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 he assigned to alcohol. Butt 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 these cannot be discussed here. Suffice 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 0-11. The graphical formula shows the same thing very plainly.
IlAtuctEs. The example considered in the pre ceding paragraphs ha, led us• among other things, to the conclusion that a molecule of ordi• nary alcohol contains a 'hydroxyl group 011.' 'the conclusion was based on the fact that it por tion of the hydrogen of alcohol and the whole of its oxygen were seen to leave the compound while the rest of the molecule remained unchanged. The molecule of alcohol, C,11,0, is accordingly conceived a, consisting of two atomic groups, C,I1, and OIL the constituent atoms of which tend to hold firmly together, although, of course. they are not altogether incapable of being separated. Such atomic groups, behaving during many transformations of the compound as it they were ju-t single atoms, are called radicles.
An example may serve to show how the small cst radicle, present in a given compound are de termined by its chemical reaction,. Let the prob lem be. To determine the atomic groups in a molecule of acetic acid, To solve the problem, acetic acid is caused to undergo a series of trail-formation- and the following facts are brought to light.
By the action of phosphorus penta-chlo ride, acetic acid. U,11,0„ is readily transformed into acetyl chloride, whose formula is In this reaction all atom of hydrogen and an atom of oxygen are together replaced by an atom of chlorine. We therefore conclude that acetic acid, like alcohol. contain, a hydroxyl group, OH.
(2) By passing, chlorine gas into hot acetic acid exposed to the direct action of sunlight, a compound called tri-chloro-acetic acid is ob tained. When boiled with water, this compound is split up and chloroform is produced. Chloro form has the formula CC1,H and hence evidently contain- the group CC],. The formation, by a simple reaction, of a compound containing the group CCI,. out of tri-chloro-acetic acid, indi cates that this acid itself must contain the group CC1,—a view fully confirmed by other reactions. And since the molecule of tri-chloro
acetic acid, C,HC1,0„. contains three chlorine atoms altogether, it is evident that tri-chloro acetic acid contains no chlorine hut what is com bined in it- group.
(3) When tri-chloro-acetic acid is treated with nascent hydrogen. all of its chlorine is replaced by hydrogen. and acetic acid is re-obtained. Since tri-chloro-acetic acid was just shown to contain no chlorine outside its CC1, group, it is evident that the substitution of hydrogen for chlorine trust result in the formation of the group CH,. The resulting compound, i.e. acetic acid, must therefore contain a methyl group CH,.
(4) From the above it is clear that acetic acid contains the radicles OH and CH,. Subtraction of thee from the entire molecule, leaves the group CO. which is evidently the third and la-t group contained in acetic acid.
We may, therefore. assign to acetic acid the ra tional, or eonstitutional, formula CH,C0.01-1. And, remembering that according to the structu ral hypothesis a carbon atom is quadrivalent, and two or more carbon atoms can be linked to each other in a molecule, we can, further, com bine the three radicles of acetic acid into the following graphical formula : It is easy to see that although this is not the only possible graphical formula corresponding the oretically to the molecular formula of acetic acid, a knowledge of the radicles, de rived by experimental investigation, eliminates the other possibilities and leaves no doubt as to what graphical formula must be accepted as representing the arrangement of the atoms with in a molecule of acetic acid. Numerous facts might be cited in further support of this for mula. For example, in the formula three hydro gens are seen to be linked to carbon immediately, while the fourth hydrogen is linked to carbon through oxygen. The formula thus teaches that one of the four hydrogen atoms must have a different function from the other three hydro g,ens. But this is also the verdict of experiment. In fact, when acetic acid combines with alka lies to form the corresponding acetates, it is found that no matter how great the excess of alkali employed, one-quarter, and only one-quar ter, of the hydrogen of acetic acid can be re placed by metal, which shows that the replace able quarter of the hydrogen has a different function from the other three-quarters. In anal ogous ways, a correct graphical formula, con structed from a given set of facts with the aid of the structural hypothesis, is always found to agree with any other fact dependent on the na ture of the compound, and this agreement per mits the trained chemist to foretell what the principal chemical properties of a compound must be, by examining its graphical formula.