Sn1/Sn2 related question

[yui_96]

"Treatment of (S)-2-bromobutane with sodium hydroxide results in the production of a compound with an (R) configuration. This reaction has most likely take place through:
A. an Sn1 mechanism
B. an Sn2 mechanism
C. an Sn1 and Sn2 mechanism in sequence
D. an Sn1 and Sn2 mechanism simultaneously

The answer given here is B. I understand that Sn1 reaction is characterized by racemization and Sn2 reaction is characterized by a change in configuration, but is it possible that both reactions occur simultaneously? Why or why not?

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[BBBallIsLife]

id say not simultaneous b/c SN2 do well w/ aprotic solvents while SN1 use protic solvents --> you run the risk of protonating the Nu

 

[esob]

For an SN1 to be the major product on a secondary you would need a weak base/weak nucleophile and you would have to keep the temperature down in that scenario or you would end up with an elimination rxn.

 

[NextStepTutor_1]

Hi @yui_96 -

This may seem obvious but I think it bears mentioning -- part of getting into the test-taking mindset is working with what a question is giving you, and trying not to read too much into it. (In general, you want to hit the sweet spot between not reading the question carefully, on one hand, and going beyond what the question gives you, on the other). By telling you that the reaction results in a product with an (R) configuration, the question is telling you that Sn2 chemistry took place. It's essentially just asking you to recognize that stereospecificity is associated with Sn2.

In terms of the bigger picture of Sn1/Sn2/E1/E2 chemistry here, remember that side reactions often take place, so the question is often more about what product will be predominant. Even though a polar protic solvent like what we see here would favor Sn1 over Sn2 chemistry, we can't consider that fact in isolation. Sn1 chemistry is much less likely to take place at a secondary carbon that at a tertiary carbon, unless there are some special circumstances like carbocation rearrangement that might favor Sn1. That alone should push us to consider Sn2. Additionally, OH- is a very good nucleophile as well as being a strong base, which also favors Sn2. As @esob points out, E2 would also be a possibility depending on temperature, but luckily for us on this question it's not given as a possible answer choice. (Elimination chemistry is not specifically included as a testable content item on the current version of the MCAT, although it may show up as a distractor answer choice -- for the MCAT, you probably want to be aware of elimination chemistry, but for specific problems, you should think Sn1/Sn2).

Hope this helps clarify things & best of luck studying!

 

[PlsLetMeIn21]

Maybe I'm missing something, but this question seems straight forward. The product has inversion from S to R and you are asked if it's either SN1, SN2, or both. Inversion is SN2.

 

[esob]

Maybe I'm missing something, but this question seems straight forward. The product has inversion from S to R and you are asked if it's either SN1, SN2, or both. Inversion is SN2.

SN1 results in both retention and inversion and there is no clarification that the compound produced is the ONLY compound. The key here is probability and a secondary alkyl halide with a strong nucleophile has the highest probability of undergoing an SN2 reaction. You could pick SN2 even if they didn't tell you that you had an inverted product if you are working on highest probability only.

 

[aldol16]

The reason why both reactions don't occur simultaneously is because you can think of the reaction as proceeding through one of two pathways. There's a fork in the road and the system will always take the road that results in the lowest free energy. And in this case, that happens to be the SN2 pathway. Also, if both were happening simultaneously, you wouldn't observed enantiopure product. You would observe a mix of R and S products that's biased towards one or the other but with both present.