Positional Isomers

[JBush]

All,
I am looking for help separating three positional isomers. The compound has a benzene ring at one end to which an NO2 group is the only other substitution. It is this NO2 group that is providing the positional isomers. The compound has a number of polar groups but is not very soluble in water. I have so far been using RPLC and getting the best separation on a 4.6x250mm 5um ymc ods-aq column. However in order to achieve baseline resolution the run time becomes quite excessive (about 60 minutes) with really broad peaks. There is one chiral center at the oppositie end of the compound however it shows no retention on an CHIRALPAK AD-RH or AS-RH columns. Any help would be greatly appreciated.

Perhaps a hilic column?

[Rob Burgess]

Not that I've done much chromatography on posistional isomers, but from what I have learnt from various texts is that normal phase chromatography may be the way to go for posistional isomers.

Also for nitro containing compounds flrorinated columns may offer the selectivity you are looking for. A number of manufacturers offer such phases now.

[JBush]

Thanks, I will go ahead and try both a normal phase chiral run as well as a normal phase silanol run. Lets hope for the best. As of later this afternoon i have the three isomers near separated on a 4.6x250mm ODS-AQ but run time is up there. Anyway thanks for the help.

[Mark Tracy]

We have noticed that nitroaromatic compounds have unusual selectivities on Dionex Acclaim PA and PA2 polar-embedded phases. Presumably other vendors' polar-embedded columns do also.

[Uwe Neue]

Another proposal could be a phenyl column such as the XBridge Phenyl column from Waters. The reason for this suggestion is that the selectivity of phenyl columns is based on pi-pi interactions, which should be stronger with your nitro-substituted phenyl ring. In addition, one would expect that thepi-pi interaction is significantly different between the different positions of the nitro group.

[Kostas Petritis]

For the same reasons (pi-pi interactions) you might want to try porous graphitic carbon columns such as Hypercarb. Actually there are several phenolic compounds that have been evaluated with this chromatographic support and I am sure that nitro substituted phenyl rings were part of these. I do not recall the exact reference but you may want to do a literature research for this. In order to narrow it down, one of the authors I think was Marie Claire Hennion...

[JBush]

Thank you all, I appreciate the quick responses. I know we have a few phenyl columns lurking around that I will absolutely try. the pi-pi interraction thought had skipped my mind. Thanks, I will certainly include a post on my final results. I'm hesitant to purchase additional columns but it may turn out necessary.

[JBush]

All,
i was able to separate the two compounds with a resolution of 1.54 through the addition of THF into my Methanol mobile phase on a 250mm ymc ODS-AQ column. Neither phenyl or CN columns seemed to help out in this case. Thanks for the suggestions.

[HW Mueller]

Neat that someone actually came back to report on results, thanks JBush.

[JBush]

its the only way any of us are really going to benefit from this forum, especially people like me who are new to the field.