[Mocook]

Hello all,

I have recently been working with porphyrins, has anyone on the forum worked with them before?
How can I determine decomposition and waste product in relation to a new reaction with them? I have ran a silica column gravitational to purify the various compounds that were produced but there were two bands that were difficult to obtain with increasing polarity, almost immobilized with a mixture of solvent system so I eluted one band with ethyl acetate and another with ethanol, any good solvent systems for polar compounds you can suggest to purify these two bands?im sure they contain multiple compounds in them

[Mocook]

The torture ...

[Arkcon]

My knee-jerk response in this sort of situation is HPLC, but maybe you can tell us more about the source of your porphyrins and what reactions you're trying to perform?

[Mocook]

Iam only familiar with traditional methods of purification, HPLC have no experience in using it. The reaction ultimate wanted product is a halogenated porphyrin derivative, my main issue/worry is to know if my reaction leads to many products or are the products decomposing on the silica during the column.

[Mocook]

i completed the isolation, but i want to confirm the fractions as pure or impure, is there another method other than TLC since im working with compounds with very similar polarities.

[Corribus]

I did my PhD work on porphyrin synthesis and properties analysis.  How easy they are to purify depends quite a bit on what the side chains are because this determines solubility.  Porphyrins with simple phenyl groups or alkoxy-substituted phenyl groups for side chains generally run down silica columns pretty easily with good separation.  On the other hand porphyrins with polyethylene glycol substitutde phenyl groups were a real pain in the butt because they streaked the whole way down the column, rendering separation very very difficult.  What I'm getting at is without knowing specifically what kind of porphyrin you are working with, it's hard to be much help.

For purification we usually did chromatographic separation on silica gel, followed by size exclusion chromatography if necessary (and always then a flash column to get rid of residual SE beads).  To determine purity of a fraction we generally used UV-vis and NMR.  If these were clean you can usually assume the fraction is at least 90-95% pure.  MS gives you a secondary confirmation of identity.  If you need a higher standard of purity you'll have to move to some chromatographic technique like HLPC.  Recrystalization as well can give higher purity levels.

If you're doing halogenation of your porphyrin, a quick way to check purity of your (halogenated) species is to check fluorescence.  Halogenated porphyrins, particularly brominated porphyrins, have usually almost totally quenched fluorescence.  Thus if you see any fluorescence in your sample, or any fluorescent trailing bands on your column, then you can be pretty sure you have some impurity, probably a nonhalogenated starting material or decomposition product.  Of course lack of fluorescence won't ensure that you've got a single species, but in general illumination with a hand-held lamp is a quick and dirty way to see if your halogenated product is reasonable pure.