AP BIOLOGY

Detailed explanation-1: -The relative energy required for complete a rotation in ethane is called the energy barrier to rotation, and it is about 3 kcal/mol. That is, the eclipsed structure is 3 kcal higher in energy than the staggered structure.

Detailed explanation-2: -Such spatial arrangements of atoms which can be converted into one another by rotation around a C−C single bond are called conformations or conformers or rotamers.

Detailed explanation-3: -Unhindered (Free) Rotations Do Not Exist in Ethane So at normal temperatures, the carbon-carbon bond is constantly rotating.

Detailed explanation-4: -However, it may be remembered that rotation around a C-C single bond is not completely free. It is hindered by a small energy barrier of 1-20 kJ mol due to weak repulsive interaction between the adjacent bonds. Such a type of repulsive interaction is called torsional strain.

Detailed explanation-5: -Experiments show that there is a 12 kJ/mol (2.9 kcal/mol) barrier to rotation in ethane.