Configurational isomers Stereoisomers diastereomers

There are two major types of stereoisomer conformational isomers and configurational isomers. Configurational isomers include optical isomers, geometrical isomers, enantiomers and diastereomers. 

Trans-1,2-dibromoethene and cis-1,2-dibromoethene are configurational isomers (they can only be interconverted by breaking and remaking a bond, in this case the pi-bond of the alkene), they are both achiral (their mirror images are identical to themselves), and they are diastereomers (although they are stereoisomers, they are not enantiomers). 

For convenience, we laid down (Sec. 4.20) the following ground rule for discussions and problems in this book unless specifically indicated otherwise, the terms stereoisomers," ""enantiomers," and ""diastereomers" will refer only to configurational isomers, including geometric isomers, and will exclude conformational isomers. The latter will be referred to as conformational isomers," con-formers, conformational enantiomers, and conformational diastereomers. ... 

We have seen here and in Section 2.11 that stereoisomers can be classified in three different ways. They may be either conformers or configurational isomers they may be chiral or achiral and they maybe enantiomers or diastereomers. 

When the stereoisomers are not interconverted by rotations along single bonds, they are referred fo as configurational isomers. These isomers are nonsuperposable mirror images and are thus enantiomers, or they are diastereomers. 

In Summary The presence of more than one stereocenter in a molecule gives rise to diastereomers. These are stereoisomers that are not related to each other as object and mirror image. Whereas enantiomers have opposite configurations at every respective stereocenter, two diastereomers do not. A molecule with n stereocenters may exist in as many as 2" stereoisomers. In cyclic compounds, cis and trans isomers are diastereomers. 

There are two types of stereoisomers conformational and configurational isomers, and these are roughly divided into two classes enantiomers (optical isomers) and diastereomers (nonoptical isomers) see Figure 1.1. 

Diastereomers include all stereoisomers that are not related as an object and its mirror image. Consider the four structures in Fig. 2.3. These structures represent fee four stereoisomers of 2,3,4-trihydroxybutanal. The configurations of C-2 and C-3 are indicated. Each stereogenic center is designated J or 5 by application of the sequence rule. Each of the four structures is stereoisomeric wife respect to any of fee others. The 2R R and 25,35 isomers are enantiomeric, as are fee 2R, iS and 25,3J pair. The 21 ,35 isomer is diastereomeric wife fee 25,35 and 2R,3R isomers because they are stereoisomers but not enantiomers. Any given structure can have only one enantiomer. All other stereoisomers of feat molecule are diastereomeric. The relative configuration of diastereomeric molecules is fiequently specified using fee terms syn and anti. The molecules are represented as extended chains. Diastereomers wife substituents on the same side of the extended chain are syn stereoisomers, whereas those wife substituents on opposite sides are anti stereoisomers. 

Note carefully the difference between enantiomers and diastereomers. Enantiomers have opposite configurations at all chirality centers, whereas diastereomers have opposite configurations at some (one or more) chirality centers but the same configuration at others. A full description of the four stereoisomers of threonine is given in Table 9.2. Of the four, only the 2S,3R isomer, [o]D= -28.3, occurs naturally in plants and animals and is an essential human nutrient. This result is typical most biological molecules are chiral, and usually only one stereoisomer is found in nature. 

The other two isomers are the (15,25) and (IR,2R) isomers, and these two also share a mirror image relationship, have the opposite configuration at both chiral centres, and are, therefore, a pair of enantiomers. From a structure with two chiral centres, we thus have four stereoisomers that consist of two pairs of enantiomers. Stereoisomers that are not enantiomers we term diastereoisomers, or sometimes diastereomers. Thus, the (15,25) and (1R,2R) isomers are diastereoisomers of the (l/f,25) isomer. Other enantiomeric or diastere-omeric relationships between the various isomers are indicated in the figure. 

Stereoisomers that are not enantiomers are called diastereoisomers. Three classes may be distinguished configurational, geometrical, and conformational isomers. Configurational diastereomers include molecules with more than one chiral center. Thus 2,3-dichlorobutane can exist in three configurationally... 

Exercise 5-13 Write structures for all the configurations possible for 2,4-dibromo-pentane. Which stereoisomers are enantiomers Which are diastereomers What combination of isomers would give a racemic mixture Which isomer is achiral ... 

Although there are formally six possible stereoisomers of an octahedral complex of general formula Ma2b2c2, for dichlorido(diazane)bis(triphenyl-phosphane)cobalt(l+) only four isomers occur, since an arrangement of the two nitrogen atoms of the hydrazine trans to each other in the complex is not possible. The two OC-6-32 isomers are chiral, they are enantiomers and have the A and C configuration, respectively. Both the other two isomers are achiral diastereomers. 

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