Question

1) It was found that dihedral angles of the hydrogens at C1 in gauche butane were not exactly 60 or 180 degrees, why is that?

2) Does increasing the size of an axial substitute always result in a higher relative strain energies for the axial conformation?

3)How do you explain the fact that the largest X(CH3)3 group has the smallest difference? How does this reduce the axial strain energy?

4)What structural features are required to generate a structure with a zero dipole moment for a molecule containing more than one highly polar bond?

Answer 1

1) At 60 degree dihedral angle ,one hydrogen of each methyl groups on C1 is closer to a hydrogen on the other methyl group. Due to the presence of two bulky CH3 group at gauche positions, the steric repulsions pushes the CH3 group somewhat more apart than expected.This increases the dihedral angle between the methyl groups .So, the dihedral angle is greater than 60 degree.

Similary ,at 180 degree ,the C1 methyl hydrogen offers steric repulsion to the hydrogen at gauche position ,so the dihedral angle is greater than expected.

2)No, , in some cases as the size of the axial substituent increases , the bond length betweeen the respective carbon and substituent increases simultaneously.Thus, the relative tortional strain for axial conformation does not change .

example : For Iodide as substituent, C-I bond length increases, and the C1 substituent does not interact with C1 substituent to offer steric repulsion

3) If the axial substituent is a larger group like X(CH3)3 then ,the bond length between the bulky group and carbon increases,thereby decreasing the bond strength or bond order.so, the energy difference between axial substituents is lesser than expected and the axial strain energy is also reduced.

4) For a molecule with more than one polar bonds, the net dipole moment can be made to be zero when those bonds oppose each other.