Which of the following is not among the standard conformations of $\mathrm{butane}$?

• A. $\mathrm{Eclipsed}$
• B. $\mathrm{Gauche}$
• C. $\mathrm{Cis}$
• D. $\mathrm{Anti}$

Answer: (C)

Verified step by step guidance

1. Understand that conformations refer to different spatial arrangements of atoms in a molecule that can be interconverted by rotation around single bonds without breaking any bonds.
2. Recall the common conformations of butane, which include the 'anti' conformation (where the two largest groups are 180° apart), the 'gauche' conformation (where the two largest groups are 60° apart), and the 'eclipsed' conformation (where groups are aligned, causing torsional strain).
3. Recognize that 'cis' and 'trans' are terms used to describe stereoisomers related to double bonds or ring systems, not conformations arising from rotation around single bonds.
4. Identify that among the options given—Eclipsed, Gauche, Cis, and Anti—the term 'Cis' does not describe a standard conformation of butane because butane does not have a double bond or ring to exhibit cis/trans isomerism.
5. Conclude that 'Cis' is not a standard conformation of butane, while Eclipsed, Gauche, and Anti are recognized conformations based on rotation around the central C-C bond.