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STEREOCHEMISTRY (Gr. umEptc, solid, a...

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Originally appearing in Volume V25, Page 892 of the 1911 Encyclopedia Britannica.
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STEREOCHEMISTRY (Gr. umEptc, solid, and chemistry), a branch of chemistry which considers the spatial arrangement of the atoms composing a molecule (see STEREO-ISOMERISM). STEREO-ISOMERISM, or STEREOMERISM, a term introduced by Victor Meyer (by way of his denomination stereo-chemistry for " chemistry in space ") to denote those cases of isomerism, i.e. the difference of properties accompanying identity of molecular formulae, where we are forced to admit the same atomic linking and can only ascribe the existing difference to the different relative position of atoms in the molecule. Historical.—Considerations concerning the relative position of atoms have been traced back as far as Swedenborg (1721); in more recent times the first proposal in this direction seems due to E. Paterno (1869), followed by Auguste Rosenstiehl and by Alexis Gaudin (1873). The step made by J. A. Le Bel and J. H. van't Hoff (1874) brought considerations of this kind in the reach of experimental test, and so led to " stereo-chemistry." The work of Louis Pasteur on molecular asymmetry in tartaric acid (186o) touched stereo-chemistry so nearly that, had structural chemistry been sufficiently developed then, stereo-chemistry might have originated fourteen years earlier; it happened, however, that Wislicenus's investigation of lacticacids (1868) immediately stimulated Van't Hoff's views. The fundamental conceptions, of Le Bel and Van't Hoff differ in that the former are based on Pasteur's notions of molecular asymmetry, the latter on structural chemistry, especially as developed by August Kekule for quadrivalent carbon. Both seem to lead to the same conclusions as to stereo-isomerism, but the latter has the advantage of allowing a more detailed insight, whereas the former, which is free from hypothetical conceptions, is of absolute reliability. As our knowledge of stereo-isomerism originated in the chemistry of carbon compounds and found the largest development there, this part will be treated first. Stereo-isomerism in Carbon Compounds. 1. The Asymmetric Carbon Atom.—Though stereo-chemistry is based on the notion of atoms, there is not the least danger that it may break down when newer notions about those atoms are introduced. Even admitting that they are of a compound nature, i.e. built up from smaller electrical particles or anything else and able to split up under given conditions, their average lapse of existence is long enough to consider them as reliable building-stones of the molecule, though these building-stones may give way now and then, as our best ordinary ones by the action of an earthquake. Another thing which stereo-chemistry abstracts beforehand is the movement of atoms, which is generally accepted to exist, but becoming less as the temperature sinks and disappearing at absolute zero. And so the following symbols, representing atoms in a fixed position, may correspond to these last circumstances, whereas at ordinary temperatures atoms may vibrate, for instance, with these fixed positions as centres. The first development from structural to stereo-chemistry was to consider the relative position of atoms in methane, CH4. Structural chemistry had proved that the four atoms of hydrogen were linked H to carbon and not to each other, thus C< H and not, for example H H—H•C , but how the four were grouped remained to decide. H The decision is derived as follows: If the four hydrogen atoms are supposed to be in a plane on one side of the carbon atom as above, two methylchlorides CHsCl should be possible, viz.: C/ CI H H Such isomeric compounds have never been found, but they appear as soon as the four atoms (or groups of atoms) to which carbon is combined are different, for example in CHFC1Br, fiuorchlorbrommethane. Then and only then two isomeric compounds have been regularly observed, and the sole notion about relative position of atoms in methane which explains this fact is that the four groups combined with carbon are placed at the summits of a tetrahedron whose centre is formed by carbon. The two possibilities are then represented by : These groupings have the character of enantiomorphism, i.e. they are non-identical mirror images. If any of the two differences in the summits is given up, for example, F substituted by Cl with the formation of CHC12Br, the enantiomorphism disappears. The isomerism corresponding to this difference in relative position is the simplest case of stereo-isomerism. The carbon atom in the special condition described, linked to four different atoms or groups, is denominated " asymmetric carbon," and will be denoted in the following formulae as C. Stereo-isomerism exists in tartaric acid, HO2C•CH(OH)•CH(OH)•CO2H (studied by Pasteur), in the lactic acid, CH3.CH•OH•CO2H (studied by Wislicenus), while the simplest case at present known is the chlorobromofluoracetic acid, C•Cl•Br•F•CO2H, obtained by Schwartz. This stereo-isomerism, due to the presence of asymmetric