Conformations Stereoisomers staggered

The staggered and eclipsed forms of ethane are conformational stereoisomers (conformational isomers, conformers) because they have the same molecular formulas and sequences of bonded elements but different spatial arrangements due to rotations around single bonds. (Actually there are an infinite number of conformational isomers (also called conformations) because there are an infinite number of degrees of rotation around the bond, but normally one only needs to be concerned with energy minima and maxima.)... 

Examine stereoisomer A, stereoisomer B and stereoisomer C. Each provides a set of three staggered conformers for that 2,3-difluorobutane. 

Spin-spin coupling in nmr, 242 Spontaneous reaction, 36 Stability, heat of combustion, 90 of radicals, 61 resonance, 24 Staggered conformation, 51 Stereoisomers, 69 Stereomers, conformational, 80 Stereoselective reactions, 91 Steric strain, 54, 126 Stilbene, 218 Strain, torsional, 54, 167 Structure of benzene, 189 cyclooctatetraene, 195 Substitution, ally lie, 105 in arenes, 205 in naphthalene, 213 nucleophilic aromatic, 215 ... 

In Sec. 3.5, we saw that there are several different staggered conformations of /i-butane, each of which lies at the bottom of an energy valley—at an energy minimum—separated from the others by energy hills (see Fig. 3.4, p. 79). Different conformations corresponding to energy minima are called conformational isomers, or conformerS. Since conformational isomers differ from each other only in the way their atoms are oriented in space, they, too, are stereoisomers. Like stereoisomers of any kind, a pair of conformers can either be mirror images of each other or not. 

FIGURE 5.8 Stereoisomers of a Ruthenium complex with sulfur-dominated coordination sphere. Staggered trans- i conformer with equatorial and axial phenyl group (a) crossed trans- H conformer with equatorial phenyl groups (b) butterfly cw-N conformer with equatorial phenyl groups (c). All molecules were geometry optimized with an MM-r force field in a Newton-Raphson algorithm [5]. 

A Fischer projection does not show the three-dimensional structure of the molecule, and it represents the molecule in a relatively unstable eclipsed conformation. Most chemists, therefore, prefer to use perspective formulas because they show the molecule s three-dimensional structure in a stable, staggered conformation, so they provide a more accurate representation of structure. When perspective formulas are drawn to show the stereoisomers in their less stable eclipsed conformations, it can easily be seen—as the eclipsed Fischer projections show—that the erythro isomers have similar groups on the same side. We will use both prospective formulas and Fischer projections to depict the arrangement of groups bonded to an asymmetric carbon. 

Following is a staggered conformation of one of the stereoisomers of 2-butanol (See Example 6.2)... 

All of these are chemically realizable. The realizations turn out to correspond to exactly three staggered conformers for each of the three stereoisomers, as shown in Figure 4.6. These correspond to the orientation functions as indicated by the numbers. 

If compounds are stereoisomers, we can make a further distinction as to isomer type. If single-bond rotation easily interconverts the two stereoisomers (as with staggered and eclipsed ethane), we call them conformers. If the two stereoisomers can be interconverted only by breaking and remaking bonds (as with cis- and from-1,2-dimethylcyclopentane), we call them configurational isomers. ... 

Stereoisomers also include conformational isomers, in which different isomers are generated through rotations about bonds. Conformational isomers are often called con-formers. Eclipsed and staggered ethane are typical examples. Note that a conformational isomer need not be an energy minimum— the eclipsed conformation of ethane is an energy maximum, for example. Conformational isomers can be either enantiomeric or diastereomeric.The two gauche forms of butane are conformational enantiomers, but the gauche and the anti forms of butane are conformational diastereomers (Rg. 4.57). 

The CH3CHXCHXCH3 molecule can exist as the stereoisomers shown here. In the echpsed conformation (meso) shown on the left (E), the molecule has a plane of symmetry that bisects the C2, C3 bond. This is a more energetic conformation than any of the three staggered conformations, but it is the only conformation of this configurational stereoisomer that has... 

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