Page:The New International Encyclopædia 1st ed. v. 04.djvu/236

* CABBON COMPOUNDS. 196 CABBON COMPOUNDS. tached to a carbon atom to which two more hydrogens and one CH3 group are linked at the sajue time. If we should substitute chlorine in tne place of one of the hydrogen atoms, we would — the formula tells us — obtiiin the same deriva- tive, no mailer 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 mouo- chloro-substitution products. Yet one, and only one such product could be obtained. Applying the same method to the graphical formula of propane, CjH, (see above), we find that there arc two different hydrogen-positions in it; so that two other formulas can be derived from it by substituting a chlorine atom in place of one hydrogen atom, viz. H H H H CI H III III H— C— L— C^— CI and H— C— C— C— H III III H H H H H H 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 CsHj group are 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 cases. If we consider a formula like the following: CI H H I I I H-C-O-C-H I I I H H H ■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 fonnulas: viz. its chlorine is linked to a carbon to which two hy- drogens and one C.Hj 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 liow they are combined with one another. If we now turn to the facts of experi- mental chemistry, we find again that chemists have really been able to prepare two, and only two niono-chloropropanes haing in common the molecular formula C,1I;C1, yet differing consider- ably in their properties. The number of examples thus showing the per- fect correspondence between theoretical fonnulas 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 actuallj- 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. CjH,0. On the other hand, the set of atoms making up H— C— C— O— H this molecule may be represented by two different graphical forumlas, viz. II U H H H— C— O— C— H and Now, an experimental investigation of the alco- hol reveals the following facts: (1) By the action of metallic sodium alcohol is transformed into a compound represented by the formula CiHjXaO. The molecular formula of the alcohol being C.lIoO, it is evident that in this transformation one-sixth of the hydrogen conUiined in the alcohol is replaced by sodium. Xo matter how great an excess of sodium is used, no more than oue-sixth of the hydrogen can thus be replaced. The transformation therefore speaks in favor of assigning to alcohol the second of the above graphical fornuilas, because the first shows no difi'erence whatever in the relative positions of the several hydrogen atoms, and only the second formula shows one hydrogen atom in a different position from the other five hydrogens. (2) By the action of phosphorus penta-chloride one-sixth of the hydrogen, together with all the oxygen contained in the alcohol, is replaced by chlorine, according to the following equation: c,H,,o + pa, = c.HsCi + poa, + hci Ethyl alcohol Mono-chloro-ethane This transfonnation, too. speaks in favor of as- signing to the alcohol the second of the above graphical formulas, for it shows that one-sixth of the hydrogen is so intimately associated with the oxygen that they readily leave the compound together. And as, further, metallic sodium re- fuses to combine with the mono-chloroethane produced by the transformation, wo 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 — H. Since, besides the two transformations just considered, all other reactions of ordinary alco- hol bring out the perfect correspondence between the chemical properties of this substance and the relations exhibited by the second graphical formula, there remains no doubt as to which of the two formulas should be assigned to alcohol. But then the first formula remains the only possible one for our ether. The correspondence between the ether and the graphical formula thus chosen to represent it, is, just as in the case of the alcohol, brought out by a number of reac- tions, but ihese cannot be discussed here. SuHice it to mention that the ether reacts neither with metallic sodium nor with phosphorus penta- chloride; which indicates that the ether does not, like the alcohol, contain a hydroxyl group — H. The graphical formula shows the same thing very plainly. R.DICLES. The example considered in the pre- ceding paragraphs has led us. among other things, to the conclusion that a molecule of ordi- nary alcohol contains a "hydroxyl group OH.' The conclusion was based on tlie fact that a por- tion of the hydrogen of alcohol and the whole of its oxygen were seen to leave the compound together, while the rest of the molecule remained inchanged. The molecule of alcohol, C,H,0, is