Boat conformation, of cyclohexane

The hydrogens on C-2 to C-3 and C-5 to C-6 bonds of the boat conformation are eclipsed. Also the hydrogens at C-1 and C-4 are so close that their van der Waals radii overlap. As a result, the boat conformation is very unstable. 

Hydrogen-hydrogen eclipsing (only front pair shown) 

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FIGURE 1.7 The boat conformation of cyclohexane, a = axial hydrogen atom and e = equatorial hydrogen atom. 

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. 

FIGURE 3.14 (a) A ball-and-spoke model and (h) a space-filling model of the boat conformation 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. 

Boat conformation of cyclohexane is free of angle strain. 

Figure 4.14 (a) The boat conformation of cyclohexane is formed by "flipping" one end of the chair form up (or down). This flip requires only rotations about carbon-carbon single bonds, (b) Ball-and-stick model of the boat conformation, (c) A space-filling model. 

Figure 4.15 (a) Illustration of the eclipsed conformation of the boat conformation of cyclohexane, (b) Flagpole interaction of the Cl and C4 hydrogen atoms of the boat conformation. 

Refer to Figure 24-9 for the chair and twist boat conformations of cyclohexane. 

In general, things are simpler than that, much to our advantage. Within the limits set by the precision of the present estimates, structural features like the chair, boat, or twist-boat conformations of cyclohexane rings, as well as the butane-gawc/ze effects or the cis-tmns isomerism of ethylenic compounds leave no recognizable distinctive trace in zero-point plus heat content energies. Indeed, whatever residual, presently... 

An important application of the Klyne-Prelog convention is the characterization of conformations of rings2. The convention not only allows classical concepts, such as the chair and boat conformations of cyclohexane, to be defined but also allows complicated situations, as is characteristic for medium rings, to be treated in a very convenient and general way. 

Figure 3.13, Chair arid —boat conformations of cyclohexane...

Fig.N. Newman projections of the chair and boat conformations of cyclohexane.

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 boat conformation of cyclohexane (Figure 3-20) also has bond angles of 109.5° and avoids angle strain. The boat conformation resembles the chair conformation except that the footrest methylene group is folded upward. The boat conformation suffers from torsional strain, however, because there is eclipsing of bonds. 

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