OCHEM #193 - 2016 Destroyer

[moose786]

"Which halide will undergo SN2 reaction most readily?"

I understand that all 4 choices suck, but I was between B & C when choosing the "most ready to undergo SN2". This is because I thought that even though tertiary halide is generally the worst scenario for SN2 to occur, you still get some of it to occur (even though minimal). Hence, I was in a toss-up between B, something that had steric hindrance preventing backside attack, versus C, which is a tertiary with a bad leaving group. I was just trying to figure out which one of the 2 would be the "lesser evil," but it turns out the answer is "none".

I'm probably forgetting some technical rules here that state that "no product will be formed" versus "very little will be formed". But I can't seem to recall them. Anyone care to explain where my thinking is flawed? Or if I'm thinking too much into it? I really don't want to be caught in the dilemma on test day if such a problem arises.

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[DentalNucleicAcid]

I think you're thinking too much into it. I think for our purposes, we're supposed to know that a tertiary leaving group will never undergo SN2, not even a little bit. Maybe it will give you a tiny bit of product in real life, I don't know, but this is the something that we were taught would never happen. Same thing for vinylic and aryl leaving groups, subsitution/elimination will never happen. Again, maybe it could give you a minute fraction of product in real life since no rule is usually 100% correct, but for a prehealth exam, I think they just want you to choose the "book" answer. On a side note, if you ever see that "none of these" or "no reaction" is ever an answer choice, beware that something might be fishy about the question, i.e. it might be a trick question.

 

[DAT Destroyer]

"Which halide will undergo SN2 reaction most readily?"

I understand that all 4 choices suck, but I was between B & C when choosing the "most ready to undergo SN2". This is because I thought that even though tertiary halide is generally the worst scenario for SN2 to occur, you still get some of it to occur (even though minimal). Hence, I was in a toss-up between B, something that had steric hindrance preventing backside attack, versus C, which is a tertiary with a bad leaving group. I was just trying to figure out which one of the 2 would be the "lesser evil," but it turns out the answer is "none".

I'm probably forgetting some technical rules here that state that "no product will be formed" versus "very little will be formed". But I can't seem to recall them. Anyone care to explain where my thinking is flawed? Or if I'm thinking too much into it? I really don't want to be caught in the dilemma on test day if such a problem arises.

Hello moose786,

You are thinking this much too deeply, and would result in the KISS of DEATH !!!!! A tertiary will not react at all in an SN2 process.......do not think it would result one in a billion......NO. The answer is No reaction !!!!! Remember......aryl halides, vinyl halides, bridged bicyclo compounds, and tertiary halides do NOT do the Sn2 process.

Hope this helps

Dr. Romano