Stereoisomers optical properties

Stereoisomers can be distinguished from one another by their different optical properties. Each member of a pair of stereoisomers will rotate plane-polarized light in different directions. 

Two stereoisomers are obtained from the reaction of HBr with (5)-4-bromo-l-pentene. One of the stereoisomers is optically active, and the other is not. Give the structures of the stereoisomers, indicating their absolute configurations, and explain the difference in optical properties. 

Table 8.3 lists some examples of optodes with simple receptor layers. Among them are calixarenes, which are interesting for ISEs also, since they are highly selective ligands for cations (Chap. 7, Sect. 7.1.2). Above all, calixarenes are highly efficient receptors for certain organic analytes. Optical properties of such systems have been used successfully for chiral recognition of stereoisomers. As an example, when a special calixarene was combined with the... 

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. 

When additional substituents ate bonded to other ahcycHc carbons, geometric isomers result. Table 2 fists primary (1°), secondary (2°), and tertiary (3°) amine derivatives of cyclohexane and includes CAS Registry Numbers for cis and trans isomers of the 2-, 3-, and 4-methylcyclohexylamines in addition to identification of the isomer mixtures usually sold commercially. For the 1,2- and 1,3-isomers, the racemic mixture of optical isomers is specified ultimate identification by CAS Registry Number is fisted for the (+) and (—) enantiomers of /n t-2-methylcyclohexylamine. The 1,4-isomer has a plane of symmetry and hence no chiral centers and no stereoisomers. The methylcyclohexylamine geometric isomers have different physical properties and are interconvertible by dehydrogenation—hydrogenation through the imine. 

Since chirality is a property of a molecule as a whole, the specific juxtaposition of two or more stereogenic centers in a molecule may result in an achiral molecule. For example, there are three stereoisomers of tartaric acid (2,3-dihydroxybutanedioic acid). Two of these are chiral and optically active but the third is not. 

Since all the physical properties of two given enantiomers are the same in the absence of a chiral, or optically active, medium, their chromatographic resolution needs a different approach from the relatively simple separation of geometrical isomers, stereoisomers or positional isomers. Two methods are used. The older technique of indirect resolution, requires conversion of the enantiomers to diastereoisomers using a suitable chiral reagent, followed by separation of the diastereoisomers on a non-chiral GC or LC stationary phase. This technique has now been largely superseded by direct resolution, using either a chiral mobile phase (in LC) or a chiral stationary phase. A variety of types of chiral stationary phase have been developed for use in GC, LC and SFC(21 23). 

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. 

The enantiomeric menthols have identical physical properties (apart from their specific rotation), but the racemates differ from the optically active forms in, for example, their melting points. Although the differences between the boiling points of the stereoisomers are small, the racemates can be separated by fractional distillation. Boiling points (in °C at 101.3 kPa) are as follows ... 

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 /). 

Enantiomers are stereoisomers that are non-superimposable mirror images of each other. EnrirTm mers have identical physical and chemical (except towards optically aQtJvp reagents) properties except for the direction in which plane-polarized light is rotated. Enantiomers account for a compound s optical activity. 

At the instant Pasteur recognized the existence of stereoisomers (objects), he also accepted the existence of stereoprocesses (operations). For the notion of isomer carries with it criteria of distinguishability among these is the possibility that a given isomer can be formed, separated, or altered in a way which differentiates it from other isomers. This applies equally to isomers with many properties in common, e.g. optical antipodes, or to those with essentially all different properties, e.g. cis-trans, syn-anti, gauche-anti, erythro-threo, or axial-equatorial pairs. Now, the stereo-path may be part of an overall conversion which, if described in some detail, we term a mechanism. Our present task is to attempt to understand those elementary or single-step processes by which stereochemical choices are made. 

Each stereoisomer in a pair of enantiomers has the property of being able to rotate monochromatic plane-polarized light. The instrument chemists use to demonstrate this property is called a polarimeter (see your text for a further description of the instrument). A pure solution of a single one of the enantiomers (referred to as an optical isomer) can rotate the light in either a clockwise (dextrorotatory, +) or a counterclockwise (levorotatory, -) direction. Thus those molecules that are optically active possess a handedness or chirality. Achiral molecules are optically inactive and do not rotate the light. 

The existence of these different practices was not sufficient to create a discipline or subdiscipline of physical chemistry, but it showed the way. One definition of physical chemistry is that it is the application of the techniques and theories of physics to the study of chemical reactions, and the study of the interrelations of chemical and physical properties. That would mean that Faraday was a physical chemist when engaged in electrolytic researches. Other chemists devised other essentially physical instruments and applied them to chemical subjects. Robert Bunsen (1811—99) is best known today for the gas burner that bears his name, the Bunsen burner, a standard laboratory instrument. He also devised improved electrical batteries that enabled him to isolate new metals and to add to the list of elements. Bunsen and the physicist Gustav Kirchhoff (1824—87) invented a spectroscope to examine the colors of flames (see Chapter 13). They used it in chemical analysis, to detect minute quantities of elements. With it they discovered the metal cesium by the characteristic two blue lines in its spectrum and rubidium by its two red lines. We have seen how Van t Hoff and Le Bel used optical activity, the rotation of the plane of polarized light (detected by using a polarimeter) to identify optical or stereoisomers. Clearly there was a connection between physical and chemical properties. 

Stereoisomers are classified by symmetry as either enantiomers or diaste-reomers. Enantiomers have identical physical properties except for the direction of optical rotation. Diastereomers are basically stereoisomers that are not enantiomers of each other. A pair of enantiomers exists for all molecules containing a single chiral center and have the opposite configuration at each of the stereo centers. The maximum number of stereoisomers for a compound with n stereo centers is T. Diastereomers, on the other hand, have the same configuration at one of the two centers and have the opposite configuration at the other. 

The two glyceraldehyde isomers of 4-13 are identical in all physical properties except that they rotate the plane of polarized light in opposite directions and form enantiomorphous crystals. When more than one asymmetric center is present in a low-molecular-weight species, however, stereoisomers are formed which are not mirror images of each other and which may differ in many physical properties. An example of a compound with two asymmetric carbons (a diastereomer) is tartaric acid, 4-16, which can exist in two optically active forms (d and L, mp 170 C), an optically inactive form (meso, mp 140 C), and as an optically inactive mixture (dl racemic, mp 206°C). 

Diastereomers are not miiror images of each other, and as such, their physical properties are different, including optical rotation. Figure 5.12 compares the physical properties of the three stereoisomers of tartaric acid, consisting of a meso compound that is a diastereomer of a pair of enantiomers. 

---