Stable eclipsed conformations

Although Fischer projections are commonly used to depict monosaccharides with many stereogenic centers, care must be exercised in using them since they do not give a true picture of the three-dimensional structures they represent. Because each stereogenic center is drawn in the less stable eclipsed conformation, the Fischer projection of glucose really represents the molecule in a cylindrical conformation, as shown in Figure 27.2. 

The conformation of simple alkenes can be considered by beginning with propene. There are two families of conformations available to terminal alkenes eclipsed and bisected conformations, as shown below for propene. The eclipsed conformation is preferred by about 2 kcal/mol and represents a barrier to rotation of the methyl group. A simple way to relate the propene rotational barrier to that of ethane is to regard the tt bond as a banana bond (see p. 7). The bisected conformation of propene is then seen to correspond to the eclipsed conformation of ethane, while the more stable eclipsed conformation corresponds to the staggered conformation of ethane. ... 

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. 

The five-membered chelate rings within this structure are in the presumably less stable eclipsed conformation. In spite of this fact, structure r-1,7-1 is the thermodynamically most stable for planar Ni(rac-1,7-CTH). This suggests that the relative stabilities of the six-membered chelate rings are more important than those of the five-membered chelate rings, and that the chair conformation of six-membered chelate rings is more stable than the twist or boat 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. 

In summary, structural data accumulated in the last decade have provided experimental confirmation of the theoretical prediction of the existence of nonstaggered, stable eclipsed conformations in several saturated compounds. 

Of the two conformations of ethane the staggered is 12 kJImol (2 9 heal mol) more stable than the eclipsed The staggered conformation is the most stable conformation the eclipsed is the least stable conformation Two main explanations have been offered for the difference in stability between the two conformations One explanation holds that repulsions between bonds on adjacent atoms destabilize the eclipsed conformation The other suggests that better electron delocalization stabilizes the staggered conformation The latter of these two explanations is now believed to be the correct one... 

A second but much less stable nonplanar conformation called the boat is shown in Eigure 3 14 Like the chair the boat conformation has bond angles that are approximately tetrahedral and is relatively free of angle strain It is however destabi hzed by the torsional strain associated with eclipsed bonds on four of its carbons The... 

Eclipsed conformation of ethane (least stable conformation)... 

FIGURE 7 9 Repre sentations of (2/ 3R) dihy droxybutanoic acid (a) The staggered conformation is the most stable but is not properly arranged to show stereochemistry as a Fischer projection (b) Rotation about the C 2-C 3 bond gives the eclipsed conforma tion and projection of the eclipsed conformation onto the page gives (c) a correct Fischer projection... 

Example Ethane is stable in the staggered (gauche) conformation. The transition state for rotating a methyl group in ethane has the eclipsed conformation. Ageometry optimization starting from an eclipsed conformation yields the transition state. 

One of the frmdamental structural facets of organic chemistry, which has been explained most satisfactorily in MO terms, is the existence of a small barrier to rotation about single bonds. In ethane, for example, it is known that the staggered conformation is about 3kcal/mol more stable than the ecl sed conformation so that the eclipsed conformation represents a transition state for transformation of one staggered conformation into another by rotation. 

Ab initio MO calculations using 4-3IG orbitals indicate that the eclipsed conformation of acetaldehyde is more stable than the staggered conformation. 

There are two families of conformations available to terminal alkenes. These are the eclipsed and bisected conformations shown below for propene. The eclipsed conformation is more stable by about 2kcal/mol. ... 

Conformations A and B are of the eclipsed type, whereas C and D are bisected. It has been determined by microwave spectroscopy that the eclipsed conformations are more stable than the bisected ones and that B is about 0.15 kcal more stable than A. MO calculations at the 6-31G level have found relative energies of 0.00, —0.25, 1.75, and 1.74kcal/mol, respectively, for A-D. ... 

Ketones also favor eclipsed conformations. The preference is for the rotamer in which the alkyl group, rather than a hydrogen, is eclipsed with the carbonyl group because this conformation allows the two alkyl groups to be anti rather than gauche. Electron diffraction studies of 3-pentanone indicate the conformation shown to be the most stable rotamer, in accord with this generalization. ... 

To find the least stable conformation, we need to rotate the front carbon atom and consider all three eclipsed conformations. The least stable conformation will be the one with the two largest groups eclipsing each other ... 

Eclipsed conformation maximum repulsive interaction between the electron pairs of the six C—H bonds => has the highest energy => least stable conformation. 

In the Group XIV compounds the two rings are not parallel, due to the influence of the metal lone pair, giving rise to a bent-sandwich molecular structure142 (C2V symmetry), different from that in other stable cyclopentadienyl derivatives, such as Fe(Cp)2, where the two rings are parallel (D5h symmetry in the eclipsed conformation and D5d symmetry in the staggered one). 

---