Question

Number of conformational isomers of \emph{n-butane:}

• A. One-anti \& one-gauche
• B. One-anti \& two-gauche
• C. Two-anti \& one-gauche
• D. Two-anti \& two-gauche

Hint:

Conformational isomers differ by rotation around single bonds. For alkanes like \emph{n}-butane, analyze using Newman projections for anti and gauche forms.

Answer 1

The Correct Option is B

- \emph{n-Butane} has the structural formula: $$ \text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_3 $$ - When viewed along the central C–C bond (C2–C3), different conformations arise due to rotation about this bond. - The key conformers are: - Anti: The two methyl groups are \(180^\circ\) apart (most stable). - Gauche: The methyl groups are \(60^\circ\) apart (less stable). - Eclipsed: The methyls and/or hydrogens are aligned (least stable — not counted here as stable conformers). - In \emph{n}-butane, due to symmetry: - There is 1 anti conformer. - There are 2 equivalent gauche conformers (mirror images of each other). - Hence, the molecule has 3 significant conformational isomers: - 1 anti + 2 gauche