Flagpole interactions

Boat cyclohexane has torsional strain and flagpole interaction. 

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. 

By flexing to the twist conformation, the boat conformation can relieve some of its torsional strain and reduce the flagpole interactions. 

The distance between their centres at normal tetrahedral angles should have been only about 1.8 A, but the sum of the van der Waals radii of the two hydrogen atoms is 2.4A. This is sometimes called bowsprit-flagpole interaction and this too makes its contribution to the increased energy of the boat form. 

In the twist boat conformation the "bow and the stern of the boat have been twisted slightly. Although this decreases the flagpole interaction and relieves some of the torsional strain, angle strain is introduced. Overall, the twist boat conformation is a little more stable than the boat conformation but not nearly as stable as the chair conformation. 

Corey and co-workers also developed parameters to evaluate boat conformations, which are often important contributors to the conformational population of cyclohexane derivatives (see above). The energies for boat conformations with one or more substituents in the flagpole positions use a new term (b) (see 210 and 211) which is derived from the bowsprit-flagpole interactions (R-H in 210 or R -R in 211). Corey s examples include 212 and 213. In 212, the axial hydroxyl group receives a value (Aqh) of 0.8 kcal moT (3.35 kJ moT, see Table 1.5) and b] = 0 for bond a and b2 = 0.60 for bond c (see typical b values in Table 1.6. The conformational energy for 212 is (0.6 x 0.8) = 0.48 kcal moT (2.01 kJ moT ). 

Here, 4 kcal of eclipsing strain is involved. In addition, a pair of hydrogens are forced to come together in a flagpole type of interaction. It is experimentally measured that each flagpole interaction (where the... 

Although it is more stable, the chair conformation is much more rigid than the boat conformation. The boat conformation is quite flexible. By flexing to a new form—the twist conformation (Fig. 4.15)— the boat conformation can relieve some of its torsional strain and, at the same time, reduce the flagpole interactions. 

Conversion of (a) a chair conformation to (b) a boat conformation. In the boat conformation, there is torsional strain due to the four sets of eclipsed hydrogen interactions and steric strain due to the one set of flagpole interactions. A chair conformation is more stable than a boat conformation. 

There are many other nonplanar conformations of cyclohexane, one of which is the boat conformation. You can visualize the interconversion of a chair conformation to a boat conformation by twisting the ring as illustrated in Figure 3.9. A boat conformation is considerably less stable than a chair conformation. In a boat conformation, torsional strain is created by four sets of eclipsed hydrogen interactions, and steric strain is created by the one set of flagpole interactions. Steric strain (also called nonbonded interaction strain) results when nonbonded atoms separated by four or more bonds are forced abnormally close to each other—that is, when they are forced closer than their atomic (contact) radii allow. The difference in potential energy between chair and boat conformations is approximately 27 kj/mol (6.5 kcal/mol), which means that, at room temperature, approximately 99.99% of all cyclohexane molecules are in the chair conformation. 

The boat conformation of cyclohexane has significant torsional strain (from eclipsing H s as well as flagpole interactions). The boat can alleviate some of its torsional strain by twisting, giving a conformation called a twist boat. 

---