Dextro isomer asymmetric

Carbinoxamine is a potent antihistaminic and is available as the racemic mixture. Carbinoxamine differs structurally from chlorpheniramine only in having an oxygen atom separate the asymmetric carbon atom from the aminoethyl side chain. The more active leva isomer of carbinoxamine has the (5) absolute configuration and can be superimposed on the more active dextro isomer (S configuration ) of chlorpheniramine. 

In general, the maximum number of optically active isomers is given by 2n where n represents the number of asymmetric carbon atoms. Thus for a compound where n = 1, as in lactic acid, there would be two stereoisomers, one the dextro and the other the laevo. For a compound with two asymmetric carbon atoms, there would be 22 = 4 stereoisomers. But if the two asymmetric carbon atoms carry exactly identical groups, as in tartaric acid, the number would be fewer than four and we know that it exists in three forms, the d the 1 and the meso. 

Optical isomerism is when two molecules are nonsuperimposable mirror images of each other (Think of a pair of gloves. They can only stack palm to palm, not one on top of the other.) Such molecules are termed asymmetric or chiral. (Note Chiral carbons have 4 different substituents.) Two optical isomers can be distinguished in their names by the prefixes dextro- or levo- according to whether a solution of the compound rotates a beam of polarized light to the right or the left. The abbreviations d- and l- are commonly used for dextro- and levo-. A new system of naming uses R- (rectus) and S- (sinister). 

The other example of note is the optically active tartaric acids (Fig. 11). Tartaric add contains two asymmetric carbon atoms. The dextro- and levo-tartaric adds are enantiomers. However, a third isomer is possible in which the two rotations due to the two asymmetric carbon atoms compensate and the molecule is optically inactive as a whole. That is, the molecule contains a plane of symmetry. This form, meso-tartaric acid, was also discovered by Pasteur, differs from the two optically active tartaric adds in being internally compensated, and is not resolvable. Thus, the tetrahedral model for carbon and the asymmetric carbon atom proposed by van t Hoff were able to completely explain the observations of Pasteur relating to the three isomers of tartaric add. 

The structures considered above have been concerned with the behaviour of the backbone of the polymer. On proceeding from polyethylene to the next member in the series of olefin polymers, polypropylene, [-CH2-CH(CH3)-] , an asymmetric centre has been introduced into the backbone, in this case the carbon bearing the methyl group. An asymmetric centre is one where it is possible to recognize two isomeric forms that are mirror images and not superimposable. These are often described as optical isomers and the terms d and I are introduced for dextro (right-) and laevo (left-) handed forms. For small molecules these isomers may be resolved optically since they will rotate the plane of polarization in opposite directions. 

That the adsorption of the cyclopentane ring seems to proceed mainly flatly in deuterium exchange on films has been stated above (see Section I,D). Of considerable interest are the investigations on asymmetric catalysis initiated by Schwab et al. (273). In their work, one of the optical isomers reacted a little faster than the other in a racemic mixture. Terent yev and Klabunovskii (274, 273) carried out the catalytic asymmetric synthesis from optically inactive substances. The reactions were accomplished on metals deposited on dextro- and levorotatory quartz. Organic optically active carriers and admixtures give a still greater effect. On this problem see Klabunovskii (276). At the present time still more active catalysts for the reaction of asymmetric hydrogenation and polymerization have been revealed (277-279). 

Enantiomer i- nan-te-o-mor [Gk enangtios + E -mer] (ca. 1929) n. Either of a pair of chemical compounds whose molecular structures have a mirror-image relationship to each other. An asymmetric molecule that is the mirror image of its stereoisomer. The two isomers are given the prefixes dextro-and leva-, e.g., d- and Mactic acid. The physical properties of pure enantiomers are equal within experimental error, yet mixtures of the two, called racemic mixtures, may have different properties. For example, 50-50 d/ -lactic acid melts 20°C lower than its pure enantiomers. 

The separation of optical isomers presents an entirely different problem. By definition, dextro and levo isomers have identical dipole moments, vapor pressures, etc. Therefore, there can be no separation unless an asymmetric environment exists within the column. In gas-liquid chromatography this condition is fulfilled when an optically active liquid is employed as the... 

In general, compounds which contain asymmetric carbon atoms rotate the plane of polarization of plane-polarized light. For this reason they are said to be optically active. When the molecular symmetry is such that the optical activity of one portion of the molecule is cancelled by that of the second portion of the molecule, the compounds are said to be internally compensated and are called meso compounds. The tartaric acid with the formula (X) is such a compound and has been known as the meso-tartaric acid. The tartaric acids identified as (VIII) and (IX) have been known as d-tartaric acid and Z-tartaric acid because of the sign of their optical rotations (dextro and levo, respectively). (The nomenclature of these acids is discussed later in this chapter.) The compounds (VIII) and (IX) are non-superimposable mirror images, called enantiomorphs. The existence of such pairs of asymmetric isomers is the fundamental basis of optical activity. The asymmetry may be in either the molecular structure or the crystal structure. Asymmetric carbon atoms are not always present in optically active molecules. 

Lactic acid exists as two position isomers, primary or /S-Iactic acid CH2OH.CH2.COOH, and secondary or a-lactic acid CH3.CHOH.COOH. This last form possesses an asymmetric carbon atom and therefore exists as a laevo- and a dextro- rotatory form. 

For every optically active dextro- or laevo-rotatory compound, a corresponding stereo-isomer, or epimer, exists, having the same formula but exactly opposite optical properties. This can only be explained by assmning that the groups attached to each asymmetric carbon atom can be arranged in either of two ways, and that the fact that chemical compounds occupy tri-dimensional space must be recognised in constructing formulae. 

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