Skew-boat conformation cyclohexane

A potential energy diagram for nng inversion m cyclohexane is shown m Figure 3 18 In the first step the chair conformation is converted to a skew boat which then proceeds to the inverted chair m the second step The skew boat conformation is an inter mediate in the process of ring inversion Unlike a transition state an intermediate is not a potential energy maximum but is a local minimum on the potential energy profile... 

The various conformations of cyclohexane are in rapid equilibrium with one another, but at any moment almost all of the molecules exist in the chair confor mation. Not more than one or two molecules per thousand are present in the skew boat conformation. Thus, the discussion of cyclohexane confor-mational analysis that follows focuses exclusively on the chair confor mation. 

FIGURE 3.12 (a) The boat and (fa) skew boat conformations of cyclohexane. A portion of the torsional strain In the boat Is relieved by rotation about C—C bonds In the skew boat. Bond rotation Is accompanied by movement of flagpole hydrogens away from each other, which reduces the van der Waals strain between them. 

FIGURE 3.15 Energy diagram showing interconversion of various conformations of cyclohexane. In order to simplify the diagram, the boat conformation has been omitted. The boat is a transition state for the interconversion of skew boat conformations. 

Energy diagram for ring inversion in cyclohexane. The energy of activation is the difference in energy between the chair and half-chair conformations. The skew boat conformations are intermediates. The boat and half-chair conformations are transition states. 

In l,l,3,3,5,5-hexaphenyl-l,3,5-trisilacyclohexane, 403, the central trisilacyclohexane system exists in a skewed boat conformation which resembles a twisted boat conformation more than a symmetrical boat form. The bond angles of Si (oc = 110°) and C (p = 117.9°) shown in Fig. 20 [170] produce an average value of a, p = 114.0° which, in comparison to the tetrahedral angle is considerably increased towards the value of 120° displayed by a planar six-membered ring. The cyclohexane ring is... 

Section 3 7 Three conformations of cyclohexane have approximately tetrahedral angles at carbon the chair the boat and the skew boat The chair is by far the most stable it is free of torsional strain but the boat and skew boat are not When a cyclohexane ring is present m a compound it almost always adopts a chair conformation... 

A modified boat conformation of cyclohexane, known as the twist boat (Fig. 1.8), or skew boat, has been suggested to minimize torsional and nonbounded interactions. This particular conformation is estimated to be about 1.5 kcal moE (6 kJ moE ) lower in energy than the boat form at room temperature. 

Skew boat (Section 3.7) An unstable conformation of cyclohexane. It is slightly more stable than the boat conformation. 

In the symmetrical boat conformation of cyclohexane, eclipsing of bonds results in torsional strain. In the actual molecule, the boat is skewed to give the twist boat, a conformation with less eclipsing of bonds and less interference between the two flagpole hydrogens. 

The conformations of the six-membered ring systems are better characterized than those of the less stable five-membered analogues. For example, the cyclohexane molecule can occur in two strainless forms, namely in the rigid chair form or in the flexible form (Fig. 2-9). The latter can exist in a variety of shapes of which only the boat and the skew boat (or twist) are regular and easily depictable on paper. The chair form is preferred energetically because it is usually free from steric interactions whereas the flexible forms are not. The half-chair conformation is possible, when a six-membered ring contains either a double bond or an oxiran ring. In the half-chair conformation four adjacent atoms are in the same plane. 

Fig. 2-9. Conformations of cyclohexane. 1, Chair 2, boat 3, skew boat 4, half-chair.

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