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Formula (1) represents each carbon atom in the molecule CH, as directly interacting with another carbon atom and with 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 C,H. 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.
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 CH.. 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 chlorinė to form the dichloride C,H.Cl. If the formula of
ethylene is 0
C, the formula of the dichloride is almost
certainly Cl —C—C— Cl; if the formula of ethylene is
H – C—C— H, the formula of the dichloride is almost
1 certainly H-C-0-H. Only two formulae are possible
| 1 Ci H
1 1 for the molecule C, HCl,; the formation of CI — C UCI
1 is much more likely than the formation of H- C C-H
because, as the maximum valency of a . 1
H carbon atom is 4, and as each carbon atom in the molecule H H 1 C-C is represented as trivalent, it is only necessary 1 1 H H for each carbon atom to interact directly with one of the chlorine atoms brought into contact with the molecule C,H,
in order to produce the molecule Ci — C- C -- Cl; but if
the molecule H-0-0-H is produced a considerable
CI 'H 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, C.H. + 2Cl2 = C, H,C1, + 2HCI], it is called ethylidene chloride. To which of these compounds must we
assign the formula H -C-C-H? Ethylidene chloride is
01 H also produced by the interaction
of phosphorus pentachloride with ethylic aldehyde; thus C,H,O+PCI,= CH.Cl, + POCI,. In this reaction one atom of oxygen has been removed from the molecule C,H,0 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 C,H_Cl, are related to the rest of the molecule in a way similar to that in which the atom of oxygen in the molecule C,H,O is related to the rest of the molecule. We shall now assume that the formula of ethylic aldehyde is
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*
1 1 H-0-0- Cl. But the compound produced is ethy
H H lidene chloride; hence the formula of ethylidene chloride is
* Compare this interaction of PCl, and C,H,0 with that of PCl, and C,H60 (methylic ether) given in par. 362.
o C Cl; and hence the formula of the isomeric
1 1 ethylene chloride is Cl —C—C— Cl, because these are the
H H only possible formulae for the molecule C,H,C1, But ethylene chloride is produced by the direct addition of chlorine to
1 1 ethylene; hence the formula of ethylene is C — C.
H H Two compounds having the molecular composition CH 365 may exist according to the hypothesis of valency: only one actually exists; but derivatives of both, e.g. chlorides, are known. It is possible that the compound the molecule of
which would have the composition H-C-C. H may be
H produced; but it is not likely, because the many attempts made to form it have all resulted in the production of the
1 isomeric compound C-C.
H H This is an illustration of the proposition, that it is not always possible to obtain every one of the isomerides of a given composition the existence of which is indicated by the hypothesis of valency; or, in other words, that the existence of a compound of specified composition is not conditioned solely by the valencies of the atoms which form the molecule of this compound.
The hypothesis of valency leads to the conception of the molecule as a structure, the parts of which are related to each other in a definite manner.
Formulae such as those given in the preceding paragraphs for ethylic alcohol, methylic ether, ethylene chloride, and ethylidene chloride, are called rational or structural formulae; they are contrasted with empirical formulae (C.H.O and C,H,C1) which express the percentage and atomic composition of molecules.
Structural formulae are attempts to summarise the chief reactions of formation and decomposition of compounds in the highly symbolical language of a special hypothesis resulting from the application of the molecular and atomic theory to the chemical phenomena of isomerism.
A structural formula may be found for any gasifiable compound the molecule of which is composed of atoms of known valencies; but the structural formula to be of any value must be the outcome of many experiments on the interactions of the compound to which it is given. The value of the formula consists in its suggestiveness of reactions, and in the extent to which it exhibits the analogies between the compound formulated and other compounds. The structural formulae of carbon compounds have been greatly developed. There can be no doubt that the chemistry of these compounds would not have advanced as it has done without the aid of structural formulae ; indeed the remarkable predictions which have been made, and verified, regarding classes of chemical changes among carbon compounds afford satisfactory evidence that the conceptions on which structural formulae are based are accurate and well founded.
It is generally possible to shew that the characteristic properties of a group of similar carbon compounds are connected with a certain arrangement of some of the atoms in the molecules of these compounds, which arrangement is common to all the members of the group, and can be expressed in a structural formula. Thus, a great many alcohols behave similarly when oxidised; the molecule of each loses 2 atoms of hydrogen thereby producing an aldehyde, and this aldehyde is then oxidised to an acid the molecule of which is composed of the same number of carbon atoms as the molecule of the alcohol. Such alcohols are called primary alcohols. The following formulae give examples of the oxidation of primary alcohols.