Chiral Compounds Are Optically Active

Enantiomers share many of the same properties, including the same boiling points, the same melting points, and the same solubilities. In fact, all the physical properties of enantiomers are the same except those that stem from how groups bonded to the asymmetric center are arranged in space. One property that enantiomers do not share is the way they interact with plane-polarized light. 

Only light oscillating in a single plane can pass through a polarizer. 

In 1815, the physicist Jean-Baptiste Biot discovered that certain naturally occurring organic compounds are able to rotate the plane of polarization of plane-polarized light. He noted that some compounds rotated it clockwise and some rotated it counterclockwise. He proposed that the ability to rotate the plane of polarization of plane-polarized light was due to some asymmetry in the molecules. It was later determined that the asymmetry was associated with compounds having one or more asymmetric centers. 

An achiral compound does not rotate the plane of polarization of plane-polarized light. 

On the other hand, when plane-polarized light passes through a solution of chiral molecules, the light emerges with its plane of polarization rotated either clockwise or 

---

A compound that rotates the plane of polarization is said to be optically active. In other words, chiral compounds are optically active and achiral compounds are optically inactive. 

Chiral compounds are optically active—they rotate the plane of polarized light achiral compounds are optically inactive. If one enantiomer rotates the plane of polarization clockwise (+), its mirror image will rotate the plane of polarization the same amount counterclockwise (—). Each optically active compound has a characteristic specific rotation. A racemic mixture is optically inactive. A meso compound has two or more asymmetric carbons and a plane of symmetry it is an achiral molecule. A compound with the same four groups bonded to two different asymmetric carbons will have three stereoisomers, a meso compound and a pair of enantiomers. If a reaction does not break any bonds to the asymmetric carbon, the reactant and product will have the same relative configuration—their substituents will have the same relative positions. The absolute configuration is the actual configuration. If a reaction does break a bond to the asymmetric carbon, the configuration of the product will depend on the mechaitism of the reaction. 

In 1847, an explanation for the source of optical activity was proposed by French scientist Louis Pasteur. Pasteur s investigation of tartrate salts (discussed later in this chapter) led him to the conclusion that optical activity is a direct consequence of chirality. That is, chiral compounds are optically active, while achiral compounds are not. Moreover, Pasteur noted that enantiomers (nonsuperimposable mirror images) will rotate the plane of plane-polarized Ught in equal amounts but in opposite directions. We will now explore this idea in more detail. 

Chiral compounds are optically active achiral compounds are optically inactive. 

The problems of charged and reactive polymers are correlated to optically active compounds and it shows an inherent property of both ordinary macromolecules as well as large range of synthetic polymers. Chiral compounds are optically active and essential for life such as proteins, polysaccharides, nucleic acids, etc. and chirality is most important for existence. About 97 % dmgs are formed from namral sources,... 

In all the amino acids shown in Table 23.3 except glycine, the a-carbon is chiral. This means that these compounds are optically active. With alanine, for example, there should be two optical isomers ... 

Chiral auxiliaries are optically active compounds which are used to direct asymmetric synthesis. The chiral auxiliary is temporarily incorporated into an organic synthesis which introduces chirality in otherwise racemic compounds. This temporary stereocentre then forces the asymmetric formation of a second stereocentre. The synthesis is thus diastere-oselective, rather than enantioselective. After the creation of the second stereocentre the original auxiliary can be removed in a third step and recycled. E. J. Corey in 1975, B. M. Trost in 1980 and J. K. Whitesell in 1985 introduced the chiral auxiliaries 8-phenylmenthoT (1.40), chiral mandelic acid (1.41) and frans-2-phenyl-l-cyclohexanoT (1.42), respectively. 

Enantiomers are compounds which have the same chemical structure but different conformations, whose molecular structures are not superimposable on their mirror images, and, because of their molecular asymmetry, these compounds are optically active. The most common cause of optical activity is the presence of one or more chiral centers which, in organic chemistry, are usually related to tetrahedral structures formed by four different groups around carbon, silicon, tin, nitrogen, phosphorus, or sulfur. 

The answers are shown in Figure 4.20. Structures (1) and (2) are enantiomeric pairs. Structures (1) and (3) and structures (2) and (3) are pairs of diastereoisomers (or diastereomers), while structure (3) is a meso compound. A meso compound is optically inactive since it possesses a plane of symmetry and is superimpos-able on its mirror image. It does, however, contain two chiral carbon atoms. This reminds us that not all compounds that contain chiral centres are optically active. 

