[RiceKernel]

Hi guys,

I don't really have a clue on where to start or if its even possible to do what I described in the title.

My inquiry originates from this practice exam question :

Which of the following molecules is achiral?
A) (2R,3R)-2,3-Dichloropentane
B) (2R,3S)-2,3-Dichloropentane
C) (2S,4S)-2,4-Dichloropentane
D) (2S,4R)-2,4-Dichloropentane
E) Two of these

The answer is D and I have no idea why. I've tried building some models but it's still hard to understand. Finding and labeling those centers are not a problem itself but I don't think I understand the significance of the R-S system.

If there's a trick that we can use , it would be much appreciated to learn here.

[Dan]

If there's a trick that we can use , it would be much appreciated to learn here.

There is a trick of sorts - the concept at work here is symmetry. If a molecule has a plane or centre of symmetry it will be superimposable on its mirror image and thus achiral. Look up "meso compound" in your textbook/online and see what you can come up with.

R/S nomenclature is used to precisely describe the configuration of asymmetric centres. The learning point is that the presence of asymmetric centres does not automatically make a molecule chiral.