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Topic: surprising answers for Sn2 vs Sn1  (Read 397 times)

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Offline OrganicH2O

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surprising answers for Sn2 vs Sn1
« on: May 08, 2021, 09:45:36 PM »
In this homework I was surprised especially by the answer for B, and slightly unsure about D, although I was leaning toward Sn2. For both of these questions, E2/E1 was not considered as a possibility. We are only supposed to consider substitution, although any thoughts about elimination are welcome.

I attached pictures of both of them.

B is a primary halide with acetate as a nucleophile and acetic acid as solvent. I was thinking it should be a slow Sn2, because the solvent is protic. Professor is saying Sn1. On a primary halide?? With no rearrangement??

And then for D, it is a secondary halide with cyanide  as a nucleophile and methanol solvent. I  was again thinking it should be a kind of slow Sn2, because the solvent should is protic. Professor is saying Sn1. The whole issue of Sn1 on secondary is controversial, and cyanide is a pretty good nucleophile. I don't remember seeing these exact conditions before in any textbook.

Anyone have any thoughts?
I have a Master's in organic chemistry and I am exposed to a LOT of different introductory organic chem classes in the course of my work, ranging from very basic to Harvard. I am here to refine my knowledge and consult with other organic chemistry nerds.

Offline rolnor

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Re: surprising answers for Sn2 vs Sn1
« Reply #1 on: May 09, 2021, 02:08:14 AM »
I agree with you, I would say d is possibly a mixture of Sn1 and Sn2 and b is pure Sn2. But also, acetic acid is a unusuall solvent so one has to look in the original paper, if there is one.

Offline Meter

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Re: surprising answers for Sn2 vs Sn1
« Reply #2 on: May 09, 2021, 05:37:33 AM »
I agree with you, I would say d is possibly a mixture of Sn1 and Sn2 and b is pure Sn2. But also, acetic acid is a unusuall solvent so one has to look in the original paper, if there is one.
Carbocation stability depends on its ability to hyperconjugate with adjacent atoms. Can hyperconjugation occur between molecules?

Offline rolnor

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Re: surprising answers for Sn2 vs Sn1
« Reply #3 on: May 09, 2021, 06:41:20 AM »
I am not sure I understand what you mean meter, what I wonder is if, with a very strong acid its maybe possible to protonate a primary alkyl bromide and this could create a carbocation. The acid could be TFMSA. But then you should see rearangement also. It would be nice to see what paper the professor has taken this example from.

Offline OrganicH2O

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Re: surprising answers for Sn2 vs Sn1
« Reply #4 on: May 09, 2021, 03:23:33 PM »
I agree with you, I would say d is possibly a mixture of Sn1 and Sn2 and b is pure Sn2. But also, acetic acid is a unusuall solvent so one has to look in the original paper, if there is one.
Carbocation stability depends on its ability to hyperconjugate with adjacent atoms. Can hyperconjugation occur between molecules?

When Rolnor says "mixture of Sn1 and Sn2", they mean two separate pathways/mechanisms. I am sure they know what hyperconjugation is.

For example, if the experimental outcome is 80% inversion and 20% retention, this could be explained by a mixture of different mechanisms. Unfortunately, this could also be explained by some kind of ion pairing argument, and other things maybe like a reversible Sn2, so there would need to be some other data to be sure.

Thank you for your input, Rolnor. If I can, I will contact the writer of the question and see if they have a reference. This would be a crapshoot, as it is possible they just made it up off the top of their head.
I have a Master's in organic chemistry and I am exposed to a LOT of different introductory organic chem classes in the course of my work, ranging from very basic to Harvard. I am here to refine my knowledge and consult with other organic chemistry nerds.

Offline Meter

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Re: surprising answers for Sn2 vs Sn1
« Reply #5 on: May 09, 2021, 03:30:02 PM »
I agree with you, I would say d is possibly a mixture of Sn1 and Sn2 and b is pure Sn2. But also, acetic acid is a unusuall solvent so one has to look in the original paper, if there is one.
Carbocation stability depends on its ability to hyperconjugate with adjacent atoms. Can hyperconjugation occur between molecules?

When Rolnor says "mixture of Sn1 and Sn2", they mean two separate pathways/mechanisms. I am sure they know what hyperconjugation is.

For example, if the experimental outcome is 80% inversion and 20% retention, this could be explained by a mixture of different mechanisms. Unfortunately, this could also be explained by some kind of ion pairing argument, and other things maybe like a reversible Sn2, so there would need to be some other data to be sure.

Thank you for your input, Rolnor. If I can, I will contact the writer of the question and see if they have a reference. This would be a crapshoot, as it is possible they just made it up off the top of their head.
It's most likely just a brain fart then. I don't see how SN1 could proceed on a primary alkyl halide unless some exotic chemistry was going on. Maybe the author ment to but the Br on the second carbon, where SN2/SN1 are both possible, but ultimately the nucleophile/solvent decided?

Offline Meter

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Re: surprising answers for Sn2 vs Sn1
« Reply #6 on: May 09, 2021, 03:31:26 PM »
Also, I didn't mean to insinuate that rolnor didn't know what hyperconjugation is. It was a geniune, open-ended question.

Offline Babcock_Hall

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Re: surprising answers for Sn2 vs Sn1
« Reply #7 on: May 10, 2021, 08:53:59 AM »
It would be surprising if (b) were SN1.  I sometimes think (just as a pedagogical issue) that we overemphasize this topic and sometimes choose examples that are murkier than is ideal.  Example (d) might fall into this category.

Offline OrganicH2O

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Re: surprising answers for Sn2 vs Sn1
« Reply #8 on: May 11, 2021, 03:27:39 PM »
To meter: sorry my my assumption. Many people on this forum ask leading questions to tease out answers. To my knowledge, hyperconjugation does not occur between molecules significantly, maybe because other general things like dipole-ion, etc are more significant there.

To Babcock: I agree about the murkiness! I would much rather see a bigger focus on questions that actually test various bits of fundamental knowledge that are more significant. The question for reaction D could be: "A young chemist proposed this reaction with cyanide and methanol and a secondary halide to their adviser. They were chastised for proposing a terrible reaction. Why?"

We should create exam problems that either show examples of reactions that actually work well, or separate problems that allow a demonstration of conceptual knowledge. This topic is already tough enough for students even in the best of circumstances.
I have a Master's in organic chemistry and I am exposed to a LOT of different introductory organic chem classes in the course of my work, ranging from very basic to Harvard. I am here to refine my knowledge and consult with other organic chemistry nerds.

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