(•••) In contrast to ethane and other alkanes studied in this chapter, there is no free rotation around any bonds in cyclopentane (shown below). Why?


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Verified step by step guidance

1. Identify the structure of cyclopentane, which is a cyclic alkane with the molecular formula C_5H_10.
2. Understand that cyclopentane forms a ring structure, which restricts the rotation around the carbon-carbon bonds.
3. Recognize that in cyclic structures, the carbon atoms are connected in a loop, creating a constraint that prevents free rotation.
4. Consider the concept of ring strain, which arises from the angles and torsional strain in the cyclic structure, further limiting rotation.
5. Conclude that the lack of free rotation in cyclopentane is due to the rigidity of the ring structure, which holds the carbon atoms in fixed positions relative to each other.

Key Concepts

Cyclic Structures

Cyclic structures, such as cyclopentane, are closed-loop molecules where carbon atoms form a ring. This configuration can introduce strain and restrict the rotation around bonds due to the fixed positions of the atoms in the ring, leading to unique properties compared to linear alkanes like ethane.

Bond Rotation and Steric Hindrance

In organic chemistry, bond rotation refers to the ability of atoms to rotate around a bond axis. In cyclopentane, steric hindrance occurs when atoms or groups are too close to each other, preventing free rotation and leading to a more rigid structure compared to alkanes, which can rotate freely around their single bonds.

Angle Strain

Angle strain arises when bond angles deviate from their ideal values, causing increased energy in a molecule. In cyclopentane, the ideal tetrahedral angle is 109.5 degrees, but the angles in the ring are slightly less, leading to angle strain that contributes to the restricted rotation and stability of the molecule.