Chiral compounds and optical activity are discussed in Section 3.3.2.1... 

Optically Inactive Chiral Compounds. Although chirality is a necessary prerequisite for optical activity, chiral compounds are not necessarily optically active. With an equal mixture of two enantiomers, no net optical rotation is observed. Such a mixture of enantiomers is said to be racemic and is designated as ( ) and not as dl. Racemic mixtures usually have melting points higher than the melting point of either pure enantiomer. 

Trialkylsilyl radicals are known to be strongly bent out of the plane (cr-type structure 5). The pyramidal structure of trialkylsilyl radicals (RaSi ) was first indicated by chirality studies on optically active compounds containing... 

A molecule that contains just one chiral carbon atom (defined as a carbon atom connected to four different groups also called an asymmetric or stereogenic carbon atom) is always chiral, and hence optically active. As seen in Figure 4.1, such a molecule cannot have a plane of symmetry, whatever the identity of W, X, Y, and Z, as long as they are all different. However, the presence of a chiral carbon is neither a necessary nor a sufficient condition for optical activity, since optical activity may be present in molecules with no chiral atom and since some molecules with two or more chiral carbon atoms are superimposable on their mirror images, and hence inactive. Examples of such compounds will be discussed subsequently. 

Chiral compounds are sometimes configurationally stable as solids and configurationally labile in solution. When optically active samples of these deriv-... 

Most of the physical properties (e.g., boiling and melting point, density, refractive index, etc.) of two enantiomers are identical. Importantly, however, the two enantiomers interact differently with polarized light. When plane polarized light interacts with a sample of chiral molecules, there is a measurable net rotation of the plane of polarization. Such molecules are said to be optically active. If the chiral compound causes the plane of polarization to rotate in a clockwise (positive) direction as viewed by an observer facing the beam, the compound is said to be dextrorotatory. An anticlockwise (negative) rotation is caused by a levorotatory compound. Dextrorotatory chiral compounds are often given the label d or ( + ) while levorotatory compounds are denoted by l or (—). 

Since all the molecules are asymmetric and have no plane of symmetry, all are optically active. Further structures I and II are enantiomorphs and so are structures III and IV. But structures III and I or IV and I are although stereoisomers but are not enantiomorphs. Such pairs of steroisomers which possess chirality but are not the mirror images of each other are called diastereomers. Thus III and IV are diastereomers of 1. So diastereomers will always be formed when the compound contains two dissimilar asymmetric carbon atoms and will exist in four stereoisomeric forms. 

Stereoisomerism in compounds with two stereo centres diastereomers and meso structure In compounds whose stereoisomerism is due to tetrahedral stereocentres, the total number of stereoisomers will not exceed 2", where n is the number of tetrahedral stereocentres. For example, in 2,3,4-trihydroxybutanal, there are two chiral carbons. The chiral centres are at C-2 and C-3. Therefore, the maximum number of possible isomers will be 2 = 4. All four stereoisomers of 2,3,4-trihydroxybutanal (A-D) are optically active, and among them there are two enantiomeric pairs, A and B, and C and D, as shown in the structures below. 

Another compound in which nitrogen is connected to two oxygens is 11. In this case there is no ring at all, but it has been resolved into ( + ) and (-) enantiomers ([a] = 3°).37 This compound and several similar ones reported in the same paper are the first examples of compounds whose optical activity is solely due to an acyclic tervalent chiral nitrogen atom. However, 11 is not optically stable and racemizes at 20°C with a half-life of 1.22 hr. A similar compound (11, with OCH2Ph replaced by OEt) has a longer half-life— 37.5 hr at 20°C. 

As with the Michael reaction (5-17) the 1,4 addition of organometallic compounds has been performed diastereoselectively517 and enantioselectively.518 In one example of the latter,519 a, (3-unsaturated sulfoxides that are optically active because of chirality at sulfur (p. 100) have given high enantiomeric excesses, e.g.,520... 

A number of biologically significant chiral compounds are synthesized from optically active functionalized alcohols. Scheme 42 shows some naturally occurring and nonnaturally occurring chiral compounds that have been synthesized from alcoholic building blocks in recent years. 

Chiral compounds are very important substances. Many natural products, medicinal compounds, and biomolecules exist as a single, optically active stereoisomer. Furthermore the opposite enantiomer or diastereomer may not have any physiological activity or may, in fact, have a detrimental physiological effect. There is therefore great interest in reactions in which only one stereoisomeric form of a compound is produced by a particular synthetic sequence. 

---