Eclipsed bonds

Eclipsed bonds are characterized by a torsion angle of 0° When the torsion angle is approximately 60° we say that the spatial relationship is gauche, and when it is 180° we say that it is anti Staggered conformations have only gauche or anti relationships between bonds on adjacent atoms... 

Conformations m which the torsion angles between adjacent bonds are other than 60° are said to have torsional strain Eclipsed bonds produce the most torsional strain staggered bonds none Because three pairs of eclipsed bonds are responsible for 12 kJ/mol (2 9 kcal/mol) of torsional strain m ethane it is reasonable to assign an energy cost of 4 kJ/mol (1 kcal/mol) to each pair In this chapter we 11 learn of additional sources of strain m molecules which together with torsional strain comprise steric strain... 

At Its most basic level separating the total strain of a structure into its components is a qualita tive exercise For example a computer drawn model of the eclipsed conformation of butane using ideal bond angles and bond distances (Figure 3 8) reveals that two pairs of hydrogens are separated by a distance of only 175 pm a value considerably smaller than the sum of their van der Waals radii (2 X 120 pm = 240 pm) Thus this conformation is destabilized not only by the torsional strain associ ated with its eclipsed bonds but also by van der Waals strain... 

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... 

FIGURE 3 14 (a) A ball and spoke model and (b) a space filling model of the boat confor mation of cyclohexane Torsional strain from eclipsed bonds and van der Waals strain involving the flagpole hydrogens (red) make the boat less stable than the chair... 

Torsional strain (Section 3 1) Decreased stability of a mole cule associated with eclipsed bonds trans (Section 3 11) Stereochemical prefix indicating that two substituents are on opposite sides of a ring or a double bond (Contrast with the prefix cis )... 

Torsional strain (Section 3.1) Decreased stability of a molecule associated with eclipsed bonds. 

Eclipsing strain (Section 3.6) The strain energy- in a molecule caused by electron repulsions between eclipsed bonds. Fxlipsing strain is also called torsional strain. 

Molecules of the simpUcity of ethane or the complexity of proteins and DNA adopt different conformations. In the case of ethane this gives rise to the notion of a staggered and eclipsed bond, whereas proteins form an array of complex structural elements and DNA - the famous double hehx. The understanding of the conformational properties of small molecules is an important factor in computational approaches contributing to drug discovery. 

Cyclobutane as a planar molecule All H atoms are eclipsed Bond angle = 90°... 

The low reactivity of l,6-anhydro-2,3,4-tri-0-benzyl-/3-D-allopyra-nose is also of interest.106 This monomer has 1,3-interaction between substituents on C-2 and C-4 in the C4(d) conformation. However, when conversion into a boat conformation, as proposed, occurs on reaction with a propagating cation, eclipsed bonds develop at C-2, C-3, and C-4. It is, therefore, not surprising that the D-allo anhydride is less reactive than those of D-mannose, D-glucose, or D-galactose. The corresponding, D-altrose derivative has only one axial substituent in addition to the anhydro ring, and these are on opposite sides of the pyranose ring, and therefore do not interact it would be expected to be, and has proved to be, of very low polymerizability.106... 

Bonds of methyl group eclipsed Bonds of methyl group staggered with those of attached carbon with those of attached carbon... 

Fig. 4. Solid-state structure of the Me3PB(C6F5)3 adduct, illustrating the nearly eclipsed bonds and close...

Out of the two conformations, the staggered conformation is the more stable as the C-H bonds and hydrogen atoms are as far apart from each other as possible. In the eclipsed conformation, both the bonds and the atoms are closer together and it can cause strain due to electron repulsion between the eclipsed bonds and between the eclipsed atoms. Therefore, majority of the ethane molecules are in the staggered conformation at any point of time. Since the energy difference between the staggered and eclipsed conformations is small and so it allows each ethane molecule to pass through an eclipsed conformation ... 

Torsional strain (Section 6.3) Destabilization resulting from eclipsed bonds. 

Eclipsed bonds are drawn slightly displaced for clarity—as though the bond were rotated by a tiny fraction... 

This form of carbon is almost invariably found with the cubic structure shown in Fig. 7-1. There is also a hexagonal form (lonsdaleite),5 found in certain meteorites and also available synthetically, in which the puckered layers are stacked in an ABAB- pattern instead of the ABCABC- pattern. The hexagonal form is probably unstable toward the cubic, since unlike the cubic, it contains some eclipsed bonds. 

The appearance of crown forms in tetroxocane and other compounds with heteroatoms in the 1,3,5 and 7 positions probably results from two stabilizing effects (a) a slightly lower eclipsing barrier for the —O—CHg— versus the —CH2—CH2 fragment in the crown there are eight partially eclipsed bonds which would benefit from relief of torsional strain (b) distorted crowns, i.e. the chair-chair and twist-chair-chair may suffer from higher dipolar repulsions than does the crown. Unfortunately, there are no conformational energy calculations available on any of the heterocyclic systems. 

Next, let us flip the left end of the molecule up (Fig. 9.4) to make the boat conformation, (Like all the transformations we shall carry out in this section, this involves only rotations about single bonds what we are making are indeed conformations.) This is not a very happy arrangement. Sighting along either of two carbon-carbon bonds, we see sets of exactly eclipsed bonds. 

---