Stereochemistry problem

[heylollipop]

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Which of the following compounds does NOT show any optical rotation of planar light?

A. 2R, 3R-dibromobutane
B. 2R, 3S-dibromobutane
C. (eliminated)
D. (eliminated)

So my thought process was that this compound would have to be a meso compound, for it to be not optically active. I chose A because I thought that the Br substituents on the same side would make the compound meso. Why is the answer B and not A?

 

[sazerac]

Which of the following compounds does NOT show any optical rotation of planar light?

A. 2R, 3R-dibromobutane
B. 2R, 3S-dibromobutane
C. (eliminated)
D. (eliminated)

So my thought process was that this compound would have to be a meso compound, for it to be not optically active. I chose A because I thought that the Br substituents on the same side would make the compound meso. Why is the answer B and not A?

Because only in B are the Br substituents on the same side.

 

[phattestlewt]

B is meso, A is not meso.

I actually have a hint that works great for this problem and most other meso compound related questions.

http://forums.studentdoctor.net/threads/a-useful-hint.1040302/

 

[SpaceBuff12]

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