carbon, is of a characteristic kind, which is in perfect accordance with the theory of its origin, being the most complete identity combined with the difference that exists between the left and right hand. All the properties which r and H CCl -H H H cannot differ in this last sense are identical, viz.: melting and boiling point, specific gravity, &c. But the crystalline form, which may show enantiomorphism, indeed shows this difference in the isomers in question; and especially the behaviour (in the amorphous state) towards polarized light differs in the sense that the plane of polarization is turned to the left by the one isomer, and exactly as much to the right by the other, so that they may be termed " optical antipodes." All these differences disappear with the asymmetric carbon, and the succinic acid, HO2C•CH2•CH2•CO2H, from tartaric acid is optically inactive and shows no stereo-isomerism. 2. Compounds with more than one Asymmetric Carbon Atom. Stereo-isomerism and the space relation of atoms in compounds with higher asymmetry can best be developed by aid of graphic representa- tions, founded on the notion of space relations in ethane, H3C.CH3. A consequence of the tetrahedral grouping in methane is the con-figuration given in fig. 3, where the six hydrogen atoms are substituted by six atoms or groups Ri,...R6. The second (above) carbon atom is sup-posed to be at the top of the lower tetrahedron, and vice versa. Each other position, obtained by turning R1R2R3 around the •C—C• axis, is also possible, but since no isomerism due to this difference of relative position, which might already show itself in ethane, has been observed, we may admit that one of the positions obtained by the above rotation is the stable one, and fig. 3 may represent it. For simplicity's sake this figure may be projected on a plane by moving R3 and R6 respectively upward and downward, with R1R2 and R,R6 as axes, which leads to the first of the four configurations representing the stereo-isomers possible in the above case. They differ in the two possible spatial arrangements of R1R2R3 and R4R6R6: R3 R3 R3 R3 I I I R1—R2 R2 —R1 RI—C—R2 R2—C—R, { I I R,—C—R6 R,—C—R6 R5—C—R4 1 I { R6 R6 RI6 R6 As one asymmetric carbon introduces two stereo-isomers and two introduce four, n asymmetric carbon atoms will lead to 2" isomers. They are grouped in pairs presenting enantiomorphic figures in space, as do the first and the last of the above symbols, which correspond to the character of optical antipodes, whereas the first and second correspond to greater differences in melting points, &c. A well-studied example is offered by the dibromides of cinnamic acid, C6H5.CHBr•CHBr•CO2H. They have been obtained by Liebermann in two antipodes melting at 92°, and two other antipodes, differing in optical rotation from the first, and melting at 195 A simplification is introduced when the structural formula shows symmetry, as is the case in RIR2R3C•CR3R2RI. The four above-mentioned symbols then are reduced to three: R3 R3 R3 R1--R2 R1—C—R2 R2— c—R1 R2—C—R1 R1—C—R2 R1—C—R2 I I I R3 R3 RIa of which the first and last show the enantiomorphism corresponding to the character of optical antipodes, while the second shows symmetry and corresponds to an inactive type. A well-studied example is offered here by tartaric acid: the two antipodes, often denoted as d and 1, have been found, viz. in the ordinary dextrogyre form and the laevogyre form, prepared by Pasteur from racemic acid, while the third corresponds to mesotartaric acid; such internally compensated compounds are generally termed " meso. 3. Cyclic Compounds:-Three or more carbon atoms may link together so as to produce ring systems such as R1CR2 R3R4C—CR6R6. It is in these cases that the principle of the asymmetric carbon, which in the above case leads to 23=8 stereo-isomers, is easily applied by means of graphical representations in a plane, derived from the space relation shown in fig. 4. The six groups, R1 . . . R6, are either under HO2C•HC—CH•CO2H for which three formulae can be deduced: — CO2H CO2H CO2H H H H H the first, where the carboxyl groups •CO2H lie on the same side of the carbon ring, called, as Von Baeyer proposed, the cis-form, the others trans-forms. The trans-forms show enantiomorphism and correspond to optical antipodes, whereas the first symbol may be considered as corresponding to mesotartaric acid, symmetrical in configuration and inactive; this third stereo-isomer has also been met with. Special attention has been given to those ring systems of the general form:— R1\ /R3—R3 \ /R2 R2' "R,—R37 ~R1 This trans-form corresponds to a cis-form, where both R2 and R1 are on the same side of the plane containing the ring. These latter are enantiomorphic in the ordinary sense of the word, but the particular feature is that the trans-form, though offering nc plane of symmetry, is yet identical with its mirror image, and thus not enantiomorphic and not corresponding to optical antipodes but to the meso-form. There correspondences have been realized by Emil Fischer in derivatives of alanine, H3C•CH(NH2).CO2H, which exists in two antipodes d and 1. Two of these molecules can be combined to a lanyl-alanine : H3C.CH •NH(COC•H•NH2•CH3) •CO2H, which, as containing two asymmetric carbons, may be had in four stereo-isomers dd, Ii, dl and 1d. In their anhydrides H\ CO—NH CH3 H3C' NH—COQ "H we meet the above type, and find that dd and 11 formed the predicted antipodes, while the anhydride of dl and ld is one and the same substance, without any optical activity. Such cases are often termed " pseudo-asymmetric. 