Light rotation of polarized

In principle, any physical property that varies during the course of the reaction can be used to follow the course of the reaction. In practice one chooses methods that use physical properties that are simple exact functions of the system composition. The most useful relationship is that the property is an additive function of the contributions of the different species and that each of these contributions is a linear function of the concentration of the species involved. This physical situation implies that there will be a linear dependence of the property on the extent of reaction. As examples of physical properties that obey this relationship, one may cite electrical conductivity of dilute solutions, optical density, the total pressure of gaseous systems under nearly ideal conditions, and rotation of polarized light. In sufficiently dilute solutions, other physical properties behave in this manner to a fairly good degree of approximation. More complex relationships than the linear one can be utilized but, in such cases, it is all the more imperative that the experimentalist prepare care-... 

FIGURE 16.3 The optically active isomers of [Co(en)3]3+(top) and their directions of rotation of polarized light. 

Pasteur thus made the important deduction that the rotation of polarized light caused by different tartaric acid salt crystals was the property of chiral molecules. The (+)- and ( )-tartaric acids were thought to be related as an object to its mirror image in three dimensions. These tartaric acid salts were dissymmetric and enantiomorphous at the molecular level. It was this dissymmetry that provided the power to rotate the polarized light. 

The rotation of polarized light.—Rotation of polarized light7 is additive for soln. of mixtures of salts which do not form double salts, and there is a marked deviation from the additive law for mixtures known to form double salts. 

Cotton-Mouton effect), NMR chemical shift and coupling constants, the optical rotation of polarized light and correlation coefficients between different properties. Extensions to incorporate long-range interactions have also been elaborated11 and it has even been possible to adapt RIS theory for the description of the dynamics of transitions between rotational isomeric states.12,13... 

Distinction should be made at this time between diastereoisomers and enantiomers. The former are characterized by the presence of at least two closely associated asymmetric centers in the molecular structure, either of which can epimerize. Altogether then there are two pairs of enantiomers for a total of four stereochemically unique individuals. Diastereoisomers have different physical properties and as a result discriminations, and even separations, can be done relatively easily. Enantiomers on the other hand differ in only one physical property, i.e. the direction of rotation of polarized light. Reaction of an enantiomeric racemic mixture with a third chiral species will produce a mixture of diastereomers therefore facilitating their identification or their separation. Early examples of this were the separations done by fractional crystallization of salts produced by a derivatization reaction with, for example, the alkaloid (-)-brucine. Fractional crystallization would never seem to be an effective analytical method yet it was used with some success in a forensic sciences context to confirm the presence of (L)-cocaine by a carefully contrived microcrystalline test. The physical properties... 

Compounds that contain an asymmetric substitutions on a carbon atoms exist in enantiomorphic forms. These differ only in the direction of rotation of polarized light. When the absolute configuration has been identified in the original report, these compounds are identified by (R)- and (S)- prefixes. Sometimes prefixes such as d-, /-, (+)-, and (-)-, which are given in the original report have been used. Very often no identification of the optical isomer is given even when the compound contains an asymmetric carbon atom Then it is assumed that the samples are racemic mixtures. However, none of these distinctions affect the densities of liquids, therefore all the isomers of this type are combined to determined the recommended values. If a compound contains more than one asymmetric carbon atom diasteieomers can exist. In principle, these might have different densities, but no examples have been found. 

To use the rotation of polarized light as a characteristic property of a compound, we must standardize the conditions for measurement. We define a compound s specific rotation [a] as the rotation found using a 10-cm (1-dm) sample cell and a concentration of 1 g/mL. Other cell lengths and concentrations may be used, as long as the observed rotation is divided by the path length of the cell (Z) and the concentration (c). 

Hydrolysis of the d-benzoyl derivative is accomplished as in the case of the 1-compound, and the d-alamne is isolated in the same way. It is very similar in its properties to the /-compound except that the slight rotation of polarized light is in the opposite direction. The synthetic d-alanine is identical in every respect with the d-alamne isolated from the hydrolytic products of silk and other proteins. 

This rotation of polarized light is an important physical property that is used in chiral detection. If a molecule has two asymmetric (chiral) carbon atoms in its structure, then it is possible to have two pairs of optically active molecules and the pairs are said to be diastereoisomers as in the second diagram. If the two asymmetric carbons have identical substitution, again there will be two pairs of diastereoisomers formed, but one pair will possess a plane of symmetry and thus be optically inactive. The inactive pair are called me so diastereoisomers. 

The rotation of polarized light can be in the clockwise or counterclockwise direction. 

In 1873 Wislicenus made the suggestion that in compounds like lactic acid and the tartaric acids in which isomers have the same structure but differ in physical properties, e.g.y in their rotation of polarized light, the only explanation is that the atoms of the molecules are dijfer-ently arranged in space. Now, in considering this suggestion in con-... 

Since the difl erence in optical rotation was observed in solution Pasteur concluded that it was characteristic, not of the crystals, but of the molecules. He proposed that, like the two sets of crystals themselves, the molecules making up the crystals were mirror images of each other. He was proposing the existence of isomers whose structures differ only in being mirror images of each other, 4nd whose properties differ only in the direction of rotation of polarized light. 

We know (Sec. 7.3) that when optically inactive reactants form a chiral compound, the product is the racemic modification. We know that the enantiomers making up a racemic modification have identical physical properties (except for direction of rotation of polarized light), and hence cannot be separated by the usual methods of fractional distillation or fractional crystallization. Yet throughout this book are frequent references to experiments carried out using-... 

Such pairs of protons are called enantiotopic protons. The environments of these two protons are mirror images of each other these protons are equivalent, and we see one nmr signal for the pair. (Like any other physical property—except rotation of polarized light—the nmr spectrum does not distinguish between mirror images.)... 

By chmdcal isomer, le Bel means isomers differing in characteristics other than their direction of rotation of polarized light. Those differing only in their effect on polarized light were often spoken of as physical isomers.—O.T.B,]... 

When a compound undergoes a reaction and maintains its molecular structure and thus its identity, the reaction is called a physical reaction. Melting, evaporation, dissolution, and rotation of polarized light are some examples of physical reactions. When a compound undergoes a reaction and changes its molecular structure to form a new compound, the reaction is called a chemical reaction. Combustion, metathesis, and redox are examples of chemical reactions. Chemical reactions can be represented by a chemical equation with the molecular formulae of the reactants on the left and the products on the right. 

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