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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: CHOH on February 26, 2008, 08:23:23 AM

Title: pKa Help
Post by: CHOH 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
Title: Re: pKa Help
Post by: Arkcon 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.
Title: Re: pKa Help
Post by: CHOH 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?
Title: Re: pKa Help
Post by: Arkcon 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.
Title: Re: pKa Help
Post by: umpapa on February 29, 2008, 09:58:29 PM
Assignment due Monday CHOH
you better hurry up  ;D
Title: Re: pKa Help
Post by: LK on March 16, 2008, 02:16:11 PM
Cyclophosphamide is basic with pKa 2.84
Title: Re: pKa Help
Post by: Borek on March 16, 2008, 02:48:40 PM
basic with pKa 2.84

Huh? About as basic as chloroacetic acid...
Title: Re: pKa Help
Post by: LK 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...
Title: Re: pKa Help
Post by: Borek 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.
Title: Re: pKa Help
Post by: LK 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...
Title: Re: pKa Help
Post by: Borek 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.

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

Acid :)
Title: Re: pKa Help
Post by: LK 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?

Title: Re: pKa Help
Post by: enahs 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?

Title: Re: pKa Help
Post by: 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?
Title: Re: pKa Help
Post by: Borek on March 16, 2008, 05:52:43 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.
Title: Re: pKa Help
Post by: LK on March 16, 2008, 06:25:33 PM
pKa 2.8 acidic => negatively charged at pH 7 (e.g. acetic acid)
pKa 2.8 basic => neutral at pH 7 (e.g. cyclophosphamide or 3-nitroaniline)

pKa 2.8 => can be anything

Title: Re: pKa Help
Post by: Borek on March 16, 2008, 06:31:56 PM
I have never heard about such use of acidic/basic adjective. Can you show us a reference?

What about hydrocyanic acid? pKa 9.31. Basic?

What about hypothetic pKa=7.00 acid, 50/50 at pH=7.0? Basic? Acidic? Neutral?