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Author Topic: difference of silica column and alumina column  (Read 448 times)

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Richard Jeong

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difference of silica column and alumina column
« on: January 21, 2019, 02:54:00 PM »

I hope you reading this have a good day.
I am synthesizing an air-sensitive compound. To recognize the optimal eluents, I did TLC with silica plate and DCM and Chloroform, respectively-I did TLC twice. While oxides of reactants and product had Rf value=0 as expected, below are Rf values that I got with silica plate;
With DCM, reactant Rf: 0.88 and product Rf: 0.0
With Chloroform, reactant Rf: 0.97 and product Rf: 0.87
So, to purify the product, I did pipet column using alumina(basic) in a glove box. First washed the reactant with DCM and eluted the product with Chloroform.
However, the yield was terrible and Chloroform solution contained the reactant.
Is it because there are differences between silica and basic alumina? Thank you for reading.
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hypervalent_iodine

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Re: difference of silica column and alumina column
« Reply #1 on: January 21, 2019, 03:54:31 PM »

Quite different, yes. To start, silica is slightly acidic whereas basic alumina is, well, basic. As a result, silica tends to stick more to basic compounds, and basic alumina to acidic compounds. Alumina is also more polar I believe. Is there any reason you decided to use alumina instead of silica when your TLC was done using a silica plate?
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Richard Jeong

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Re: difference of silica column and alumina column
« Reply #2 on: January 21, 2019, 04:33:52 PM »

Quite different, yes. To start, silica is slightly acidic whereas basic alumina is, well, basic. As a result, silica tends to stick more to basic compounds, and basic alumina to acidic compounds. Alumina is also more polar I believe. Is there any reason you decided to use alumina instead of silica when your TLC was done using a silica plate?

Well, thanks! The reason that I used alumina instead of silica is according to a reference that dealt with similar compounds. Luckily, I did not discard DCM solution and it looks like I can recover the product.
Thanks to you, for the next trial, I am planning to do TLC with alumina plate. Or, I would try column with silica.
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hypervalent_iodine

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Re: difference of silica column and alumina column
« Reply #3 on: January 21, 2019, 07:23:38 PM »

Quite different, yes. To start, silica is slightly acidic whereas basic alumina is, well, basic. As a result, silica tends to stick more to basic compounds, and basic alumina to acidic compounds. Alumina is also more polar I believe. Is there any reason you decided to use alumina instead of silica when your TLC was done using a silica plate?

Well, thanks! The reason that I used alumina instead of silica is according to a reference that dealt with similar compounds. Luckily, I did not discard DCM solution and it looks like I can recover the product.
Thanks to you, for the next trial, I am planning to do TLC with alumina plate. Or, I would try column with silica.

Is the alumina plate basic too? I think that if you get the separation you need with silica, there's no reason you can't use that. Switching to alumina seems unnecessarily complicated.
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