CARBON COMPOUNDS. 197 CARBON COMPOUNDS. accordingly conceived as consisting of two atomic groups, C;U; and Oil, the constituent atoms of which tend to hold tirmly 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 just single atoms, are called radicles. An example may serve to show how the small- est radicles present in a given compound are de- termined by its cliemical reactions. Let the prob- lem be. To determine the atomic groups in a molecule of acetic acid, C.HjO,. To solve the problem, acetic acid is caused to undergo a series of transformations and the following facts are brought to light. { 1 ) By the action of phosphorus penta-chlo- ride, acetic acid. C^HiO,, is readily transformed into acetyl chloride, whose formula is C2H3OCI. In this reaction an atom of hydrogen and an atom of oxygen are together replaced by an atom of chlorine. We therefore conclude that acetic acid, like alcohol, contains a hydroxyl group, OH. (2) By passing chlorine gas into hot acetic acid exposed to the direct action of sunlight, a compound called trichloroacetic acid is ob- tained. When boiled with water, this compound is split up and chloroform is produced. Chloro- form has the formula CCI3H and hence evidently contains the group CCI3. The formation, by a simple reaction, of a compound containing the group CC'lj, out of tri-chloro-acetie acid, indi- cates that this acid itself must contain the group CCl- — a view fully confirmed by other reactions. And since the molecule of tri-ehloro- acetic acid, C.HCl^O™. contains three chlorine atoms altogether, it is evident that tri-chloro- acetic acid contains no chlorine but what is com- bined in its CCl, group. (3) When trichloroacetic acid is treated with nascent hydrogen, all of its chlorine is replaced by hydrogen, and acetic acid is re-obtained. Since tri-chloro-aeetic acid was just shown to contain no chlorine outside its CCI3 group, it is evident that the substitution of hydrogen for chlorine must' result in the formation of the group C'Hj. The resulting compound, i.e. acetic acid, must therefore contain a methyl group CH3. (4) From the above it is clear that acetic acid contains the radicles OH and CH3. Subtraction of these from the entire molecule, CjH.O;, leaves the group CO, which is evidently the third and last group contained in acetic acid. We may, therefore, assign to acetic acid the ra- iionnl, or constitutional, formula CH3CO.OH. 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: H 0=0 H— C— H ! H Acetic acid 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, C.HiO;, 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 miglit 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 o.xygen. The formula thus teaches that one of the four hydrogen atoms must have a different function from the other three hydro- gens. 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 fimction 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- tiire 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. UxsATLTSATED COMPOUNDS. Thus far we have referred only to compounds in which the valen- cies of the several atoms are completely satisfied, so that the molecules are incapable of taking on any more atoms. Thus, in marsh-gas, CH„ the four valencies of the carbon atom are evidently satisfied by four atoms of hydrogen. In ethane, C;He, three valencies of each carbon atom are evidently satisfied by hydrogen atoms, while the fourth valency of each carbon atom is satisfied by the other carbon atom. Compounds thus con- taining the maximum possible number of atoms of hydrogen, or of other elements, are said to be saturated. JIany other compounds, however, are known, in which this is not the case. Thus, while ethane, C^Hs, contains six hydrogen atoms, the gaseous compound known as ethylene contains only four hydrogen atoms in combination with two atoms of carbon, the formula of ethylene being C,H,. Three different graphical formulas might rep- resent a molecule of ethylene ; viz, H " H H I I I H— C H— C— H— C II I and T H— C H— C— H— C According to the first, two valencies of each carbon atom are satisfied by the other carbon atmn. According to the second, one valency of each carbon atom remains unsatisfied. Accord- ing to the third, the carbon atom is not quadri- valent, but tri-valent. Xow, the last two formu- las must be rejected in view of the following fact: A great deal of ingenuity has been spent, by .some of the most celebrated organic chemists, in efforts to produce a compound of molecular formula OH, and similar compounds, the exist- I H
