TBR Organic Chem II Phase I

[DPTinthemaking15]

This section tore me a new one (13/25), but it is helpful learning from my mistakes. Anyways, I have one question that I cant understand. Here it is:

18. Which of the following will lead to a product mixture composed of more than fifty percent of one of the enantiomers (the product mixture not being racemic)?

A. Carrying the reaction out with a chiral solvent
B. Carrying the reaction out with a chiral catalyst
C. Carrying the reaction out at a lower temperature
D. Carrying the reaction out at a higher concentration

I eliminated answer choices A and C, but I have a hard time understanding the reasoning between B and D. Here is TBR's explanation about concentration "The concentration does not affect the alignment of the molecules in the transition state, only the frequency with which the reactants collide to form the transition state."

My thought process: I assumed if we are creating a mixture that is not racemic, we will have an SN2 mechanism that will occur. A SN2 mechanism depends upon the k[nucl] [Elect], correct? Shouldn't a higher concentration=a better product mixture? Or should is a chiral cataylst the gold standard for creating a better mixture?

Thank you in advance for your help!

---

Members don't see this ad.

 

[MCATLasagna]

Hey, my understanding may not be a 100% on this but the way I think about it is:
A catalyst lowers the EA to get to the TS. If the catalyst is chiral then the majority of the TS intermediate will also have to be chiral leading to the product being mainly once enantiomer.


***Also since it's asking about enantiomeric excess I'd assume this is in the context of an SN1 ****
An SN2 will usually give you close to a majority of the opposite enantiomer

 

[DPTinthemaking15]

Hey, my understanding may not be a 100% on this but the way I think about it is:
A catalyst lowers the EA to get to the TS. If the catalyst is chiral then the majority of the TS intermediate will also have to be chiral leading to the product being mainly once enantiomer.


***Also since it's asking about enantiomeric excess I'd assume this is in the context of an SN1 ****
An SN2 will usually give you close to a majority of the opposite enantiomer

Hey! Thank you for the response, but can you help with a few gray areas. In a SN1 reaction a racemic mixture forms, so how will it produce enantiomeric excess? I would assume that it would make a half-and-half mixture. For some reason I cannot wrap my head around this.

 

[aldol16]

You really shouldn't be using such a specific example for a general question like this. Yes, SN2 reactions do form enantiomerically-specific products. But that's not the only reaction ever that does that. Here's a general rule: in order for chirality to be introduced into a system, something that is participating in the reaction itself must be chiral. Simply increasing the concentration doesn't change chirality. (And in your example above, increasing the concentration would change the rate of the reaction - the reaction still proceeds with the exact same stereospecificity, you're just making it go faster) So in this case, only a chiral catalyst can induce chirality - and in fact you should understand that this is how nature is able to function. Nature uses protein catalysts with defined chiralities that results in us being able to synthesize chiral products. Without chiral enzymes, we would never be able to form chiral products.