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See also:STEREOCHEMISTRY (Gr. umEptc, solid, and See also:chemistry) , a See also:branch of chemistry which considers the spatial arrangement of the atoms composing a See also:molecule (see STEREO-See also:ISOMERISM).
STEREO-ISOMERISM, or STEREOMERISM, a See also:term introduced by See also:Victor See also:Meyer (by way of his See also: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.
See also:Historical.—Considerations concerning the relative position of atoms have been traced back as far as See also:Swedenborg (1721); in more See also:recent times the first proposal in this direction seems due to E. See also:Paterno (1869), followed by Auguste Rosenstiehl and by See also:Alexis Gaudin (1873). The step made by J. A. Le See also:Bel and J. H. See also:van't Hoff (1874) brought considerations of this See also:kind in the reach of experimental test, and so led to " stereo-chemistry." The See also:work of See also: 1. The See also:Asymmetric Carbon See also: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 See also:compound nature, i.e. built up from smaller See also:electrical particles or anything else and able to split up under given conditions, their See also:average See also:lapse of existence is See also:long enough to consider them as reliable See also:building-stones of the molecule, though these building-stones may give way now and then, as our best See also:ordinary ones by the See also:action of an See also:earthquake. Another thing which stereo-chemistry abstracts beforehand is the See also: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, See also:CH4. Structural chemistry had proved that the four atoms of See also: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 See also:plane on one See also: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 See also: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 See also: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 See also:tetrahedron whose centre is formed by carbon. The two possibilities are then represented by : These groupings have the See also:character of enantiomorphism, i.e. they are non-identical See also:mirror images. If any of the two See also: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 See also:case of stereo-isomerism. The carbon atom in the See also:special See also: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)•See also:CO2H (studied by Pasteur), in the lactic acid, See also:CH3.CH•OH•CO2H (studied by Wislicenus), while the simplest case at See also: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 See also:complete identity combined with the difference that exists between the See also:left and right See also: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 See also:form, which may show enantiomorphism, indeed shows this difference in the isomers in question; and especially the behaviour (in the amorphous See also:state) towards polarized See also: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 " See also:optical See also:antipodes." All these differences disappear with the asymmetric carbon, and the succinic acid, HO2C•See also: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, See also:H3C.CH3. A consequence of the See also:tetrahedral grouping in methane is the See also: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 See also:top of the See also:lower tetrahedron, and See also:vice versa. Each other position, obtained by turning R1R2R3 around the •C—C• See also: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 See also:stable one, and fig. 3 may represent it. For simplicity's See also: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—See also: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, See also:C6H5.CHBr•CHBr•CO2H. They have been obtained by See also: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 See also: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 See also:link together so as to produce See also: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 See also:lie on the same side of the carbon ring, called, as Von See also:Baeyer proposed, the cis-form, the others trans-forms. The trans-forms show enantiomorphism and correspond to optical antipodes, whereas the first See also:symbol may be considered as corresponding to mesotartaric acid, symmetrical in configuration and inactive; this third stereo-isomer has also been met with. Special See also:attention has been given to those ring systems of the See also: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 See also:image, and thus not enantiomorphic and not corresponding to optical antipodes but to the meso-form. There correspondences have been realized by Emil See also:Fischer in derivatives of alanine, H3C•CH(See also: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. See also: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 See also: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 See also: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 See also:double See also:sodium and ammonium See also:salt crystallized from its aqueous See also: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 See also: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 See also: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 See also:water, while under 27° the opposite See also:change occurs. This temperature limit, generally called transition-point, was discovered by Van't Hoff and Van See also: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 See also:half-way between R1 and R2, R3 and R,, R6 and R6. One of the most See also: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 See also:conine was separated by Ladenburg by means of d-tartaric acid, and one of these antipodes proved to be identical with natural conine. See also:Aldehydes and See also:ketones on the other hand may be split up by their combinations with an active See also: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 See also:strychnine. It is in this case also that the transition-point forms the limit of possible separation, determined by Ladenburg and G. See also: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 See also: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. See also:McKenzie, who chiefly worked out this method, found, for example, that the salt of methylethylmalonic acid, C(CH3) (C2H5) (CO2H)2, with the active See also:brucine forms on See also: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. Additional information and CommentsThere are no comments yet for this article.
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