Question

Isomers which can be interconverted through rotation about C–C single bond are

• A. diastereomers
• B. enantiomers
• C. conformers
• D. chain isomers
• E. position isomers

Hint:

Single bond → free rotation → conformers Double bond → restricted rotation → geometrical isomers

Answer 1

The Correct Option is C

Concept: Isomerism refers to compounds having the same molecular formula but different arrangements of atoms. A special type of stereoisomerism arises due to rotation about a single bond, known as conformational isomerism. The different spatial arrangements obtained due to rotation about a C–C single bond are called conformers.

Step 1: Nature of C–C single bond A carbon-carbon single bond is a sigma bond ($\sigma$ bond), which allows free rotation. This is because sigma bonds are cylindrically symmetrical and do not restrict movement of bonded atoms.

Step 2: What happens during rotation? When rotation occurs about a C–C bond, the relative positions of atoms/groups attached to the carbons change continuously. These different spatial arrangements are called conformations.

Step 3: Definition of conformers The isomers which differ only due to rotation about a single bond and can interconvert without breaking any bond are called: \[ Conformers (Conformational isomers) \]

Step 4: Example (Ethane molecule) Consider ethane (C$_2$H$_6$):
• When hydrogen atoms are staggered → staggered conformation (more stable)
• When hydrogen atoms align → eclipsed conformation (less stable) These two forms are interconverted simply by rotating one CH$_3$ group relative to the other.

Step 5: Energy consideration
• Staggered form → minimum energy
• Eclipsed form → maximum energy Despite energy differences, interconversion is easy at room temperature.

Step 6: Elimination of wrong options
• Diastereomers: differ in spatial arrangement but not interconverted by simple rotation
• Enantiomers: mirror images, require bond breaking to interconvert
• Chain isomers: differ in carbon skeleton
• Position isomers: differ in position of functional group None of these involve simple bond rotation. Final Conclusion: \[ \boxed{\text{Conformers are formed by rotation about C–C single bond}} \]