Orgo EK question

[-----MD]

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I am working through a passage in orgo EK - passage 1 in-class exam for lecture 2 question 26- and I'm confused about one answer. The question is:
If (R,R)- 2, 3 dichlorobutane is found to have a specific rotation of -25.66 then which of the compound has a specific rotation of +25.66. Firstly: aren't meso compounds optically inactive? And let's say they can be optically active (which I thought it can't) As far as I understand the only one to have a +/- rotation with the same degree would be the (s,s) form of the meso compound. The passage says retention of configuration is held, so I understand why ss cant form. But how can a diastereomer polarize light to the same angle if it only has one mirror image chiral carbon?

I am missing something, or am I just confused about configurations?

Thanks

( I am also gonna try to attach a pic of the question - but I'm on mobile, so I can't promise it'll work) thanks

 

[orangetea]

Okay maybe some clarification of a few concepts will make understanding this question a bit easier.

Meso compounds: 1.have an internal plane of symmetry, 2. have two or more substituents( in R and S configuration)
So if a compounds meets BOTH criteria then yes you are correct is will NOT rotate plane polarized light.

Enantiomers: These are stereoisomers that differ at BOTH stereocenters.


So now to the question, the passage states that A, B, C are optically active. C and D are stereoisomers.
Had you figured out the correct structures of the products you could have noticed that compound D has S stereochemistry and like you stated it will not be formed since the stereochemistry was retained.

Therefore we are left with C since A and B are not 2,3 dichlorobutanes.

hope that was clear enough! 🙂

 

[-----MD]

Ohhh
I misunderstood the question. I thought the RR mentioned is the d compound and thusly assumed the SS is the one that can't form due to retention. I guess a good Orgo tip is to draw any compounds mentioned instead of just assumptions. :/


So the question was find the S,S compound... That makes their explanation a lot clearer.

Thanks 🙂