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Topic: deuterium vs hydrogen abstraction  (Read 2337 times)

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Offline scientific

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deuterium vs hydrogen abstraction
« on: July 11, 2017, 12:02:57 AM »
Hello,

Please see the attached image.

I came across this question, and I was able to cross off 2/4 of the answers. I don't understand the final solution however. I believe this is an E2 reaction, and operated under the assumption that D was not ideal to remove.

(A) makes no sense because an H and the D get abstracted instead of the Br
(B) the configuration made no sense

I chose (C) because I thought isotopes made reactions occur more slowly. Or is deuterium more acidic than hydrogen?

Offline phth

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Re: deuterium vs hydrogen abstraction
« Reply #1 on: July 11, 2017, 01:09:05 AM »
It will be C  if there is a syn-periplanar elimination deuterium. it will be D if there is an anti-periplanar elimination of deuterium.  Therefore, APP>SPP and the answer is D.  Syn-periplanar eliminations are possible but they are significantly higher in activation energy than the anti configuration.

a) not possible
b) SPP= high energy
c) SPP= high energy
d) answer

The H vs D isotope effect is the trick part of the question.  It is relatively insignificant.

Offline Babcock_Hall

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Re: deuterium vs hydrogen abstraction
« Reply #2 on: July 11, 2017, 10:21:51 AM »
@OP, Do you know what an antiperiplanar arrangement is?  An oversimplified theory suggests that breaking a C-D bond will be up to sevenfold slower than breaking a C-H bond, but many factors contribute to the magnitude of the effect on the rate of the reaction.  I agree with pgk that this kinetic isotope effect (KIE) can be ignored in solving this problem.  I think that the deuterium is just functioning as a label here.
« Last Edit: July 11, 2017, 12:54:05 PM by Babcock_Hall »

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