Specific rotation compounds

Specific Rotation. Optical rotation is caused by individual molecules of the optically active compound. The amount of rotation depends upon how many molecules the light beam encounters in passing through the tube. When allowances are made for the length of the tube that contains the sample and the sample concentration, it is found that the amount of rotation, as well as its direction, is a characteristic of each individual optically active compound. 

Specific rotation is the number of degrees of rotation observed if a 1-dm tube is used and the compound being examined is present to the extent of 1 g per 100 mL. The density for a pure liquid replaces the solution concentration. 

The molecular ellipticity is analogous to specific rotation in that two enantiomers have exactly opposite values of 0 at every wavelength. Two enantiomers will thus show CD spectra having opposite signs. A compound with several absorption bands may show both... 

For the identification of limonene, one of the most useful compounds is the crystalline tetrabromide, Cj(,HjgBr. This body is best prepared as follows the fraction of the oil containing much limonene is mixed with four times its volume of glacial acetic acid, and the mixture cooled in ice. Bromine is then added, drop by drop, so long as it becomes decolorised at once. The mixture is then allowed to stand until crystals separate. These are filtered off, pressed between porous paper, and recrystallised from acetic ether. Limonene tetrabromide melts at 104 5° and is optically active, its specific rotation being + 73 3°. The inactive, or dipeutene, tetrabromide melts at 124° to 125°. In the preparation of the tetrabromide traces of moisture are advisable, as the use of absolutely anhydrous material renders the compound very diflftcult to crystallise. 

Caryophyllene nitrosite is an interesting compound. It has the formula CJ5H24N2O3, and was first produced by Schreiner and Kremers. It is form by treating a mixture of equal volumes of the sesquiterpene and petroleum ether with a concentrated solution of sodium nitrite and glacial acetic acid. It crystallises in fine blue needles when re-crvstallised from alcohol it melts at 115° and has a specific rotation -h 103°. 

Baker and Smith have isolated an alcohol of the formula C,oH,gO from the cajuput oil, distilled from the leaves of Melaleuca uncinata. The alcohol, which is probably an open-chain compound, forms snow-white crystals, melting at 72 5°, and having a specific rotation + Sfi fifi . 

To express optical rotations in a meaningful way so that comparisons can be made, we have to choose standard conditions. The specific rotation, [a ]D, of a compound is defined as the observed rotation when light of 589.6 nanometer (nm 1 nm = 10-9 m) wavelength is used with a sample pathlength / of 1 decimeter (dm 1 dm = 10 cm) and a sample concentration C of 1 g/mL. (Light of 589.6 nm, the so-called sodium d line, is the yellow light emitted from common sodium lamps.)... 

Specific rotation, [a]D (Section 9.3) The optical rotation of a chiral compound under standard conditions. 

Coal, structure of, 517 Coal tar, compounds from, 517 Cocaine, specific rotation of, 296 structure of. 64, 916 structure proof of. 875 synthesis of, 915... 

One of the terms for describing enantiomer composition is optical purity. It refers to the ratio of observed specific rotation to the maximum or absolute specific rotation of a pure enantiomer sample. For any compound for which the optical rotation of its pure enantiomer is known, the ee value may be determined directly from the observed optical rotation. 

The wavelength dependence of specific rotation and/or molecular ellipticity is called the Cotton effect. The Cotton effect can provide a wealth of information on relative or absolute configurations. The sign of the Cotton effect reflects the stereochemistry of the environment of the chromophore. By comparing the Cotton effect of a compound of known absolute configuration with that of a structurally similar compound, it is possible to deduce the absolute configuration or conformation of the latter. 

As this synthesis started from an achiral starting material, compound 199 must be resolved to secure enantiomerically pure intermediates for the synthesis of taxol. Treatment of (+ )-diol 199 with excess ( lA)-( )-camphanic chloride in methylene chloride in the presence of Et3N forms two diastereomeric monoesters for chromatographic separation. Enantiomerically pure diol 199 can be regenerated from the ester in 90% yield with a specific rotation of +187 (c = 0.5, CHC13). 

The slightly impure product is recrystallized from petroleum ether (b.p. 30-60°) containing 0.5 per cent of anhydrous ether in a continuous extractor, using 100 cc. of solvent per 6-7 g. of compound. One crystallization is generally sufficient to give a pure product. The specific rotation of pure tetiamethyW-glucose prepared by this method is approximately [a] d + 81.3° (Note 4). 

As in the case of other chiral compounds, the optical and enantiomeric purity of chiral organosulfur compounds can be determined by various methods (241). The simplest and most common method for the determination of optical purity of a mixture of enantiomers is based on polarimetric measurements. However, this method requires a knowledge of the specific rotation of the pure enantiomer. In the... 

The polarimetric method, in combination with the results of chemical correlation, made it possible to determine the optical purity of a range of chiral sulftnates (105-107), thiosulfinates (35,105), and sulfinamides (83) with the sulfur atom as a sole center of chirality. These compounds were converted by means of Grignard or alkyl-lithium reagents into sulfoxides of known specific rotations. This approach to the determination of optical purity of chiral sulfinyl compounds has at least two limitations. The first is that it cannot be applied to sterically hindered compounds [e.g., t-butyl /-butanethio-sulfinate 72 does not react with Grignard reagents]. Second, this... 

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