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Originally appearing in Volume V25, Page 894 of the 1911 Encyclopedia Britannica.
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C4H606, and quinine-tartrate to both. With non-electrolytes relations are less evident. One general observation is that non-saturation, especially cyclic structure, augments rotatory power. The saturated compounds, hydrocarbons, alcohols, ethers, amines and acids rarely show specific rotations higher than 1 o°, and some of them, as mannite, CH2OH(CHOH)4CH2OH, for instance, show such small values that only a more thorough investigation, due to the theoretical probability of rotatory powers in asymmetric natural products, has detected the optical activity. Unsaturated compounds generally show larger rotative powers; amyl alcohol with -5° produces an aldehyde with 15°; succinic (diamyl) ether with 9° produces fumaric ether with 15°, &c. Cyclic configuration especially leads to the highest values known: the lactic acid with 3° leads to a lactone with -86°, H3C•CH•000 I 000.HCI •CH3; mannosaccharic acid, HO2C(CHOH)4CO2H, to a dilactone (with two rings, formed by the loss of two molecules of water) with 202°, whereas the original acid only shows a small rotation. A second conception, which connects rotation with configuration in non-electrolytes, is due to Alexander Crum Brown and P. A. Guye. It starts from the simple assumption that, as rotatory power is due to the difference of the four groups around the asymmetric carbon, so its amount may correspond to the amount in this. So, generally speaking, take some property, denoted by K1, K4 respectively, a function: (K1—K2) (K1—K3) (K1—K4) (K2—K2) (K2—K4) (K3—K4) would express what is wanted. It becomes zero when two groups are equal; it changes its sign, retaining its value, when Kl is inter-changed with K2, &c. The chief difficulty in application is to point out that property which is here dominating. It has been supposed to be weight, and then the above expression divided by (Kid-K2+K3+K4)6 might be proportional 'to specific rotation. This explains, for instance, that in the homologous series of glyceric HO CH2OH ethers >C< , augmenting the heaviest group, .0O2R, first H CO2R augments the specific rotation, which then passes through a minimum (the theoretical limit being zero) : Ether of methyl, ethyl, propyl, butyl, hexyl, octyl, [4]3--4.8°, -9.2°, -12.9°, -13.2°,—11.3°,—Io.2°. But the serious objection is met that groups of equal weight and different structure often allow considerable rotatory power as in methyl acetylamygdalate, with -146°, though in the formula C6H5HC(OC2H30) (CO2CH3) the third and fourth groups are of equal weight. It is in this way especially that other properties might be tested, such as volume or density, and perhaps qualities related to light, such as refractive power and the dielectric constant. At-tempts to connect the rotatory power of a compound with more asymmetric carbons to the action of each of these separately, i.e. by the so-called optical superposition have not been very successful. In the four stereo-isomeric acids CO2H(CHOH)3CH2OH of the following configurations CO2H CO2H CO2H CO2H HCOH HCOH HOCH HOCH 7. Doubly-Linked Carbon Atoms.—When carbon atoms are doubly linked, as in derivatives of ethylene, H2C:CH2, the two tetrahedra representing the four groups around each carbon may be supposed to have two summits combined, as was supposed with one in simple linking. Fig. 5 represents this supposition, from which follows that the six atoms in question are situated in a plane and may be represented by a plane figure: R1•C•R2 R3•C•R4. The chief consequence is that as soon as the two atoms or groups attached to each carbon are different, two stereo-isomers may be looked for: Rl•C•R2 R1•C•R2 II II R1.C•R2 R2•C•Rl. Such has been found to be the case, fumaric and maleic acids, H•C•CO2H H•C•CO2H HO2C•C•H H•C:•CO2H, forming the oldest and one of the most simple examples; the simplest is a-chlorpropylene (H3C)HC :CCIH. The nature of this stereo-isomerism is quite different from that in antipodes. There is no enantiomorphism in the supposed con-figurations, and so no. rotatory power, &c., in the corresponding compounds, which, on the other hand, show differences far deeper than antipodes do, having different melting points, solubility, heat of formation, chemical properties, &c., behaving in these as ordinary isomers. These isomers, having some relation to those in cyclic compounds, may be also denoted as cis-(maleic) and trans-(fumaric) forms, a close analogy existing indeed in those ring systems of which the simplest type is: this has been realized in the 1, 3-tetramethylene dicarboxylic acids, which exist in a trans- and cis-form: When When two double carbon linkings are present, as in H2C:C:CH2, the four hydrogen atoms form the summits of a tetrahedron according to the development in fig. 4; and consequently the introduction of different groups may bring enantiomorphism and optical antipodes. This has been realized in the compound I-methyl-cyclo-hexylidene-4-acetic acid (formula I.), first prepared by W. H. Perkin and W. J. Pope in 1908, and resolved into its components by fractional crystallization of its brucine salt by Perkin, Pope and Wallach. The substance resolved by W. Marckwald and R. Meth in 1906, which was regarded as this acid, was really the isomeric I-methyl-A3-cyclo-hexene-4-acetic acid (formula II.), which contains asymmetric carbon atoms (see Journ. Chem. Soc., 1909, 95, p. 1791; cf. ibid., 1910, 97, p. 486). H3C CH2•CH2\ H H3C C/ H \CH2•CH2~~ CO2H, H I. II. 8. Numerical Value of Optical Rotation.