Question

Rotational angle required to get maximum stable conformer from minimum stable conformer in n-butane is:
1. ${360}^{\circ }$
2. ${180}^{\circ }$
3. ${120}^{\circ }$
4. ${240}^{\circ }$

Answer 1

Hint: As we know that different conformations are the arrangement of two atoms in a molecule that are found to differ by rotation. And the study of energy between different conformations is called conformational analysis.

Complete Solution :
- By rotating the back carbon $C_2-C_3$ about axis by an angle of ${180}^{\circ }$, the resulting conformation is called as Anti conformation or staggered conformation.
- We can see from the conformation drawn below:

- We can see from the above staggered conformation that all the groups on $C_2$ and $C_3$ carbons are maximum apart. Therefore, it is found that the torsional and the steric interactions are minimum. ( the bulkier two$C{H}_3$ groups are at ${180}^{\circ }$).
- Hence, it is found that this is the most stable and the most preferred conformation of n-butane. Therefore, its potential energy is considered as zero and the energies of all other conformation is determined relative to this.
- Hence, we can say that the correct option is (1), that is the Rotational angle required to get maximum stable conformer from minimum stable conformer in n-butane is ${180}^{\circ }$
So, the correct answer is “Option 2”.

Note: - We should note the main difference in between eclipsed and staggered conformation. In eclipsed conformation, the carbons are aligned in such a way that the hydrogens are lined up with each other.
- In staggered conformation atoms are found to be equally spaced from each other.