Properties of Enantiomers Optical Activity

Tell whether the two structures in each pair represent enantiomers or two molecules of the Review Problem 5,13 same compound in different orientations. 

The molecules of enantiomers are not superposable and, on this basis alone, we have concluded that enantiomers are different compounds. How are they different Do enantiomers resemble constitutional isomers and diastereomers in having different melting and boiling points The answer is no. Pure enantiomers have identical melting and boiling points. Do pure enantiomers have different indexes of refraction, different solubilities in common solvents, different infrared spectra, and different rates of reaction with achiral reagents The answer to each of these questions is also no. 

Many of these properties (e.g., boiling points, melting points, and solubilities) are dependent on the magnitude of the intermolecular forces operating between the molecules (Section 2.13), and for molecules that are mirror images of each other these forces will be identical. We can see an example of this if we examine Table 5.1, where boiling points of the 2-butanol enantiomers are listed. 

Physical Properties of 2-Butanol and Tartaric Acid Enantiomers 

Mixtures of the enantiomers of a compound have different properties than pure samples of each, however. The data in Table 5.1 illustrate this for tartaric acid. The natural isomer, (+)-tartaric acid, has a melting point of 168-170°C, as does its unnatural enantiomer, (—)-tartaric acid. An equal mixture tartaric acid enantiomers, (+/—)-tartaric acid, has a melting point of 210-212°C, however. 

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Multiple Chiral Centers. The number of stereoisomers increases rapidly with an increase in the number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound has two chiral centers but both centers have the same four substituents attached, the total number of isomers is three rather than four. One isomer of such a compound is not chiral because it is identical with its mirror image it has an internal mirror plane. This is an example of a diaster-eomer. The achiral structure is denoted as a meso compound. Diastereomers have different physical and chemical properties from the optically active enantiomers. Recognition of a plane of symmetry is usually the easiest way to detect a meso compound. The stereoisomers of tartaric acid are examples of compounds with multiple chiral centers (see Fig. 1.14), and one of its isomers is a meso compound. 

Diasteroisomers, also known as geometric isomers, have different relative orientations of their metal-ligand bonds. Enantiomers are stereoisomers whose molecules are nonsuperposable mirror images of each other. Enantiomers have identical chemical and physical properties except for their ability to rotate the plane of polarized light by equal amounts but in opposite directions. A solution of equal parts of an optically active isomer and its enantiomer is known as a racemic solution and has a net rotation of zero. 

Properties of enantiomers Enantiomers share same physical properties, e.g. melting points, boiling points and solubilities. They also have same chemical properties. However, they differ in their activities with plane polarized light, which gives rise to optical isomerism, and also in their pharmacological actions. 

The single most important physical property that differentiates enantiomers is their ability to rotate the plane of plane polarized light. This property is called optical activity and is displayed only by chiral molecules. Thus, stereoisomers which are also chiral are known as optical isomers. Chiral molecules that rotate polarized light in a clockwise fashion are termed dextrorotatory (d) while those that rotate the beam counterclockwise are levorotatory (/). Enantiomers have optical rotations of die same magnitude but of different signs (d or /). 

Circularly polarized (laser) light is widely used not only to study the absorption properties of enantiomers, but also to generate optically active compounds via enantioselective photochemical process. 

It was Pasteur, in the middle of the 19th century, who first recognized the breaking of chiral symmetry in life. By crystallizing optically inactive sodium anmonium racemates, he separated two enantiomers of sodium ammonium tartrates, with opposite optical activities, by means of their asymmetric crystalline shapes [2], Since the activity was observed even in solution, it was concluded that optical activity is due to the molecular asymmetry or chirality, not due to the crystalline symmetry. Because two enantiomers with different chiralities are identical in every chemical and physical property except for optical activity, in 1860 Pasteur stated that artificial products have no molecular asymmetry and continued that the molecular asymmetry of natural organic products establishes the only well-marked line of demarcation that can at present be drawn between the chemistry of dead matter and the chemistry... 

W. H. Pirkle, The nonequivalence of physical properties of enantiomers in Optically active solvents. Differences in nuclear magnetic resonance spectra. I, /. Am. Chem. Soc. 88 (1966), 1837. 

Experimental investigations concerning the differences between sensory properties of linalool enantiomers were performed [61]. Its enantiomers display different spectra of sensory properties. The same authors [61] carried out estimations of the above-mentioned compoimds on human beings. The therapeutical effects of aroma of each optically active linalool were investigated before and after hearing environmental... 

Enantiomers have identical chemical properties except toward optically active reagents. The two lactic acids are not only acids, but acids of exactly the same strength that is, dissolved in water at the same concentration, bo h ionize to exactly the same degree. The two 2-methyl-1-butanoIs not only form the same products—alkenes on treatment with hot sulfuric acid, alkyl bromides on treatment with HBr, esters on treatment with acetic acid —but also form them at exactly ihe same rate. This is quite reasonable, since the atoms undergoing attack in each case are influenced in their reactivity by exactly the same combination of substituents. The reagent approaching either kind of molecule encounters the same environment, except, of course, that one environment is the mirror image of the other. 

The identity of most physical properties of enantiomers has one consequence of great practical significance. They cannot be separated by ordinary methods not by fractional distillation, because their boiling points are identical not by fractional crystallization, because their solubilities in a given solvent are identical (unless the solvent is optically active) not by chromatography, because they are held equally strongly on a given adsorbent (unless it is optically active). The separation of a racemic modification into enantiomers—the resolution of a racemic modification—is therefore a special kind of job, and requires a special kind of approach (Sec. 7.9). 

Stereoisomers can be classified into two types enantiomers and dia-stereomers. Enantiomers (mirror images) have identical physical and chemical properties and therefore are not separated on the conventional reversed-phase stationary phases. Their separation will not be discussed. Diastereomers are isomers which are not mirror images of the parent. They have slightly different physical and chemical properties and can often be separated on conventional stationary phases. There are two classes of diastereomers optically active isomers when the API has two or more stereocenters and non-optically active geometric isomers, such as cis-trans, syn-anti, etc. Stereoisomers of chiral molecules must be included in the peak set. 

The crystals formed through these two mechanisms must be diastereomeric, but it is not possible to distinguish between these two possibilities by measuring physical properties such as optical activity. If resolution occurs, and the monolayer crystal domains are enantiomerically pure, the chiral images observed from the racemic material must be identical to those obtained from one or the other of the pure enantiomers. Over 100 images were analyzed for each material [enantiomerically pure (R)-... 

Organic compounds which possess a chiral center, i.e., a carbon atom with four different ligands, have the lowest possible symmetry [point group Cl (1)]. Two isomers with different absolute configurations, R or S, can occur for a molecule with one chiral center. The pure chiral compound with R-configuration is the enantiomer of the pure compound with -configuration, and vice versa. Enantiomers possess different signs of the optical activity, but their phase transition temperatures and other physical properties are equal. However, the 1 1 mixture of two enantiomers, the racemic mixture, is nonchiral. Its optical activity is zero, and its transition temperatures can deviate from the pure chiral compounds. 

Most of the physical properties of enantiomers are the same. A major exception is their interaction with plane-polarized light One enantiomer will rotate the polarization plane clockwise (dextrorotatory), the other counterclockwise (levorotatory). This phenomenon is called optical activity. The extent of the rotation is measmed in degrees and is expressed by the specific rotation, [a]. Racemates and meso compounds show zero rotation. The enantiomer excess or optical purity of an unequal mixture of enantiomers is given by... 

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