User 5508363905
10-06-2006 00:33:40
Some of my colleagues have challenged the pKa values Marvin calculates for ephedrine, N-methylephedrine and norephedrine stating that the basicity should follow the general trend of tertiary > secondary > primary. Marvin shows the reverse trend. Is there an explaination for this behavior?
User 851ac690a0
13-06-2006 10:01:01
Dear Carl,
With the improved pKa calculator I obtained the next values:
ephedrine: 9.52 (9.60)
N-methyl ephedrine 9.06 (9.30)
norephedrine 9.37 (smaller than 9.44)
Literature values are given in parenthesis.
Jozsi
User 5508363905
13-06-2006 15:39:26
Any ideas as to why the basicity of the tertiary is lower than the primary and secondary? The secondary is the most basic. Is there some special circumstance/structure in this molecule to explain this?
Anyone?
Where did you get your pKa values (reference)?
When will this improved calculator be available?
I still get anomolous pKa values for nucleobases (adenine, etc.).
Is there a beta available?
ChemAxon 43e6884a7a
13-06-2006 17:58:24
Carl,
Józsi is on holiday until the end of next week. He will send a detailed response when he is back. Some comments until then:
| Quote: |
| Any ideas as to why the basicity of the tertiary is lower than the primary and secondary? The secondary is the most basic. Is there some special circumstance/structure in this molecule to explain this? |
I think this is usually the case: secondary amines are stronger than tertiary amines because of steric effects. (Józsi will correct me if I am wrong) | Quote: |
| When will this improved calculator be available? |
We will release Marvin after fixing a few bugs (not related to pKa calculation). Hopefully very soon. | Quote: |
| Is there a beta available? |
The latest test release is at http://www.chemaxon.com/shared/alpha/marvin/, but this doesn't look like to produce the same numbers that Józsi posted, so please wait for the release.
User 851ac690a0
28-06-2006 16:06:09
Dear Carls,
I obtained our pKa data from our "in house" database.
If you calculates solvent accesible surface area increments of atoms of ephedrine derivatives you will obtain the next values:
prim. amin: 7.01 [Angstrom]
sec. amin: 8.71 [Angstrom]
tert. amin : 5.48 [Angstrom]
These values are in good agreement with calculated pKa order.
Jozsi