For $2-\mathrm{butene}$, which conformation has the lowest energy due to minimized steric interactions?

• A. A syn conformation, where all substituents are eclipsed
• B. Cis conformation, where the two methyl groups are on the same side of the double bond
• C. Trans conformation, where the two methyl groups are on opposite sides of the double bond
• D. A gauche conformation, where the methyl groups are staggered but not directly opposite

Answer: (C)

Verified step by step guidance

1. Identify the molecule in question: 2-butene, which has a carbon-carbon double bond between the second and third carbons, with methyl groups attached to these carbons.
2. Understand the types of conformations possible around the double bond: because of the double bond's rigidity, rotation is restricted, so conformations refer mainly to the relative positions of substituents across the double bond (cis or trans).
3. Recall that in alkenes, the cis conformation has substituents (like methyl groups) on the same side of the double bond, while the trans conformation has them on opposite sides.
4. Consider steric interactions: substituents on the same side (cis) experience more steric hindrance due to their proximity, increasing the molecule's energy, whereas substituents on opposite sides (trans) minimize steric repulsion, lowering the energy.
5. Conclude that the trans conformation of 2-butene has the lowest energy because the two methyl groups are positioned opposite each other, minimizing steric interactions and thus stabilizing the molecule.