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Topic: Potassium and Cesium carbonates - Pka  (Read 27997 times)

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Offline Nescafe

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Potassium and Cesium carbonates - Pka
« on: September 20, 2012, 08:01:31 AM »
Hi,

Anyone know the Pka of these bases? I am guessing the pkaof Cs2CO3 is around 10-11? that of potassium carbonate is around 7-8?

Nescafe.

Offline Dan

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Re: Potassium and Cesium carbonates - Pka
« Reply #1 on: September 20, 2012, 08:55:50 AM »
For a compound to have a pKa, it must have a proton:

AH  ::equil:: A- + H+

I assume your question is referring to the pKa of the conjugate acids, i.e. CsHCO3 and KHCO3 (i.e. bicarbonate)?

Quote
I am guessing the pkaof Cs2CO3 is around 10-11? that of potassium carbonate is around 7-8?

Why?
My research: Google Scholar and Researchgate

Offline Nescafe

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Re: Potassium and Cesium carbonates - Pka
« Reply #2 on: September 20, 2012, 06:22:24 PM »
For a compound to have a pKa, it must have a proton:

AH  ::equil:: A- + H+

I assume your question is referring to the pKa of the conjugate acids, i.e. CsHCO3 and KHCO3 (i.e. bicarbonate)?

Quote
I am guessing the pkaof Cs2CO3 is around 10-11? that of potassium carbonate is around 7-8?

Why?

Yes my assumption was that when you ask for a pKa of a compound with no protons it is asking for its conjugate acid or as Clayden likes to write, pKah. We have two negative charges, when the first proton is picked up I imagine it will become less basic than before.

My reasoning is simply comparative. acetic acid has a pKa of ~4.3. I assumed if if we have two negative charges close by it would be more unstable than just having a methyl, and therefore more basic, even after one gets protonated it should still be basic but less than it initially was. So by some imagination I assumed the pKah to be ~10-12. I also know it is not as strong as lets say potassium tert butoxide which forms a stable complex (pkah ~18) after protonation. Thus why I went for the middle. So if you know what it is I would love to know as I plan to use it as a base in my SN2.

Nescafe.

Offline Babcock_Hall

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Re: Potassium and Cesium carbonates - Pka
« Reply #3 on: September 20, 2012, 06:52:17 PM »
Potassium carbonate is K2CO3, whereas cesium carbonate is Cs2CO3.  My impression is that potassium ions and cesium ions are not very different from each other, and I would be surprised if there was a big difference in their basicities.  Are you thinking of bicarbonates?

Offline Nescafe

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Re: Potassium and Cesium carbonates - Pka
« Reply #4 on: September 20, 2012, 07:50:40 PM »
Yes the bicarbonates, any idea of their Pkas?

Offline Dan

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Re: Potassium and Cesium carbonates - Pka
« Reply #5 on: September 21, 2012, 02:59:10 AM »
Quote
I am guessing the pkaof Cs2CO3 is around 10-11? that of potassium carbonate is around 7-8?

OK, my question is: why do you guess that cesium carbonate is 1000 times more basic than potassium carbonate?
My research: Google Scholar and Researchgate

Offline AWK

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Re: Potassium and Cesium carbonates - Pka
« Reply #6 on: September 21, 2012, 04:27:51 AM »
Hi,

Anyone know the Pka of these bases? I am guessing the pkaof Cs2CO3 is around 10-11? that of potassium carbonate is around 7-8?

Nescafe.
As a bases work CO32- or HCO3-. Its protolysis constants can be easily calculated from Ka2 and Ka1 of carbonic acid, respectively.
AWK

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