Chromatography Forum

i think the (Cl) in the ring increase the polarity ?

If you know some book can teach me to understand this conceptes i would be very grateful.

Regards

Xie

China

If you want to ESTIMATE the polarity on basis of single bond dipoles or even on basis of electronegativities it would help if you studied chemistry for ~10 to ~15 years, later concentrating on quantum mechanics. I don´t think this would help too much, though, in predicting the conformation and thus polarity of a molecule which is adsorbed on the stat. phase.
On more practical terms: The C-I bond is also quite polar, yet an iodinated (on the phenol) methyltyrosine has a much higher rt on a C-18 than the methyltyrosine. The C-I dipole seems to counteract another dipole, also, the iodine might figure in the absorption on the stat. phase. I was even more surprised to see that a second I on the ring increases the rt even more, extensively. If I rememeber correctly a similar situation holds with baclofen (a chloro-phenylbutyric acid) in comparison to a derivative with two chlorines on the ring. I don´t think that anybody predicted that either.

There are some modeling tools available on the web for calculating polar surface area which has been shown in some cases to correlate to retention on reversed phase columns.

http://www.daylight.com/meetings/emug00/Ertl/index.html

Is one example.

Part of the problem with all of this is that the approximation "more polar compounds elute earlier in reversed-phase" is just that: an approximation. It has the same general validity as "men are taller than women".

Other factors enter into retention: degree of solvation (of both the analyte and the stationary phase), and secondary interactions with residual silanols are just two of the possibilities that come to mind.

Thanks to you all my friends .I really have to read some organic books

thanks again