4. Isolation of Optical Antipodes.—The optical antipodes are often found as natural products, as is the case with the ordinary or d-tartaric acid; generally only one of the two forms appears, the second form (and, more generally both forms) being obtained synthetically. This is a problem of particular difficulty, since the artificial production of a compound with asymmetric carbon, from another which has no asymmetric carbon, always produces the two antipodes in equal quantity, and these antipodes, by their identity in most properties, e.g. melting and boiling point, solubility, and also on account of their analogous chemical behaviour, cannot be separated by customary methods, the application of which is rendered still more difficult by the formation of a so-called racemic compound. The method called " spontaneous separation " was first observed by Pasteur with racemic acid, which in its double sodium and ammonium salt crystallized from its aqueous solution in two enantiomorphic forms, which could be separated on examination. One of the two proved to be the ordinary sodium-ammonium-tartrate, the other its laevogyre antipode; thus l-tartaric acid was discovered, and racemic acid proved to be a combination of d- and l-tartaric acid. The further examination of this particular transformation showed that it had a definite temperature limit. Only below 27° is Pasteur's observation corroborated, while above 27° a racemate appears; these changes are due to a chemical action taking place at the given temperature between the solid salts 2C4O6H,NaNHe4H2O (C4O6H4NaNH4)2.2H2O+6H20, one molecule of the d- and one of the l-tartrate forming above 27°, the racemate with loss of water, while under 27° the opposite change occurs. This temperature limit, generally called transition-point, was discovered by Van't Hoff and Van Deventer. It is the limit where the possibility of spontaneous separation begins, and is relatively rare, so that this way of separation is an exceptional one, most antipodes forming a racemic compound stable at all temperatures that come into question. The use of optically active compounds in separating antipodes or above the plane in which the carbon ring is supposed to be situated, and this may be indicated by the following symbol: Rd ga where the carbon atoms are supposed half-way between R1 and R2, R3 and R,, R6 and R6. One of the most simple examples is offered by the trimethylenedicarboxylic acids CH2 Rd FIG. 4. is of the greatest value. The general principle is that the compounds which the d- and 1-form give with a different active compound, for instance d producing dd and ld, are by no means antipodes and so exhibit the ordinary differences, e.g. in solubility, which allow separation. It was in this way that Pasteur split up racemic acid by cinchonine. This method has since been applied to the most various acids; bases may be split in an analogous was ; artificial conine was separated by Ladenburg by means of d-tartaric acid, and one of these antipodes proved to be identical with natural conine. Aldehydes and ketones on the other hand may be split up by their combinations with an active hydrazine, &c., and so this method is by far the most fruitful. The formation of a racemic compound built up from dd and ld has also been observed in the so-called partial racemate. An example is the racemate of strychnine. It is in this case also that the transition-point forms the limit of possible separation, determined by Ladenburg and G. Doctor to be 30°. Such partial racemic combination however occurs only in exceptional cases, else it would have invalidated this method, as it did spontaneous separation. A different way of using active compounds in producing antipodes consists in the so-called asymmetric synthesis. The method consists in the introduction of an active complex before that of the asymmetric carbon; both stereo-isomers need not then form in the same quantity. W. Marckwald and A. McKenzie, who chiefly worked out this method, found, for example, that the salt of methylethylmalonic acid, C(CH3) (C2H5) (CO2H)2, with the active brucine forms on heating the corresponding salt of d- and 1-methylethylacetic acid C(CH3) (C2H5)H(CO2H), with the 1-antipode in slight excess. 5. Configuration of Stereo-isomers.—The conception of asymmetric carbon not only opens the possibility of determining when and how many stereo-isomers are to be expected, but also allows a deeper insight into the relative position of atoms in each of them. The chief indication here lies in the configuration of the meso-type, already given for mesotartaric acid; the corresponding alcohol, the natural sugar erythrite, which produces this acid by oxidation, consequently corresponds to CH2OH H—LOH H---OH
End of Article: STEREOCHEMISTRY (Gr. umEptc, solid, and chemistry)
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