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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: xshadow on February 18, 2020, 03:35:24 PM

Title: Kinetic isotope effect of this molecule...
Post by: xshadow 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 :

(https://i.imgur.com/DvyZ7La.jpg)

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!!

Title: Re: Kinetic isotope effect of this molecule...
Post by: hollytara 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.

Title: Re: Kinetic isotope effect of this molecule...
Post by: Babcock_Hall 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.

Title: Re: Kinetic isotope effect of this molecule...
Post by: xshadow 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

Title: Re: Kinetic isotope effect of this molecule...
Post by: Babcock_Hall 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.

Title: Re: Kinetic isotope effect of this molecule...
Post by: xshadow on February 21, 2020, 10:35:04 AM

Quote from: Babcock_Hall 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.

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

Title: Re: Kinetic isotope effect of this molecule...
Post by: Babcock_Hall 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.