stereoisomers

[I GET BUCKETS]

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the only thing I knew before the explanation is that the molecule above is S. But can someone explain how I'm supposed to know if it's + or - since that can only be determined experimentally?

 

[humblethinker]

Hopefully this strategy makes sense...

a) induction b/c dextroamphetamine is S, this makes levoamphetamine R, therefore A must be true.
c) dextrorotatory compounds are indicated with a +, therefore C must be true.
d) d=dextrorotatory therefore answer choice D must be true.

But think about the problem like this....we know A is true based on induction in the problem. The real decision is to figure out between b,c,d which one is not true. If you look at d, if this is considered true, then c must be true (d = dextro = +). Considering D and C rely on each other, {[from D, d=dextro] = [from C, dextro = +]} therefore, D and C = true. Using strategic thinking, B is the outlier that is not reliant on any other choice.