In the lowest energy (anti) conformation of 2-bromobutane, which group is gauche to the ${\mathrm{Br}}_{}$ group?

• A. No group is gauche to the ${\mathrm{Br}}_{}$ group in the anti conformation
• B. The ethyl group (${\mathrm{CH}}_2$${\mathrm{CH}}_3$)
• C. The hydrogen atom
• D. The methyl group (${\mathrm{CH}}_3$)

Answer: (D)

Verified step by step guidance

1. Identify the structure of 2-bromobutane, which has a bromine (Br) substituent on the second carbon of a butane chain. The groups attached to C-2 are Br, a methyl group (CH₃), an ethyl group (CH₂CH₃), and a hydrogen atom (H).
2. Recall that the anti conformation is the lowest energy staggered conformation where the largest groups on adjacent carbons are positioned 180° apart to minimize steric hindrance. In 2-bromobutane, the Br group is anti to the ethyl group (CH₂CH₃).
3. Understand that gauche interactions occur when two groups are 60° apart in a staggered conformation. In the anti conformation, the group 60° from Br is gauche to it.
4. Determine which group is gauche to Br by looking at the Newman projection down the C1–C2 bond or C2–C3 bond, depending on the perspective. Since Br is anti to the ethyl group, the groups gauche to Br are the methyl group (CH₃) and the hydrogen atom (H).
5. Compare the sizes of the groups gauche to Br. The methyl group (CH₃) is larger than hydrogen, so the methyl group is the one gauche to Br in the lowest energy anti conformation.