Enantiomers specific rotation

Specific rotation is a physical property of a substance just as melting point boil mg point density and solubility are For example the lactic acid obtained from milk is exclusively a single enantiomer We cite its specific rotation m the form [a]o =+3 8° The temperature m degrees Celsius and the wavelength of light at which the measure ment was made are indicated as superscripts and subscripts respectively... 

Cholesterol when isolated from natural sources is obtained as a single enantiomer The observed rotation a of a 0 3 g sample of cholesterol in 15 ml of chloroform solution contained in a 10 cm polarimeter tube is -0 78° Cal culate the specific rotation of cholesterol... 

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... 

It is convenient to distinguish between enantiomers by prefixing the sign of rotation to the nfflne of the substance. For exanple, we refer to one of the enantiomers of 2-butanol as (-l-)-2-butanol and the other as (—)-2-butanol. Optically pure (-l-)-2-butanol has a specific rotation [a]o of +13.5° optically pure (—)-2-butanol has an exactly opposite specific rotation [a]o of —13.5°. 

Enantiomers must have equal and opposite specific rotations. Diastereomers can have different rotations, with respect to both sign and magnitude. Thus, as Eigure 7.8 shows, the (2/ ,3/ ) and (2.S,3.S) enantiomers (I and II) have specific rotations that aie equal in magnitude but opposite in sign. The (2/, 3S) and (2S,3/i) enantiomers (III and IV) likewise have specific rotations that aie equal to each other but opposite in sign. The magnitudes of rotation of I and II aie different, however, from those of their diastereomers III and IV. 

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. 

Define plane-polarized light, optical rotation, optical activity, asymmetric carbon atom, enantiomers, racemic mixture, polarimeter, and specific rotation. 

For a nonracemic mixture of enantiomers prepared by resolution or asymmetric synthesis, the composition of the mixture was given earlier as percent optical purity (equation 1), an operational term, which is determined by dividing the observed specific rotation (Mobs) of a particular sample of enantiomer with that of the pure enantiomer ( max), both of which were measured under identical conditions. Since at the present, the amount of enantiomers in a mixture is often measured by nonpolarimetric methods, use of the term percent optical purity is obsolete, and in general has been replaced by the term percent enantiomeric excess (ee) (equation 2) introduced in 197163, usually equal to the percent optical purity, [/ ] and [5] representing the relative amounts of the respective enantiomers in the sample. 

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... 

An interesting method for the estimation of optical purity of sulfoxides, which consists of the combination of chemical methods with NMR spectroscopy, was elaborated by Mislow and Raban (241). The optical purity is usually determined by the conversion of a mixture of enantiomers into a mixture of diastereomers, the ratio of which may be easily determined by NMR spectroscopy. In contrast to this, Mislow and Raban used as starting material for the synthesis of enantiomeric sulfoxides a diastereomeric mixture of pinacolyl p-toluenesulfinates 210. The ratio of the starting sulfinates 210 was 60.5 39.5, as evidenced by the H NMR spectrum. Since the Grignard reaction occurs with full stereospecificity, the ratio of enantiomers of the sulfoxide formed is expected to be almost identical to that of 210. This corresponds to a calculated optical purity of the sulfoxide of 20%. In this way the specific rotations of other alkyl or aryl p-tolyl sulfoxides can conveniently be determined. 

Since the early times of stereochemistry, the phenomena related to chirality ( dis-symetrie moleculaire, as originally stated by Pasteur) have been treated or referred to as enantiomericaUy pure compounds. For a long time the measurement of specific rotations has been the only tool to evaluate the enantiomer distribution of an enantioimpure sample hence the expressions optical purity and optical antipodes. The usefulness of chiral assistance (natural products, circularly polarized light, etc.) for the preparation of optically active compounds, by either resolution or asymmetric synthesis, has been recognized by Pasteur, Le Bel, and van t Hoff. The first chiral auxiliaries selected for asymmetric synthesis were alkaloids such as quinine or some terpenes. Natural products with several asymmetric centers are usually enantiopure or close to 100% ee. With the necessity to devise new routes to enantiopure compounds, many simple or complex auxiliaries have been prepared from natural products or from resolved materials. Often the authors tried to get the highest enantiomeric excess values possible for the chiral auxiliaries before using them for asymmetric reactions. When a chiral reagent or catalyst could not be prepared enantiomericaUy pure, the enantiomeric excess (ee) of the product was assumed to be a minimum value or was corrected by the ee of the chiral auxiliary. The experimental data measured by polarimetry or spectroscopic methods are conveniently expressed by enantiomeric excess and enantiomeric... 

A carbon atom with four different groups attached is chiral. A chiral carbon rotates plane-polarized light, light whose waves are all in the same plane, and has an enantiomer (non-superimposable mirror image). Rotation, which may be either to the right (dextrorotatory) or to the left (levorotatory), leads to one optical isomer being d and the other being 1. Specific rotation (represented... 

A meso compound has a specific rotation of polarised light of 0°. A racemate is an equimolar mixture of two enantiomers and its specific rotation is also 0°. 

The optical purity (P. synonymous with optical yield) is defined as the specific rotation ([a]) of an enantiomeric mixture, divided by the specific rotation ([amax], maximum specific rotation) of the pure enantiomer (either enantiomer the sign of the rotation is ignored for convenience)... 

Under identical conditions the specific rotation of an enantiomer has the same magnitude, but the opposite sign, as its antipode. 

The maximum specific rotation [a]mal (absolute specific rotation) of one of the pure enantiomers must be known with certainty and it must be confirmed by an independent method. 

A requirement for determination of optical purity (P) is that the specific rotation of the pure enantiomer, [a]max, is known with certainty. This maximum rotation can be established by calculation, e.g., via competitive reaction methods41,42, or by direct determination employing an enantiomerically pure sample. Enantiomerically pure samples are generally believed to arise from enzymatic reactions performed on biogenic substrates, an assumption which in some instances is incorrect31, or are obtained in most cases by crystallization2. As many direct, nonchiroptical methods are available for determining enantiomeric purities, maximum rotations can also be extrapolated from the specific rotation of a sample of known ee. 

It should be noted that the enantiomeric excess in the product was determined in most cases by measuring the specific rotation of the neat liquid11, although GC methods are now available13. It is known that this rotation value is temperature sensitive ( + 0,18/deg) so that the temperature of measurement must be measured carefully and specified a value for enantiomer-ically pure 3-phenyl-l-butene of [a] 2 +5.92 (neat) is recommended (the / -enantiomer has a negative rotation). 

The specific rotation [a]f is an inherent physical property of an enantiomer which, however, varies with the solvent used, temperature T in °C), and wavelength of light used (A), It is defined as the observed rotation per unit length of light path, per unit concentration (for a solution) or density (for a pure liquid) of the enantiomer thus ... 

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