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C4H606
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C4H606 , and See also:quinine-tartrate to both.
With non-electrolytes relations are less evident. One See also:general observation is that non-saturation, especially cyclic structure, augments rotatory See also:power. The saturated compounds, See also:hydrocarbons, See also:alcohols, See also:ethers, See also:amines and acids rarely show specific rotations higher than 1 o°, and some of them, as mannite, See also:CH2OH(CHOH)4CH2OH, for instance, show such small values that only a more thorough investigation, due to the theoretical See also:probability of rotatory See also:powers in See also:asymmetric natural products, has detected the See also:optical activity.
Unsaturated compounds generally show larger rotative powers; amyl See also:alcohol with -5° produces an aldehyde with 15°; succinic (diamyl) See also:ether with 9° produces fumaric ether with 15°, &c. Cyclic configuration especially leads to the highest values known: the lactic See also: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 See also:water) with 202°, whereas the See also:original acid only shows a small rotation.
A second conception, which connects rotation with configuration in non-electrolytes, is due to See also: The See also:chief difficulty in application is to point out that property which is here dominating. It has been supposed to be See also: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 See also:series of glyceric HO CH2OH ethers >C< , augmenting the heaviest See also:group, .0O2R, first H CO2R augments the specific rotation, which then passes through a minimum (the theoretical limit being zero) : Ether of methyl, See also: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 See also:formula C6H5HC(OC2H30) (CO2CH3) the third and See also:fourth groups are of equal weight. It is in this way especially that other properties might be tested, such as See also:volume or See also:density, and perhaps qualities related to See also:light, such as refractive power and the See also:dielectric See also:constant. At-tempts to connect the rotatory power of a See also:compound with more asymmetric carbons to the See also: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 See also: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 See also:ethylene, See also:H2C:See also: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 See also: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 See also:case, fumaric and maleic acids, H•C•CO2H H•C•CO2H HO2C•C•H H•C:•CO2H, forming the See also:oldest and one of the most simple examples; the simplest is a-chlorpropylene (See also:H3C)HC :CCIH. The nature of this stereo-See also:isomerism is quite different from that in See also:antipodes. There is no enantiomorphism in the supposed See also:con-figurations, and so no. rotatory power, &c., in the corresponding compounds, which, on the other See also:hand, show See also:differences far deeper than antipodes do, having different melting points, solubility, See also:heat of formation, chemical properties, &c., behaving in these as See also:ordinary isomers. These isomers, having some relation to those in cyclic compounds, may be also denoted as cis-(maleic) and trans-(fumaric) forms, a See also:close See also:analogy existing indeed in those See also: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-See also:form: When When two See also:double carbon linkings are See also:present, as in H2C:C:CH2, the four See also:hydrogen atoms form the summits of a See also: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. See also:Perkin and W. J. See also:Pope in 1908, and resolved into its components by fractional See also:crystallization of its See also:brucine See also: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. See also: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 See also:wave-length of the light in question, D being that of the See also:sodium See also: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, See also:relating to rotatory power, is that for electrolytes, i.e. salts in aqueous See also: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 — See also:Cor zero in See also:total. This supposition is in so far related to that of Crum Brown and Guye that it admits that the smallest conceivable See also:change, i.e. stereo-isomeric change, in one group does not See also: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 See also: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 See also:stable, with a pronounced tendency to change over into fumaric acid ; this goes hand in hand, according to a general See also:rule, with smaller heat of formation,- See also:lower melting point and increased solubility. In the cyclic compounds analogous phenomena occur. The formation of See also: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).,See also:CH3, H02C•CH2•CHOH•(CH2)°-1CHs,H02C•(CH2)s.CHOH(CH2)°-2CHs,&c., the y-compounds most easily form a See also:lac-See also:tone, though in the a-series carboxyl and hydroxyl run nearer. The See also:tetrahedral arrangement, how-ever, as shown in fig. 6, explains that A, one of the groups attached to the carbon See also:atom Cl, is fairly near C6, one of the Froups attached to the carbon atom C, (the See also:angle A being 35°) ; A would correspond to the hydroxyl forming See also:part of carboxyl around Cl; C6 to the hydroxyl linked with the carbon atom in the 7-position. A third See also:consideration on analogous ground is that of " steric hindrance." It was introduced by See also:Victor See also: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 See also:access to the latter. This See also: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 See also:acetone at loo°) takes See also: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 See also: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 See also: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 See also:sulphur, See also:selenium, See also:tin and See also:silicon, when these elements behave as quadrivalent. S. See also:Smiles (Journ. Chem. Soc., 1900, 77, pp. 1072, 1174; 1905, 87, p. 450) split up such derivatives of methylethyl-thetine as See also: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. See also:Neville (Journ. Chem. Soc., 1902, 18, p. Additional information and CommentsThere are no comments yet for this article.
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