2011 dat destroyer OC #89

[dental29]

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Can someone please explain how there are 3 stereoisomers and not 2? Isn't compound III meso?

 

[DrRoyal Pains]

Meso compound my friend. It has an internal plane of symmetry. Look out for that stuff.

 

[dental29]

Yes I understand that. But I dont understand the solution. It shows 3 compounds. Isnt it supposed to be 2 stereoisomers because the other is meso?

 

[DrRoyal Pains]

No. Remember there are 2^n stereoisomers, with n being the number of chiral centers.

 

[dental29]

then how do you get 3?

 

[DrRoyal Pains]

The meso compound does not count as two different isomers. It is optically inactive. For example, one form could have the designation RR, but its other form will have the designation SS. Here, I got this off of chemwiki:In general, a meso compound should contain two or more identical substituted stereocenters. Also, it has an internal symmetry plane that divides the compound in half. These two halves reflect each other by the internal mirror. The stereochemistry of stereocenters should "cancel out". What it means here is that when we have an internal plane that splits the compound into two symmetrical sides, the stereochemistry of both left and right side should be opposite to each other, and therefore, result in optically inactive. Cyclic compounds may also be meso.

 

[dental29]

thanks! so basically its not 4 stereoisomers because 2 out of the 4 are meso and the meso is hence only counted for once.

 

[DrRoyal Pains]

Exactly 👍. Be aware of meso's though. Just remember if you have 2 chiral centers with the same substituent and an internal plane of symmetry, its probably meso.

 

[dental29]

speaking about symmetry do you know how the compound in choice a) of question #100 is symmetrical and compound in choice d) is not?