Page:The New International Encyclopædia 1st ed. v. 18.djvu/650

STEREO-CHEMISTRY. 556 STEREO-CHEMISTRY. models, why only one acetic acid, CHj(COOH), is possible; why only one mono-chlor-acetie acid, CHj(Cl) (COOH), is possible; why only one di- chlor-aeetic acid, CHCl j( COOH), is possible; wiiy only one trichloracetic acid, CClj(COOH), is possible; whv two optically isomeric ehloro- bronio-acetic kcids, C{H) (CI) (Br) (COOH), are possible: and why two optically isomeric chloro- bromo-fluor-acetic acids, C(C1) (Br) (F) (COOH), are possible. As already pointed out above, optical isomers are invariably found to possess precisely the same chemical and physical properties and to differ only with regard 'to the direction in which they rotate tlie plane of polarized light. The question now suggests itself, How can such iso- mers be separated from each other when mixed? This problem is so much the more important be- cause all reactions by which compounds with asymmetric carbon atoms may be produced from compounds containing no such atoms cause the simultaneous production of both Optical isomers in precisely equal quantities; so that a separa- tion is required whenever optically active com- pounds are to be prepared artificially (in nature either one or the other of a pair of optical isomers is often found isolated ) . Three methods have thus far been found for effecting the sepa- ration. First, it is possible, in many cases, and under certain conditions of temperature, to sepa- rate the isomers mechanically — in those cases, namely, in which the two crystallize from their solutions separately. In all such cases the two isomers are found to form enantiomorphous crys- tals. Below, or sometimes above, the point or interval of temperature at which this takes place, the two isomers usually crystallize together, forming a double compound, the so-called 'ra- cemic' modification of the given compound. The racemic modification is optically inactive be- cause its two components tend to rotate the plane of polarized light to the same extent in opposite directions. The second method of separating optical isomers is applicable only to acids and bases. If a mixture of two optically isomeric acids is treated with an optically active base, two salts result dift'ering more or less con- siderably in solubility, and therefore capable of being separated by fractional crystallization. The result is .similar when a mixture of two optically active isomeric bases is treated vith an optically active acid. Finally, the third method is based on the fact that certain processes of fermentation often destroy one of the optical isomers and leave the other intact. Thus, levo-ghieose may he prepared by subjecting to fermentation its mix- ture with rfr.7'/ro-glucose. the latter alone being afl'ected by the fermentation. The reason of such facts is not yet understood. It has been assumed by some that the living organisms (e.g. Penicil- lium glaucum) causing fermentation are capable of discriminating, by a sort of instinct, between the isomers, and. while feeding on one, reject the other. But the lifeless enzvmes (q.v.) ob- tained from ferments have been sho«Ti to exercise the same action as the ferments themselves: and hence the peculiar action of the latter may be as- sumed to be due to the purely chemical proper- ties of their enzymes. Passing now to the consideration of compounds whose molecules contain more than one asym- metric carbon atom, the most important case to be mentioned is that of tartaric acid, (COOH) C"(H) (OH) •C(H) (OH) (COOH), with two asymmetric carbons in its molecule. By the use of models like those mentioned above, it is easy to convince one's self that three diflerent ar- rangements are possible: (1) a right-handed arrangement of the groups around either of the asymmetric carbons; (2) a left-handed arrange- ment of the groups around either of the asym- metric carbons; (3) a right-handed arrangement around one, and a left-handed arrangement around the other asjTumetric carbon. Corre- sponding to these are the well-known dextro- rotatory, iei;o-rotatoi'j', and inactive modifications of the acid, the last named being inactive because one-half of its molecule rotates the plane of po- larized light to the same extent in one direction as the other half does in the opposite direction. Besides these there is the racemic modification ('racemic acid'), which is a double compound of the dextro-rotatory and levo-rotatory tartaric acids (q.v.). The compounds containing more than two asymmetric carbon atoms include the sugars (q.v.) — an important class of compounds wliose theory could not have been developed, and many of which could hardly have been discovered without the concepts of stereo-chemistry. Geometrical Isomerism. It may be seen from the preceding paragraphs that the funda- mental hypothesis of stereo-chemistry, viz. that the valencies of a carbon atom act symmetrically around it in space, while indispensable for cor- relating optical isomers, also throws additional light on the phenomena of chemical isomerism proper. Take, for example, again the case of methylene chloride, CH^Clj. Were optical isomer- ism unknown and the stereo-chemical hypothesis non-existent, the question as to why only one modification of this compound is possible would be answered by pointing out that the four carbon valencies are identical and that, hydrogen and chlorine atoms being univalent, only one mode of linking the atoms is possible. If it were fur- ther asked, "But how do we know that two methylene chlorides might not exist, in whose molecules the atoms are linked similarly, but arrancicd diflferently ?" — chemists would answer: "Of course, such a state of things is not incon- ceivable: only in all the innumerable cases thus far discovered, the doctrine of the linking of atoms is sufficient, the number of known isomeric compounds never exceeding the number of pos- sible modes of linking; and so we deem it un- necessary to enter into speculations as to the arrangement of atoms in space." From the standpoint of stereo-chemistry the answer is thorough : only one methylene chloride is pos- sible, (1) because the four valencies of carbon are identical ; (2) because only one mode of link- ing the atoms is possible; (3) because only one arrangement of the atoms in space is possible, as may be readily demonstrated by the use of tetrahedron models. It is, therefore, clear that stereo-chemistry not only explains, and hence permits of foreseeing, all possible cases of optical isomerism, but also explains thoroughly why more cases of chemical isomerism than are explained and foretold by the doctrine of the linking of atoms are not pos- sible. But stereochemistry has still another im- portant application. Its fundamental hypothesis applied to the so-called 'unsaturated' compounds