Enantiomers Stereoisomers that differ only

Enantiomers Stereoisomers that differ only by being nonsuper-imposable mirror images of each other, like left and right hands also called optical isomers. 

One of the problems attaching to the HPLC separation of peptides is the analysis of stereoisomers (enantiomers and diastereoisomers), that is, of peptides that differ only in the configuration of their amino acid residues. 

Molecules that differ only in the spatial arrangement of some of their atoms are called stereoisomers. Stereoisomers with an asymmetric carbon atom have two non-superimposable minor-image forms called enantiomers. Most asymmetric molecules in living organisms have only one stereoiso-meric form. 

Stereoisomers Isomers of the same constitution that differ only in the position of atoms and ligands in space i.e., enantiomers and diastereomers). 

Stereoisomers (Sections 1.13B, 4.9A, 4.13, 5.2B, and 5.14) Compounds with the same molecular formula that differ only in the arrangement of their atoms in space. Stereoisomers have the same connectivity and, therefore, are not constitutional isomers. Stereoisomers are classified further as being either enantiomers or diastereomers. 

We can tell a lot even from the schematic coded picture of Figure 4.32. First of all, (R-R) and (R-S) are clearly neither identical nor enantiomers (nonsuperimposable mirror images), because the mirror image of (R-R) is (S-S), not ( -5). Yet these molecules are stereoisomers— they differ only in the arrangement of their constituent parts in space. There is a new word for such molecules—they are diastereomers, stereoisomers that are not mirror images of each other—in other words, stereoisomers but not enantiomers. To sum up, there are two kinds of stereoisomers enantiomers (nonsuperimposable mirror images) and diastereomers. 

Structures I and III are stereoisomers, but they are not enantiomers. Stereoisomers that are not enantiomers are called diastereomers. The pairs II and III, I and IV, and II and IV are diastereomers. In contrast to enantiomers, which have the same chemical and physical properties, diastereomers have different chemical and physical properties. For example, the enantiomers I and II both are liquids at room temperature and are very soluble in ethanol. The enantiomers III and IV both melt at 130 °C and are only slighdy soluble in ethanol. 

The minor images of bromochlorofluoromethane have the sane constitution. That is, the atoms are connected in the sane order. But they differ in the anangement of then-atoms in space they are stereoisomers. Stereoisomers that are related as an object and its nonsuperimposable minor image are classified as enantiomers. The word enantiomer describes a paiticulai- relationship between two objects. One cannot look at a single molecule in isolation and ask if it is an enantiomer any more than one can look at an individual human being and ask, Is that person a cousin Fuithennore, just as an object has one, and only one, minor image, a chiral molecule can have one, and only one, enantiomer. 

Some molecules have more than one chirality center. Enantiomers have opposite configuration at all chirality centers, whereas diastereomers have the same configuration in at least one center but opposite configurations at the others. Epimers are diastereomers that differ in configuration at only one chirality center. A compound with n chirality centers can have a maximum of 2n stereoisomers. 

Figure 9.3 Stereoisomers of the D-aldoses. D-Ribose and D-arabinose differ only in their configuration about a single carbon atom (carbon 2) and are examples of epimers. Diastereoisomers are stereoisomers which are not enantiomers of each other but are chemically distinct forms, the eight D-hexoses being examples. Some, however, are also epimers of each other, for example D-allose and D-altrose. The number of aldoses in the L series is equal to that of the d series and each compound is an enantiomer of one in the other series.

Amphetamine and methamphetamine occur as structural isomers and stereoisomers. Structural isomers are compounds with the same empirical formula but a different atomic arrangement, e.g., methamphetamine and phentermine. Stereoisomers differ in the three-dimensional arrangement of the atoms attached to at least one asymmetric carbon and are nonsuperimposable mirror images. Therefore, amphetamine and methamphetamine occur as both d- and L-isomeric forms. The two isomers together form a racemic mixture. The D-amphetamine form has significant stimulant activity, and possesses approximately three to four times the central activity of the L-form. It is also important to note that the d- and L-enantiomers may have not only different pharmacological activity but also varying pharmacokinetic characteristics. 

An immediate consequence of Pasteur s law is that the relationship between enantiomers is established by symmetry alone and does not require any knowledge of molecular bonding connectedness (constitution). This is in contrast to diastereomers, the other class of stereoisomers Diastereomers are not related by symmetry, and their relationship can be defined only by first specifying that their constitutions are the same—otherwise, there would be nothing to distinguish them from constitutional isomers. Thus enantiomers, which have identical scalar properties and differ only in pseudoscalar properties, have more in common with homomers than with diastereomers, while diastereomers, which differ in all scalar properties, have more in common with constitutional isomers than with enantiomers.51, 52 It therefore makes more sense, in an isomer classification scheme, to give priority to isometry rather than to constitution.52 In such a scheme there is no need for the concept stereoisomer the concept retains its usefulness only because it normally proves convenient, in chemical reaction schemes, to combine enantiomers and stereoisomers in a common class. 

When molecules composed of the same constituents have the same structural formulas but differ only with respect to the spatial arrangement of certain atoms or groups of atoms, they are defined as stereoisomers. Stereoisomers can be optical isomers or geometrical isomers. Optical isomers are members of a set of stereoisomers, at least two of which are optically active or chiral geometrical isomers are members of a set of Stereoisomers that contains no optically active members. If the relationship between optical isomers is one of nonsuperimposable mirror images, the isomers are defined as enantiomers. Molecules having at least one pair of enantiomers are considered chiral. Optical isomers not related to each other as enantiomers are diastereomers. 

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