the interaction of chlorine with the hydrocarbon ethane CH, [thus, CH + 2C1, C2HCl, + 2HC1], it is called ethylidene chloride. To which of these compounds must we 6 = 4 ΟΙ H = also produced by the interaction of phosphorus pentachloride with ethylic aldehyde; thus C.H2O+ PCI,CH,CI, + POCI ̧. In this reaction one atom of oxygen has been removed from the molecule C,H ̧O and 2 atoms of chlorine have been put in its place; therefore, unless distinct reasons can be shewn to the contrary, it is likely that the 2 atoms of chlorine in the molecule C2HCl, are related to the rest of the molecule in a way similar to that in which the atom of oxygen in the molecule C2HO is related to the rest of the molecule. We shall now assume that the formula of ethylic aldehyde is H 0 H-C C. This formula rests on a large number of H H reactions; there is very little doubt as to its correctness. Now the replacement of the atom of oxygen in this molecule by 2 atoms of chlorine will produce the molecule* H C C-Cl. But the compound produced is ethy H H lidene chloride; hence the formula of ethylidene chloride is * Compare this interaction of PC1, and C2HO with that of PC1, and CHO (methylic ether) given in par. 362. |