Question

Statement I: Staggered conformation of ethane is more stable than the eclipsed conformation.
Statement II: The torsional strain in staggered conformation is more.
Read the above statements and choose the correct answer from the options given below.

• A. Both Statement I and Statement II are false
• B. Both Statement I and Statement II are true
• C. Statement I is true but Statement II is false
• D. Statement I is false but Statement II is true

Hint:

Remember the energy profile for ethane's rotation. The energy is lowest (most stable) at the staggered conformations (60\(^\circ\), 180\(^\circ\), 300\(^\circ\)) and highest (least stable) at the eclipsed conformations (0\(^\circ\), 120\(^\circ\), 240\(^\circ\)). The energy difference is due to torsional strain.

Answer 1

The Correct Option is C

Step 1: Understanding Conformations and Stability.
Conformations are different spatial arrangements of atoms in a molecule that can be interconverted by rotation about single bonds. The stability of a conformation depends on its potential energy; lower energy means higher stability.
Step 2: Analyzing Statement I.
In ethane (CH\(_3\)-CH\(_3\)), the two main conformations are staggered and eclipsed.

• Staggered conformation: The hydrogen atoms on the back carbon are positioned exactly between the hydrogen atoms on the front carbon. This arrangement minimizes the repulsive forces between the electron clouds of the C-H bonds.
  
• Eclipsed conformation: The hydrogen atoms on the back carbon are directly behind the hydrogen atoms on the front carbon. This arrangement maximizes the repulsive forces.
  

Due to minimum repulsion, the staggered conformation has the lowest potential energy and is therefore the most stable. So, Statement I is true.
Step 3: Analyzing Statement II.
Torsional strain is the repulsion between the electron clouds of bonds on adjacent atoms when they pass by each other during rotation.

• In the eclipsed conformation, the C-H bonds are closest to each other, leading to maximum torsional strain.
  
• In the staggered conformation, the C-H bonds are farthest apart, resulting in minimum torsional strain.
  

Statement II says that the torsional strain in the staggered conformation is more (implying maximum), which is incorrect. Torsional strain is minimum in the staggered conformation. So, Statement II is false.
Step 4: Final Answer:
Statement I is true, and Statement II is false.