Conformations Stereoisomers eclipsed

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.)... 

The aldotetroses are the four stereoisomers of 2 3 4 trihydroxybutanal Fischer pro jections are constructed by orienting the molecule m an eclipsed conformation with the aldehyde group at the top The four carbon atoms define the mam chain of the Fischer projection and are arranged vertically Horizontal bonds are directed outward vertical bonds back... 

FIGURE 7.10 Stereoisomeric 2,3-butanediols shown in their eclipsed conformations for convenience. Stereoisomers (a) and (b) are enantiomers of each other. Structure (c) is a diastereo-mer of (a) and (b), and is achiral. It is called meso-2,3-butanediol. 

Compounds such as PhCHBr-CHBrPh have two asymmetric centres, each of which can have the R or the S configuration. There will be three stereoisomers (reactions 3.18 and 3.19), which can be designated S,S (14), R,R (15), and R,S (12) (S,R will be the same as R,S because of the symmetry of the molecule). The R,R and S,S forms will be optically active and rotate polarized light in opposite directions by an equal amount. Synthesis will normally give a 50/50 mixture of the two isomers, the racemic or ( + ) mixture. The R,S or nteso isomer will be optically inactive since it is identical with its mirror image (the mirror plane in 12 can be seen in the eclipsed conformation obtained by rotation of the front of the molecule clockwise by 60° round the central C-C bond). 

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. 

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). 

Fischer projections do not attempt to represent energy minimum conformations. Instead, they are designed to help distinguish the stereoisomers. An analogy would be the representation of substituted ethanes in an eclipsed conformation, called the sawhorse stmcture (p. 65). Fischer projections are used because they are simple to draw and read. We will see soon how to translate them into more accurate representations of molecules. 

In each of the Fischer projections shown above, the horizontal bonds project out of the paper toward the viewer and the vertical bonds extend behind the paper away from the viewer. Groups can rotate freely about the carbon-carbon single bonds, but Fischer projections show the stereoisomers in their eclipsed conformations. 

When perspective formulas are drawn to show the stereoisomers in their less stable eclipsed conformations (those shown next), we can easily see that the erythro enantiomers have similar groups on the same side. We will use both perspective formulas and Fischer projections to depict the arrangement of groups bonded to an asymmetric center. 

The missing stereoisomer is the mirror image of 1 because 1 and its mirror image are the same molecule. This can be seen more clearly when the perspective formulas are drawn in eclipsed conformations or when Fischer projections are used. 

In the case of the acyclic compound, the meso compound is the stereoisomer with a plane of symmetry when drawn as an eclipsed conformer (B). For the cyclic compounds, the meso compound is the cis isomer (D and F). 

The particular pair of stereoisomers that is formed depends on whether the reactant is a cis alkene or a trans alkene. Syn addition of H2 to a cis alkene forms only the erythro enantiomers. (In Section 4.11 we saw that the erythro enantiomers are the ones with the hydrogens on the same side of the carbon chain in the eclipsed conformers.)... 

The stereochemical rationale put forth for the observed stereoisomers involves examination of four transition states (I-IV). Adducts 52 and 53 would arise from the chair-like conformations I and II respectively (Scheme 2.7). The formation of 52 as the major product was anticipated since the related transition-state conformation II leading to 53 is destabilized by an eclipsing interaction between Ha and Hb. Not surprisingly, the octahydro-quinoline products, 54 and 55, derived from the two boatlike conformations, III and IV, are not detected. 

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