The respective configutations of a and b are...? enter image description here

I tried to find the answer for a (for me is: 2S, 3S) but I'm not sure of my results, can someone help me?

enter image description here


1 Answer 1


Here is my sage advice to you, Silvia95. Do not redraw structures!!! Use the method described here. Transcribing structures to place the lowest priority group in the rear will invariably lead to errors. Leave the structure where it is and use your hands as tools to determine chirality. Make a "thumbs up" gesture with either hand and point the thumb from the asymmetric carbon to the lowest priority group. Your fingers should point in descending order from highest priority to the next to lowest priority. If not, you are using the wrong hand. Right hand, R; left hand, S.

C2 of dibromide a has the hydrogen in the rear. The fingers of your left hand show Br>C3>C1>C2 in dibromide b has the hydrogen in the front. Now the thumb of your left hand points toward you and Br>C3>C1. Using the same technique for C3 of a and b, your right hand fingers reveals Br>C2>C4. These two structures are identical, (2S,3R)-2,3-dibromopentane. If you rotate either structure by 180o about a vertical axis in the plane of the screen, the two structures will superimpose.

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    $\begingroup$ Of course, there's a shortcut for this particular problem if you've cultivated some skill in visualization. It is possible to recognize directly that the two figures represent the same diastereomer: they are related to each other by a 180-degree rotation about an axis parallel to vertical direction of the page, and (pure) rotations preserve hand. With that fact in mind, one can observe that only one of the answers offered has the needed property. $\endgroup$ Commented May 22, 2021 at 17:16
  • $\begingroup$ Clearly, I'm well-aware of their identity. My pet-peeve is the way textbooks teach the rules for R/S and the usual consequences of novices redrawing structures. $\endgroup$
    – user55119
    Commented May 22, 2021 at 18:49
  • $\begingroup$ I can't speak much to the current crop of textbooks, but I will say that back in the day, it was the method you describe in this answer that I learned. $\endgroup$ Commented May 22, 2021 at 19:23

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