Page:EB1911 - Volume 14.djvu/916

Rh that in the compound of higher molecular weight no new links of carbon with carbon are introduced, the trioxymethylene being probably O whereas honey-sugar corresponds to CH2OH·CHOH·CHOH·CHOH·CHOH·CHO, each point representing a linking of the carbon atom to the next. This observation is closely related to the above-mentioned resistivity of the carbon-link, and corroborates it in a special case. As carbon tends to hold the atom attached to it, one may presume that this property expresses itself in a predominant way where the other element is carbon also, and so the linkage represented by –&thinsp;C&thinsp;–&thinsp;C&thinsp;– is one of the most difficult to loosen.

The conception of metamerism, or isomerism in restricted sense, has been of the highest value for the development of our notions concerning molecular structure, i.e. the conception as to the order in which the atoms composing a molecule are linked together. In this article we shall confine ourselves to the fatty compounds, from which the fundamental notions were first obtained; reference may be made to the article : Organic, for the general structural relations of organic compounds, both fatty and aromatic.

A general philosophical interest is attached to the phenomena of isomerism. By Wilhelm Ostwald especially, attempts have been made to substitute the notion of atoms and molecular structure by less hypothetical conceptions; these ideas may some day receive thorough confirmation, and when this occurs science will receive a striking impetus. The phenomenon of isomerism will probably supply the crucial test, at least for the chemist, and the question will be whether the Ostwaldian conception, while substituting the Daltonian hypothesis, will also explain isomerism. An early step accomplished by Ostwald in this direction is to define ozone in its relation to oxygen, considering the former as differing from the latter by an excess of energy, measurable as heat of transformation, instead of defining the difference as diatomic molecules in oxygen, and triatomic in ozone. Now, in this case, the first definition expresses much better the whole chemical behaviour of ozone, which is that of “energetic” oxygen, while the second only includes the fact of higher vapour-density; but in applying the first definition to organic compounds and calling isobutylene “butylene with somewhat more energy” hardly anything is indicated, and all the advantages of the atomic conception—the possibility of exactly predicting how many isomers a given formula includes and how you may get them—are lost.

To Kekulé is due the credit of taking the decisive step in introducing the notion of tetravalent carbon in a clear way, i.e. in the property of carbon to combine with four different monatomic elements at once, whereas nitrogen can only hold three (or in some cases five), oxygen two (in some cases four), hydrogen one. This conception has rendered possible a clear idea of the linking or internal structure of the molecule, for example, in the most simple case, methane, CH4, is expressed by

It is by this conception that possible and impossible compounds are at once fixed. Considering the hydrocarbons given by the general formula CxHy, the internal linkages of the carbon atoms need at least x &minus; 1 bonds, using up 2(x &minus; 1) valencies of the 4x to be accounted for, and thus leaving no more than 2(x + 1) for binding hydrogen: a compound C3H9 is therefore impossible, and indeed has never been met. The second prediction is the possibility of metamerism, and the number of metamers, in a given case among compounds, which are realizable. Considering the predicted series of compounds CnH2n+2, which is the well-known homologous series of methane, the first member, the possible of isomerism lies in that of a different linking of the carbon atoms. This first presents itself when four are present, i.e. in the difference between C&thinsp;–&thinsp;C&thinsp;–&thinsp;C&thinsp;–&thinsp;C and With this compound C4H10, named butane, isomerism is actually observed, being limited to a pair, whereas the former members ethane, C2H6, and propane, C3H8, showed no isomerism. Similarly, pentane, C5H12, and hexane, C6H14, may exist in three and five theoretically isomeric forms respectively; confirmation of this theory is supplied by the fact that all these compounds have been obtained, but no more. The third most valuable indication which molecular structure gives about these isomers is how to prepare them, for instance, that normal hexane, represented by CH3·CH2·CH2·CH2·CH2·CH3, may be obtained by action of sodium on propyl iodide, CH3·CH2·CH2I, the atoms of iodine being removed from two molecules of propyl iodide, with the resulting fusion of the two systems of three carbon atoms into a chain of six carbon atoms. But it is not only the formation of different isomers which is included in their constitution, but also the different ways in which they will decompose or give other products. As an example another series of organic compounds may be taken, viz. that of the alcohols, which only differ from the hydrocarbons by having a group OH, called hydroxyl, instead of H, hydrogen; these compounds, when derived from the above methane series of hydrocarbons, are expressed by the general formula CnH2n+1OH. In this case it is readily seen that isomerism introduces itself in the three carbon atom derivative: the propyl alcohols, expressed by the formulae CH3·CH2·CH2OH and CH3·CHOH·CH3, are known as propyl and isopropyl alcohol respectively. Now in oxidizing, or introducing more oxygen, for instance, by means of a mixture of sulphuric acid and potassium bichromate, and admitting that oxygen acts on both compounds in analogous ways, the two alcohols may give (as they lose two atoms of hydrogen) CH3·CH2·COH and CH3CO·CH3. The first compound, containing a group COH, or more explicitly O = C–H, is an aldehyde, having a pronounced reducing power, producing silver from the oxide, and is therefore called propylaldehyde; the second compound containing the group –C·CO·C– behaves differently but just as characteristically, and is a ketone, it is therefore denominated propylketone (also acetone or dimethyl ketone). And so, as a rule, from isomeric alcohols, those containing a group –CH2·OH, yield by oxidation aldehydes and are distinguished by the name primary; whereas those containing CH·OH, called secondary, produce ketones. (Compare
 * Organic.)

The above examples may illustrate how, in a general way, chemical properties of isomers, their formation as well as transformation, may be read in the structure formula. It is different, however, with physical properties, density, &c.; at present we have no fixed rules which enable us to predict quantitatively the differences in physical properties corresponding to a given difference in structure, the only general rule being that those differences are not large.

Perhaps a satisfactory point of view may be here obtained by applying the van der Waals’ equation A(P + a/V2)(V &minus; b) = 2T, which connects volume V, pressure P and temperature T (see ). In this equation a relates to molecular attraction; and it is not improbable that in isomeric molecules, containing in sum the same amount of the same atoms, those mutual attractions are approximately the same, whereas the chief difference lies in the value of b, that is, the volume occupied by the molecule itself. For what reason this volume may differ from case to case lies close at hand; in connexion with the notion of negative and positive atoms, like chlorine and hydrogen, experience tends to show that the former, as well as the latter, have a mutual repulsive power, but the former acts on the latter in the opposite sense; the necessary consequence is that, when those negative and positive groups are distributed in the molecule, its volume will be smaller than if the negative elements are heaped together. An example may prove this, but before quoting it, the question of determining b must be decided; this results immediately from the above quotation, b being the volume V at the absolute zero (T = 0); so the volume of isomers ought to be compared at the absolute zero. Since this has not been done we must adopt the approximate rule that the volume at absolute zero is proportional to that at the boiling-point. Now taking the isomers H3C·CCl3(Mv = 108) and ClH2·CHCl2(Mv = 103), we see the negative chlorine atoms heaped up in the left hand