Two staggered Newman projections of 2,3-dibromobutane, viewed along C2-C3, are given. Projection 1: front carbon has Br at 12, CH3 at 4 and H at 8 o'clock; rear carbon has CH3 at 6, Br at 10 and H at 2 o'clock. Projection 2: front carbon has CH3 at 12, H at 4 and Br at 8 o'clock; rear carbon has Br at 2, H at 6 and CH3 at 10 o'clock. They represent:

the same stereoisomer in two different conformations

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Two staggered Newman projections of 2,3-dibromobutane, viewed along C2-C3, are given. Projection 1: front carbon has Br at 12, $CH X_{3}$ at 4 and H at 8 o'clock; rear carbon has $CH X_{3}$ at 6, Br at 10 and H at 2 o'clock. Projection 2: front carbon has $CH X_{3}$ at 12, H at 4 and Br at 8 o'clock; rear carbon has Br at 2, H at 6 and $CH X_{3}$ at 10 o'clock. They represent:

Aconstitutional isomers
Bthe same stereoisomer in two different conformations
Cenantiomers
Ddiastereomers

Answer: B

Assign each carbon with the exchange method (swap the lowest priority group H with the group pointing along the viewing axis). Projection 1 gives R at the front carbon and R at the rear carbon. Projection 2 also gives R at the front and R at the rear. Both are (2R,3R), and they differ only by a rotation about C2-C3 (Br/Br gauche in 1, anti in 2), so they are conformers of one stereoisomer.

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Answer: B

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