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75. In thus trying to use the theory of valency as a guide towards determining the structures of isomeric molecules, we have found it on the whole advantageous to limit this theory in various ways.

I. The theory is applied in strictness only to molecules of gases.

II. The valency of an atom is defined as a number which expresses the maximum number of other atoms between which and the given atom there is direct action and reaction in a molecule; this number is determined by the study of certain defined classes of molecules.

III. Isomerism is regarded as correlated with varying relative positions of atoms, not with variations in the distances between identically arranged atoms, in any molecule.

Applying the theory as thus limited, and for the most part to compounds of carbon, we found that the structural formula of classes of carbon compounds can be so far generalised as to admit of the assertion that the molecules of the members of any one class are characterised by the presence of a special atomic group which may be called the class-group; and that hence the first step in assigning a structural formula to a new compound is to determine, by a comparison of the reactions of this compound with those of known substances belonging to various classes, the class to which it belongs : having done this, we then eliminate from the possible structural formulæ those which do not contain the characteristic group of the class in which our compound is placed. Finally, we choose from the remaining formulæ that one which best summarises the reactions of the compound molecule under consideration and its relations to other molecules.

We found that a wide knowledge of the characters of classes of compounds is required on the part of him who would employ this method with success, and also that the chemist


that each atom can act directly on only a limited number of other atoms in a molecule, we are obliged to regard the atoms which form any molecule as performing constant but regulated movements, and not-as might be supposed by a careless, or superficial reader of the atomic explanation of isomerisn—as in absolutely fixed positions within the molecule.

has constantly to be on his guard against drawing too rigid conclusions. A new compound may represent a new class, hence a new class-group has to be determined by comparing the reactions of the new compound with those of others the classification of which is fairly settled, and also by seeking to obtain other representatives of the new class. The discovery and study of new compounds apparently belonging to a known class may lead to a revision of the general formula assigned to the class, and perhaps to a division of the class into subclasses, each characterised by its own group.

76. The application of the theory of valency to determine the most probable of many possible formulæ is evidently a matter of no little difficulty. Certain generalisations are usually adopted as guides in interpreting the results of the study of the chemical habitude' of molecules. The principal generalisations are these.

(1) “Those atoms which are obtained as an undecomposed group in the analysis of a compound, are contained in the molecule of that compound as a group of directly com'bined atoms.

(2) 'When a group of atoms passes from one com'pound molecule to another, the relative arrangement of these ‘atoms is not, as a rule, altered.'

(3) When an atom, or group of atoms, replaces another 'atom or group of atoms of equal valency with itself, the re'placing atom, or group, occupies (as a rule) the same position * relatively to the other atoms in the molecule as was occupied 'by the atom, or group of atoms, which it has replaced?'

77. Many of the reactions given on pp. 144—146, as illustrative of methods for assigning structural formulæ to given compounds, also serve as illustrations of the use of these generalisations; one or two further illustrations will be given here. Two isomerides, (1) CH

(2) CH,

CH, and





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having each the formula C,H,O are theoretically possible'. Two compounds having this formula are known. One of these (alcohol) is acted on by potassium or sodium thus,

(a) CH3O+K=CH,KO+H potassium (or sodium) does not act on the substance thus formed : alcohol is acted on by phosphorus pentachloride thus,

(6) C,H,0+PC1=C,H,CI+ POCI, + HCI. The second isomeride (methyl ether) is not acted on by potassium or sodium but reacts with phosphorus pentachloride thus,

C,H,O+PC;=2CH,Cl+ POCIz. The first formula generalises the reactions of alcohol, the second generalises the reactions of methyl ether; thus

(a) CH,



OH OK one, and only one, hydrogen atom is represented in the formula as indirectly bound (through an oxygen atom) to a carbon atom; (6) CH



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the group OH is replaced by the atom Cl, which being of cqual valency is regarded as occupying the place in the molecule relatively to the other atoms, formerly occupied by the

group OH.

The second formula H,C-0-CH, assigned to methyl ether, represents all the hydrogen atoms as directly acting on atoms of carbon, they have all the same function; but the oxygen atom is linked only to carbon, if it is replaced by two monovalent atoms, e.g. by chlorine, the molecule can no

1 See Lothar Meyer, loc. cit. pp. 252 el seq.

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longer hold together but separates into two molecules, each having the structure CI – CHg.

When the molecule HO – CH, -CH, is oxidised, it loses two atoms of hydrogen, producing C,H,O, which is then changed, by taking up one atom of oxygen, into the new molecule C,H,O,. Probably the simplest way in which these changes can be represented in structural formulæ is (1) CH

(2) CH, Сн,

CH,- H2

+0 C-0.


OH But when the molecule C,H,O, is acted on by phosphorus pentachloride it yields C,H,OCI, and this is unacted on by the same reagent: C,H,O, is a monobasic acid, when its sodium salt is heated with caustic soda it is decomposed thus,

C,H,NaO,+NaHO=NaCO3+CH. These reactions are all expressed by the formula' O-C-CH,

OH which is therefore adopted as the structural formula for acetic acid. But when the compound C,H,O (intermediate between alcohol and acetic acid) is acted on by phosphorus pentachloride it yields C,H,Cl,, and not C,H,Cl as might be expected if the formula OH-C-CHg, provisionally assigned to it, were correct. From synthetical and analytical reactions, C,H,CI, may be shewn to be best represented by the structural formula Cl, = CH – CH,; assuming this formula, and remembering that the reaction to be explained, viz. formation of this compound from C,H,O, consists in the replacement of

1 Thus,
(1) CH,
CH, (2) CH,


C-0+ PCIE = C-0 + &c. C-O + Na-OH = H + Na,CO3.



One of the carbon atoms in the original molecule remains associated with 3 atoms of hydrogen throughout both processes of change, hence we conclude that the molecule of acetic acid contains the group CII,.

one divalent oxygen atom by two monovalent chlorine atoms, we apply generalisation (3), par. 76, and conclude that the structure of the molecule C,H,O is best represented by the formula 0 – CH – CH,.

The oxidation of alcohol must then it appears be represented thus in structural formulæ,

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Another and somewhat more complex illustration, taken from the so-called ' aromatic'carbon compounds, will serve to shew that the generalisations stated in par. 76, although widely applicable, must yet be used with great caution. Assuming the generally adopted structural formula for benzene' (CH), viz.





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the existence of three, and only three isomeric dichloro- or
dibromo-, &c., benzenes becomes possible, viz.



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1 See Armstrong and Groves, loc. cit. pp. 260-63; also pp. 270–74. See also post, pp. 163-165, par. 81.

? The fact that this formula is generally used rather than the more complex formula originally proposed by Kekulé with alternate 'doubly' and 'singly-linked'

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