destroyer orgo #37

[atlanta213]

Do you guys know why the answer is B which is 3 stereo isomers?

The answer key says it should be 3 because of meso compound even though it has 2 chiral centers. Originally, 2 chiral centers should have 4. However, I believe there are 2 meso compounds including 3 methyl goes up and goes down? Therefore, I believe the answer should be 2 instead of 3?

Could you guys explain this problem please!!!!!!!!!!!!!

Thanks in advance.

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[imeddie]

in order for it to be meso, the two methyl groups need to be on the same side. If one is wedge and the other is dash, then its not meso.

Since meso compounds do not have an enantiomer, its 2^2 - 1 = 3 stereoisomers

 

[atlanta213]

Thanks for your reply. However, I am still confused. As you said, in order to be meso compound, methyl group should be on the same side. 2 methyl group could be both dash or both wedge. Isn't that 2 meso compound? 2^2 -2= 2?

 

[ryry]

both wedge and both dash count as same compound (flip one of them, and u will see)

so basically u have (R,S), (S,R) plus the meso, which makes it 3 isomers.

 

[jdent]

Drawing the "4" possibilities you'll see that 2 of them are exactly the same- the two that are meso. The remaining two left over are the only 2 sterioisomers there. basically do the 2n rule and then make sure their are no meso compounds.