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Topic: What reagents would you use to achieve this transformation?  (Read 1838 times)

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Offline ostudent

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Hi everyone,

I just learned very basic knowledge about substitution, elimination, and addition reactions.
The problem is that I cannot solve any of these practice questions, but I'll just post one to see if I'm missing something(s) fundamental.

Thought process:
1. Eliminate, but E2 or E1?
       - regiochemistry: no information in this case since there is only one possibility. correct?
       - primary substrate. what does this tell me?
       - good leaving group... E1?
       - E2 is faster.
       - reagent: NaOEt??? wrong!
2. Add H and Cl. H on less substituted to achieve more stable carbocation; markovnikov addition. Thus, reagent: HCl. This part was fine...

How should I think for step 1? The real answer for the reagent is "t-BuOK or NaH" which are both strong bases. However, I believed t-BuOK was a sterically hindered base i.e. favors Hofmann i.e. assumes E2 mechanism. As drawn below, I don't see the possibility for Zaitsev or Hofmann products.

Thank you so so much for your time and feedback.

Offline spirochete

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Re: What reagents would you use to achieve this transformation?
« Reply #1 on: June 30, 2018, 01:47:54 AM »
There are two effects from a Sterically hindered strong base:

1) The effect you already mentioned, where a hoffman alkene is often favored as the result of an E2 reaction

2) The slowing down of a competing Sn2 reaction, which increases the yield of E2 elimination product. The Sn2 rate is very sensitive to steric hindrance, while E2 is much less sensitive. So E2 is more likely to "win the race" in this situation.

There is only one possible elimination product in the example you gave. So the product can't be described as either Zaitsev or Hoffman. But it is ideal to use a reagent that will minimize competing E2 reaction.

Sodium Hydride has a similar effect, but it's non nucleophilic for different, more complicated reasons. The end result is still a strong favoring of E2 over Sn2.


Offline spirochete

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Re: What reagents would you use to achieve this transformation?
« Reply #2 on: June 30, 2018, 03:10:10 PM »
Second to last paragraph has a big typo: Should say "minimize competing Sn2"!!

Offline ostudent

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Re: What reagents would you use to achieve this transformation?
« Reply #3 on: July 03, 2018, 12:39:44 PM »
Thank you! v. helpful explanation & all makes sense.

Offline ostudent

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Re: What reagents would you use to achieve this transformation?
« Reply #4 on: July 07, 2018, 05:29:59 PM »
Follow-up question. Hi all, this post is related to the above on how to prioritize for mechanism predicting.

"What reagents would you use to accomplish the following transformation?"
(see attached photo)
My Thought Process: Use NaOH for Sn2 mechanism because this is simple and substrate is primary.

Actual Answer: Reagents to use are t-BuOK (implying E2) then BH3 THF then H2O2, NaOH (implying adding H and OH antimarkovnikov)

Where did I go wrong? If I am able to choose the reagent, wouldn't the reaction proceed through simplest mechanism and a reagent that favors such mechanism?

Offline spirochete

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Re: What reagents would you use to achieve this transformation?
« Reply #5 on: July 11, 2018, 06:58:23 PM »
I think a primary halide with a non-sterically hindered strong base like hydroxide should give Sn2 as the major product, at least under some circumstances (maybe not too hot, and polar aprotic solvent just to maximize Sn2 and minimize E2). I believe this is the standard for the ACS final for undergraduates.

NaH would be an exception because it is non-nucleophilic, but you're not asking about that.

I don't know if the listed answer (elimination followed by anti-mark addition) would give a better yield. It seems beyond the scope of the Sophomore organic class to worry about such things. It would be a good question for your professor.





 

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