—To express the value of optical rotation either specific or molecular rotation may be chosen, the first being the deviation caused by a layer of 1 decimetre in length when the substance in question is supposed to be present with specific gravity 1, the latter is this value multiplied by one-hundredth of the molecular weight. Specific rotation is indicated by [a]D, where the suffix indicates the wave-length of the light in question, D being that of the sodium line, and t the temperature; [M]ID is the corresponding value of molecular rotation. Both values vary with the solvent used, and probably are most adapted to solve problems touching relations of rotatory power and configuration, when they apply to extreme dilution in the same liquid. One of the most general rules, relating to rotatory power, is that for electrolytes, i.e. salts in aqueous solution, viz. the limiting rotation in dilute solution only depends on the active radicle. Oudemans found that for such active bases as quinine in its salts with hydro- HOCH HCOH HCIH HOCH HOCH HCOH HCOH HOCH H2C0H HCCOH H2COH H2COH l-arabonic d-ribonic d-lyxonic d-xylonic acid, acid, acid, acid. We might suppose the upper asymmetric carbon to produce a rotation •- A or — A, the other B and C. The rotations then wereA—B—C,A+B+C, —A—B+C and — A+B — Cor zero in total. This supposition is in so far related to that of Crum Brown and Guye that it admits that the smallest conceivable change, i.e. stereo-isomeric change, in one group does not influence the rotation caused by the asymmetric carbon attached to it. It has not been tested in this case, but substances as propyl- and isopropyl-glycerate only differ in specific rotation from — 12.9° to CH2•CH %C•CH2•COZH CH2•CH/ -11.8°, and might prove identical in the same solvent ; the sharpest test might be afforded by propylisopropylacetic acid. 9. Steric Hindrance.-The difference in the relative positions of atoms not only explains the different behaviour of optical antipodes, as has been indicated, but also gives some indication where no optical activity is concerned. In the stereo-isomerism of ethylene compounds, taking maleic and fumaric acid as examples, space relations chiefly indicate that in one of the two the carboxyl groups CO2H are nearer. Such seems indeed to characterize maleic acid. It easily gives an an- HC —CO hydride of the cyclic formula 11 >0 and, inversely, when cyclic HC—CO compounds such as benzene are broken down by oxidizing agents, it is maleic and not fumaric acid that appears. On the other hand the presence of the two negative carboxyls makes maleic acid the stronger acid but less stable, with a pronounced tendency to change over into fumaric acid ; this goes hand in hand, according to a general rule, with smaller heat of formation,- lower melting point and increased solubility. In the cyclic compounds analogous phenomena occur. The formation of lactones, i.e. cyclic anhydrides derived from oxy-acids by interaction of hydroxyl and carboxyl, presents one of them. In the oxy-acids of the fatty series a particular feature is that from the isomers, denoted as a, $ and y, &c. HO2C•CHOH(CH2).,CH3, H02C•CH2•CHOH•(CH2)°-1CHs,H02C•(CH2)s.CHOH(CH2)°-2CHs,&c., the y-compounds most easily form a lac-tone, though in the a-series carboxyl and hydroxyl run nearer. The tetrahedral arrangement, how-ever, as shown in fig. 6, explains that A, one of the groups attached to the carbon atom Cl, is fairly near C6, one of the Froups attached to the carbon atom C, (the angle A being 35°) ; A would correspond to the hydroxyl forming part of carboxyl around Cl; C6 to the hydroxyl linked with the carbon atom in the 7-position. A third consideration on analogous ground is that of " steric hindrance." It was introduced by Victor Meyer's acid, having two substituents (X and 4') in the immediate neighbourhood of carboxyl: CpzH E Y H\ 3H are unable to form ethers in the ordinary way, by treating with methyl alcohol and hydrochloric acid, whereas the isomers having only one of the substituents Y in 4 (X in 6) readily do ; it was suggested that the presence of X and Y near CO2H prevented the access to the latter. This argument has not been completely established, but a large amount of quantitative corroboration has been brought together by N. A. Menschutkin, who has found that in alcohols the more the hydroxyl group is surrounded by substituents (for instance CH3) the slower esterification (with acetic anhydride in acetone at loo°) takes place, the ratio of rates being Methyl alcohol H3C•OH . Ethyl alcohol H3C•CH2.OH Dimethyl carbinol (H3C)2CH.OH Trimethyl carbinol (H3C)3C.OH . Stereo-isomerism in Other Elements. Phenomena analogous to those observed in carbon compounds might also exist in derivatives of other quadrivalent elements; and only the relative stability of carbon-compounds makes every form of isomer, which often is unstable, more easily obtainable in organic chemistry. Nevertheless it has been possible to obtain stereo-isomers with different elements, but, as expected from the above, especially in derivatives containing carbon. Some of them have the character of optical antipodes and are more easily considered from a theoretical point of view; others have not. 1. Optically Active Stereo-isomers.—Most closely related to the phenomena with carbon are those with sulphur, selenium, tin and silicon, when these elements behave as quadrivalent. S. Smiles (Journ. Chem. Soc., 1900, 77, pp. 1072, 1174; 1905, 87, p. 450) split up such derivatives of methylethyl-thetine as C2H6\ ~CH2•CO•CsH5 CH3~ "Br obtained by condensing methylethyl sulphide with w-bromacetophenone, by means of the salt with d-bromocamphosulphonic acid, into optical antipodes. W. J. Pope and A. Neville (Journ. Chem. Soc., 1902, 18, p. 198) succeeded in the same way with a selenium compound C6Hs~ CH2CO2H >Se
End of Article: C4H606

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