December 07, 2019, 03:54:57 AM
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Topic: How much acetic acid is too much for a silica column?  (Read 507 times)

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Offline Babcock_Hall

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How much acetic acid is too much for a silica column?
« on: November 18, 2019, 01:57:09 PM »
My student accidentally prepared some ethyl acetate with 4% acetic acid, not 1 %.  He was supposed to use 4% methanol and 1 % acetic acid (it's a long story).  Is 4% too much in terms of possibly dissolving the silica?

Offline kamiyu2550

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Re: How much acetic acid is too much for a silica column?
« Reply #1 on: November 18, 2019, 10:14:45 PM »
I personally think no. It won't dissolve for 4% HOAc with 96% EA.

Indeed, theoretically methanol or ethanol can dissolve silica column. I once used ethanol to push our my stuff on column. Nothing dissolved.

By the way, usually the addition of a little HOAc additive is for basic alumina column when your stuff is acidic.

Offline hypervalent_iodine

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Re: How much acetic acid is too much for a silica column?
« Reply #2 on: November 19, 2019, 12:02:33 AM »
Indeed, theoretically methanol or ethanol can dissolve silica column.

I believe this is a myth, albeit a very pervasive one. Silica isn't dissolved by methanol, but it can cause for it to shed fine particles that are able to move through the column. (https://selekt.biotage.com/blog/does-methanol-really-dissolve-silica-during-flash-column-chromatography)

Offline kamiyu2550

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Re: How much acetic acid is too much for a silica column?
« Reply #3 on: November 19, 2019, 02:34:29 AM »
Yes, very true

Offline wildfyr

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Re: How much acetic acid is too much for a silica column?
« Reply #4 on: November 19, 2019, 03:23:42 PM »
As long as the column is working fine, the acetic acid won't hurt anything. Think about it, silica gel is just tiny pieces of very porous glass. Glassware does fine containing 100% acetic acid!

Offline kamiyu2550

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Re: How much acetic acid is too much for a silica column?
« Reply #5 on: November 19, 2019, 08:23:39 PM »
Just try to avoid acetic acid if possible. Use some other polar solvents like DCM/MeOH or EtOH instead.

Acetic acid has a strong smell. With high boiling point (118 oC), it remains with your product, very hard to remove and you can smell the poor acid from your product.

Offline hypervalent_iodine

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Re: How much acetic acid is too much for a silica column?
« Reply #6 on: November 19, 2019, 08:54:31 PM »
Just try to avoid acetic acid if possible. Use some other polar solvents like DCM/MeOH or EtOH instead.

Acetic acid has a strong smell. With high boiling point (118 oC), it remains with your product, very hard to remove and you can smell the poor acid from your product.

Sometimes MeOH / DCM doesn't quite cut it. It's not that hard with enough vacuum or by the addition of toluene, and can be quite useful when you are separating difficult acids.

Offline Babcock_Hall

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Re: How much acetic acid is too much for a silica column?
« Reply #7 on: November 20, 2019, 03:39:59 PM »
We thought that we had carboxylic acid present.  In the past we have used 1% acetic acid in the eluting solvent, presumably to ensure that the product is in the protonated form.

Offline kamiyu2550

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Re: How much acetic acid is too much for a silica column?
« Reply #8 on: November 20, 2019, 08:56:31 PM »
We thought that we had carboxylic acid present.  In the past we have used 1% acetic acid in the eluting solvent, presumably to ensure that the product is in the protonated form.

Then it seems you are running an alumina column, not silica gel column.

1% acetic acid is best described as "additive". It is not something to tune the polarity of the eluting solvent.

Offline wildfyr

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Re: How much acetic acid is too much for a silica column?
« Reply #9 on: November 20, 2019, 10:12:32 PM »
Kamiyu, with highly polar eluents, I think you'd be surprised what can increase polarity. I see things sometimes in carbohydrate or natural product syntheses where the eluent is things like 90% DCM 8% MeCN 2% Acetic acid.

Offline Babcock_Hall

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Re: How much acetic acid is too much for a silica column?
« Reply #10 on: November 26, 2019, 09:05:10 AM »
We were indeed running a silica column, but it may have been chloroform/methanol/acetic acid, possibly patterned after Janecka et al., J. Med. Chem. 1994,37, 2238-2241.

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