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Topic: pKa Help  (Read 17800 times)

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Offline CHOH

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pKa Help
« on: February 26, 2008, 08:23:23 AM »
Does anyone know the pKa for Cycloblastin (cyclophosphamide) and Tykerb (lapatinib ditosylate)? I can't even locate these in the Product Monograph or Prescription Information from the manufacturers themselves
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Offline Arkcon

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Re: pKa Help
« Reply #1 on: February 26, 2008, 08:40:42 AM »
A recent copy of The Merck Index has the pKa's and solubility data for all approved pharmaceuticals.
« Last Edit: February 27, 2008, 05:03:55 PM by Arkcon »
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Offline CHOH

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Re: pKa Help
« Reply #2 on: February 29, 2008, 06:20:03 AM »
I do not have access to the Merck Index. Is anyone able to check the pKa for me?
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Offline Arkcon

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Re: pKa Help
« Reply #3 on: February 29, 2008, 08:05:48 AM »
Not me.  The Merck Index was at work, and I don't work there anymore. :( If you have samples of those drugs, someone should have a reference book for you.  The US(EU, J, B -- pick your favorite flavor) Pharmacopeia also has this info, and a Google search might not fail, either.

*[EDIT]*

Well, a quick Google does fail me, so you'll want to get your hands on a pharmaceutical reference in the nearest university library.  I suppose, if you're metaphorically chained to a lab with a computer, you could download one of the many programs that try to deduce pKa from structure.  They should predict the pKa close enough for HPLC mobile phase selection or solubility.  But I wouldn't use the result for a quantitative titration -- not for a large molecule.
« Last Edit: February 29, 2008, 03:10:48 PM by Arkcon »
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Offline umpapa

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Re: pKa Help
« Reply #4 on: February 29, 2008, 09:58:29 PM »
Assignment due Monday CHOH
you better hurry up  ;D
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Offline LK

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Re: pKa Help
« Reply #5 on: March 16, 2008, 02:16:11 PM »
Cyclophosphamide is basic with pKa 2.84
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Offline Borek

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Re: pKa Help
« Reply #6 on: March 16, 2008, 02:48:40 PM »
Quote from: LK on March 16, 2008, 02:16:11 PM
basic with pKa 2.84

Huh? About as basic as chloroacetic acid...
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Offline LK

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Re: pKa Help
« Reply #7 on: March 16, 2008, 03:09:35 PM »
ionized at low pH and neutral at high pH... 50% ionized at pH 2.84... which is a value calculated by ACD  V8.14... so approximate...
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Offline Borek

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Re: pKa Help
« Reply #8 on: March 16, 2008, 03:26:41 PM »
You didn't get it. With pKa around 3 it is acidic, not basic... Would you call acetic acid basic? And it is much weaker.
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Offline LK

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Re: pKa Help
« Reply #9 on: March 16, 2008, 03:47:29 PM »
but acetic acid is neutral at low pH and charged at high pH (negatively)... and that's  why I would call it acidic with pKa 4.7

how would you call a compound positively charged at low pH and neutral at high pH with pKa 2.8?

or do you mind that I said basic instead of weak base? sorry, I really can't get what you mean...
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Offline Borek

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Re: pKa Help
« Reply #10 on: March 16, 2008, 03:54:08 PM »
If it has so low pKa it must dissociate in water solutions as acid. Whether it is

HA+ <-> H+ + A

or

HA <-> H+ + A-

or

HA- <-> H+ + A2-

it is all the same - solution is acidified. Charge doesn't matter. pKa does.

Quote from: LK on March 16, 2008, 03:47:29 PM
how would you call a compound positively charged at low pH and neutral at high pH with pKa 2.8?

Acid :)
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Offline LK

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Re: pKa Help
« Reply #11 on: March 16, 2008, 04:40:42 PM »
o.k.

but if I wrote only pKa is 2.8 and HCOH wants to use this value let's say for knowing if the compound is charged or neutral at the pH 7... how could he know?

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Offline enahs

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Re: pKa Help
« Reply #12 on: March 16, 2008, 04:55:36 PM »
Quote
but if I wrote only pKa is 2.8 and HCOH wants to use this value let's say for knowing if the compound is charged or neutral at the pH 7... how could he know?


Uhh, look at the pKa?

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Offline LK

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Re: pKa Help
« Reply #13 on: March 16, 2008, 05:02:07 PM »
so he looks at pKa, sees that it is 2.8 and?
is it charged or neutral at pH 7?
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Offline Borek

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Re: pKa Help
« Reply #14 on: March 16, 2008, 05:52:43 PM »
Quote from: LK on March 16, 2008, 05:02:07 PM
so he looks at pKa, sees that it is 2.8 and?
is it charged or neutral at pH 7?

You can't tell. It can be anything.
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