SN2-Inversion of Configuration

[G1SG2]

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Compound A is a pure enantiomer with one chiral center having an absolute configuration of R. If the chiral center is the location of an SN2 reaction, then the product will be:

A. pure compound B, which is exclusively R
B. pure compound B', which is exclusively S
C. a racemic mixture of Compounds B (which is exclusively R) and B' (which is exclusively S).
D. pure Compound B (which is exclusively R) or pure Compound B' (which is exclusively S); there is insufficient information to determine which one is actually formed.

I picked B because I thought of inversion of configuration...anytime I'm faced with an SN2 reaction, I NEVER try to figure out the absolute configuration of the product...I automatically assume that the absolute configuration will be opposite to that of my reactant. But the answer is D. The explanation is: because the leaving group is replaced by a nucleophile, group prioritization may change such that the absolute configuration of the product is the same as that of the reactant. Therefore, choice D is the answer, since the product will be entirely R or S.

Should I be taking a closer look at the nucleophile/resulting product when I have an SN2 reaction, as the priorities can possibly change?

 

[letaps]

yea i was confused by this question as well, i chose B too because i thought anytime you have an SN2 reaction, the product is inverted...

 

[Doodl3s]

Think, when you do an SN2 you're kicking off a group and adding a new group. So its possible the new group is no longer the same 'priority number' as the group you kicked off. For example, lets say you kick off a Br- for a CH3O-... The O that now took the place of Br is a much lower atomic number, and something like a connected fluorine make take priority! and therefore change the whole numbering scheme. (D)

 

[minutemen11]

YES you always need to consider what is being substituted, trick question that shows up sometimes on the mcat.. i remember a similar question like this in my kaplan class, is that where you got it from?

 

[G1SG2]

YES you always need to consider what is being substituted, trick question that shows up sometimes on the mcat.. i remember a similar question like this in my kaplan class, is that where you got it from?

Nope, this is a TPR question. Thanks for the responses, everyone!

 

[sv3]

Yeah i still don't know why the prep books say inversion - what exactly is being inverted? You can't tell unless you know the identity of all the groups on the chiral carbon right?

 

[Podalarius]

http://en.wikipedia.org/wiki/File:BromoethaneSN2reaction-small.png

That reaction involves an inversion because the groups that were on the left are now on the right. Given a pure reactant, you should not make a racemic mixture with this mechanism. Nevertheless, you may go from R to R, or S to S, if the priority of the groups changes.

 

[matth87]

Tricky! Nice question!