[Viking]

So I'm testing different solvents for a reaction, and products ran in pyridine always seem to have an easily cracked column.

I using 60 micrometer silica gel.

I'm not sure if it makes pack more?
The column does pack down more after more pressure is applied after being cracked.
Does silica gel go bad after sitting for to long?
Did anyone else experience this with pyridine and flash chromatography?

[kamiyu]

1) Suggest you add 3-5% triethylamine in your eluent. Otherwise, your basic stuff will stick with the acidic silica.

2) Yes, silica absorbs water in the air and it loses activity on standing in air. But this takes some time. Don't worry too much about this.

[opsomath]

"Cracked" means that either you're not getting all your air out before running the column, or else the column solvent is actually producing enough heat to vaporize itself when forced over the silica. To avoid both these problems, use the slurry method to make a column.

[Viking]

I am using the "slurry method" to pack my column.
I used excesses solvent for the last column that I ran and still it cracked.

I have yet to add triethyl amine to my solvent though

[kamiyu]

Viking:

1) What does your "crack" exactly mean? Does it mean that you saw some void volumne formed in the column?

2) If so, then I think your problem is that you used hexane for the "slurry method". This is what the textbook says but, personally, I think this is wrong. You should use the eluent mixture for the "slurry method" Under most cases, you won't see crack. The only exception is that the interaction of your sample with silica is exceptionally strong and there is still large heat given out. But this is rare.

[Dan]

So, my understanding so far is that:

1. You slurry pack (presumably with hexane, pet ether or similar).
2. Load compound.
3. Elute with a pyridine-containing eluent.
4. You get voids.

Questions:

What is the eluent in step 3?

I think the voids are probably due to an exotherm when pyridine interacts with silica (as has already been suggested). I would advise pre-equilibrating the silica with a pyridine-containing eluent (e.g. 1% pyridine/hexane - but it depends on your eluent from step 3) before loading and starting the column.

[Nescafe]

If you have a good rotavapor/high vac you should be able to get rid of most of the pyridine assuming your compound is not also volatile. Is it not possible for you to do a mildly acidic work up to get rid of the pyridine?

[kamiyu]

I do not think pyridine is a good choice

TEA is much better and normal.