Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: scientific on July 11, 2017, 12:02:57 AM
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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?
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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.
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@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.