When we examine the graphical formula of ethane. (see above). we find again that the several positions of the hydrogen atoms are identical: each hydrogen atom is, namely, at tached to a carbon atom to which two more hydrogens and one CH, group are linked at the sante time. If we should substitute chlorine in tne place of one of the hydrogen atoms. we would —the formula tells us—obtain the same deriva tive, no matter which particular portion of the hydrogen is displaced. In this case, too, different and ingenious methods have actually been em ployed by chemists, with a view to displacing different portions of the hydrogen by chlorine and thus possibly producing two different mono chloro-substitution products. Yet one, and only one such product could he obtained.
Applying the same method to the graphical formula of propane. (see above), we tind that there are two different hydrogen-positions in it so that two other formulas can be derived from it by substituting a chlorine atom iu place of one hydrogen atom, viz.
H 11 HClH •• -- — The difference between the two formulas, ex pressed in words, is as follows: in the first formu la the chlorine is linked to a carbon atom to which two hydrogens and one C,H, group ate linked at the same time: in the second formula the chlorine is linked to a carbon atom to which one hydrogen atom and two CH, groups are linked at the same time. These are the only possible eases. If we consider a formula like the following; which may seem. at first sight, to differ from either of the above two formulas, we have no difficulty in observing that it is characterized precisely as the first of those formulas: viz. its chlorine is linked to a carbon to which two hy drogens and one group are linked at the same time. In general, a graphic formula is not meant to convey an image of the configuration of the atoms within a molecule: all it is expected to show is what atoms and groups of atoms exist in the molecule and how they are combined with one another. If we now turn to the facts of experi mental ehemi,try. we find again that chemists have really been able to prepare two, and only two mono-ehloro-propane; having in common the molecular formula CH,C1, yet differing consider ably in their properties.
The number of examples thus showing the per fect correspondence between theoretical formulas and the results of experimental investigation might be multiplied almost indefinitely. A fur
ther question, however. remains to be answered: supposing two or more graphical formulas corre spond to the same molecular formula. and the several thus possible compounds have actually been prepared—how do we know which formula corresponds to which compound? This question is usually answered by a study of the reactions and methods of preparing the compounds. The two substances di-methyl ether and ordinary alcohol may serve. as an example. Both have the same molecular formula.
On the other hand, the set of atoms making up this molecule may be represented by two different graphical formulas. viz.
Now, all experimental investigation of the alco hol reseals the following facts: (1) By the action of metallic sodium alcohol is transformed into a compound represented by the formula C,I1,Na0. The molecular formula of the alcohol being C,I1,0, it is evident that in this transformation one-sixth of the hydrogen contained in the alcohol is replaced by sodinin. No matter how great an excess of sodium is used, no more than one-sixth of the hydrogen can thus be replaced. The transformathm therefore speaks in favor of assigning to alcohol the second of the above graphical formulas, because the first shows no difference whatever in the relative position: of the several hydrogen atoms. and only the second for la shows one hydrogen atom in a different position from the other live hydrogens.
(2) By the action of phosphorus penta-ehloride one-sixth of the hydrogen, together with all the oxygen contained in the alcohol. is replaced by chlorine, according to the following equation: C,11„0 + = Hel Ethyl alcohol This transformation. too, speaks in favor of as signing to the alcohol the second of the above graphical formula,. tor it shows that one-sixth of the hydrogen is su intimately associated with the oxygen that they readily leave the compound together. And as, further. metallic sodium re fuses to combine with the mono-ehloro-ethane produced by the transformation, we conclude that the portion of the hydrogen of alcohol which is replaceable by sodium must be the same as the portion which we have just seen to be intimately associated with oxygen. That portion is evident ly represented in the graphical formula by the hydrogen atom of the 'hydroxyl' group 0-11.