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Topic: Kinetic isotope effect of this molecule...  (Read 1442 times)

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Offline xshadow

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Kinetic isotope effect of this molecule...
« on: February 18, 2020, 03:35:24 PM »
Hi

I have some doubt about the "type"" of KIE  that I have when I label the green carbon :



Is the breaking of a C=C (only the π-bond part)  considered a primary isotope effect or a secondary isotope effect ?!?
(breaking the double bond the carbon  changes its hybridation  sp2-->sp3)


Thanks!!

Offline hollytara

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Re: Kinetic isotope effect of this molecule...
« Reply #1 on: February 18, 2020, 07:24:34 PM »
I think it is primary - you are breaking a bond to the isotopically substituted atom. 

But 12C vs 13C gives a small effect even for primary.


Offline Babcock_Hall

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Re: Kinetic isotope effect of this molecule...
« Reply #2 on: February 18, 2020, 08:25:00 PM »
The first step is a bond-making step.  I would also refer to the associated kinetic isotope effect as a primary isotope effect.

Offline xshadow

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Re: Kinetic isotope effect of this molecule...
« Reply #3 on: February 19, 2020, 01:36:07 PM »
Ook...thanks

So also the breaking of a π C=C is a primary isotope effect

Because usually I've seen σ C-H broken for 1° KIE and I didn't know of  an example like this...


Thanks

Offline Babcock_Hall

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Re: Kinetic isotope effect of this molecule...
« Reply #4 on: February 19, 2020, 03:15:46 PM »
Maybe I am confused.  Which carbon is labeled with C-13?  There are some inverse secondary isotope effects involving C-D bonds; therefore, there might be an analogous phenomenon for carbon depending on the position of the label.
« Last Edit: February 19, 2020, 03:27:20 PM by Babcock_Hall »

Offline xshadow

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Re: Kinetic isotope effect of this molecule...
« Reply #5 on: February 21, 2020, 10:35:04 AM »
Maybe I am confused.  Which carbon is labeled with C-13?  There are some inverse secondary isotope effects involving C-D bonds; therefore, there might be an analogous phenomenon for carbon depending on the position of the label.

Hi

Labelled only the green carbon of the figure...the one witch -Br and -OCH3 bonded

No other atom labelled (hydrogen pther carbon)

That green carbon:

-Changes hybridization (usually this gives a secondary effecy)

-one of its bonds is broken (primary effect)

So a primary effect + a secondary one?(but i see only the primary )

Thanks

Offline Babcock_Hall

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Re: Kinetic isotope effect of this molecule...
« Reply #6 on: February 22, 2020, 09:26:14 AM »
We are moving beyond my knowledge of kinetic isotope effects.  I think that the labeled carbon will see effects both from the bonds made or broken and the change in hybridization, and I imagine that the primary effect is larger.  However, I am not absolutely sure how this will play out.  If the carbon with the negative charge were labeled instead, I think that you would only see the secondary isotope effect.
« Last Edit: February 22, 2020, 12:26:48 PM by Babcock_Hall »

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