Question

The twisted boat conformation of cyclohexane is actually a slightly lower energy

conformation than the boat conformation. Which of the following accurately describes one

factor involved in this structural behavior?

A) lower angle strain in the boat

B) higher torsional strain in the twisted boat

C) fewer unfavorable steric factors in the twisted boat

D) increased rotational freedom in the boat

E) lower bond strain in the boat

The answer key says it’s C, but please explain it if it’s right (I think it relates to lower torsion strain). Thanks.

Answer 1

The boat conformer of cyclohexane has 1.0 kcal/mol higher in energy than the twisted boat conformer. In boat conformers, the hydrogen bonds, marked as red, are destabilizing due to torsional strain. But, in twisted boat conformer, the amount of torsional strain is released slightly as the marked hydrogens (blue arrow) are displaced and now placed at a longer distance minimizing the torsional strain. Obviously, twisting of the boat form increases angle strain, but it decrases tortional strain in more extent and ultimately leading to lowering of energy.

Torsional strain is a result of repulsion between bonding electron clouds and, therefore, it is just another form of steric crowding. So, the answer is option (C)