Recently we attempted to purify a molecule that moved rapidly on silica TLC plates even at 100% hexane. We did not have alumina TLC plates; therefore, we ran a pipet column using alumina and obtained satisfactory preliminary results. We plan to scale up next week. What makes alumina more suitable for separating compounds of low polarity? I have searched through a few practical organic chemistry books, but so far I have not found a good discussion of this question.

Silica has more surface area per gram than alumina does, suggesting to me that one may need a larger amount of alumina than silica for a given mass of starting material. What other practical differences are there?

6 hours ago, BabcockHall said:

Recently we attempted to purify a molecule that moved rapidly on silica TLC plates even at 100% hexane. We did not have alumina TLC plates; therefore, we ran a pipet column using alumina and obtained satisfactory preliminary results. We plan to scale up next week. What makes alumina more suitable for separating compounds of low polarity? I have searched through a few practical organic chemistry books, but so far I have not found a good discussion of this question.

Is the condition of the alumina relevant ?

Abstract

This paper reports a systematic study on the relationship between surface structure and wetting state of ordered nanoporous alumina surface. The wettability of the porous alumina is dramatically changed from hydrophilicity to hydrophobicity by increasing the hole diameter, while maintaining the hole interval and depth. This phenomenon is attributed to the gradual transition between Wenzel and Cassie states which was proved experimentally by comparing the wetting behavior on these porous alumina surfaces. Furthermore, the relationship between surface wettability and hole depth at a fixed hole interval and diameter was investigated. For those porous alumina with relatively larger holes in diameter, transition between Wenzel and Cassie states was also achieved with increasing hole depth. A capillary-pressure balance model was proposed to elucidate the unique structure-induced transition, and the criteria for the design and construction of a Cassie wetting surface was discussed. These structure-induced transitions between Wenzel and Cassie states could provide further insight into the wetting mechanism of roughness-induced wettability and practical guides for the design of variable surfaces with controllable wettability.

https://pubmed.ncbi.nlm.nih.gov/18702472/

In terms of practicality, I would say the key factor for me is the activity of the alumina, rather than just the surface area per gram. I may be wrong, however, and I’d welcome any corrections on this point. As far as I know, the water content makes a significant difference, particularly with alumina. Things suddenly run more smoothly With very non-polar substances because not everything rushes straight through or gets stuck when the alumina is slightly less active.

On top of that, silica is slightly acidic and interacts quite differently. Alumina in contrast can be neutral, acidic or basic, and I believe this can affect sensitive or weakly polar compounds. This is precisely why, I wouldn’t just use more material when scaling up and try to keep the grade, activity and moisture content as consistent as possible. Otherwise, it quickly ceases to be a nice experiment.

Studiot, the paper that you cited looks quite useful; thank you. Aliceinwonderland, we used neutral alumina, 70-230 mesh, activity level I. It had been sitting on the shelf for a while; therefore, it might have picked up water from the atmosphere. As a practical matter, the particles settled quickly, meaning that the technique we use to pour silica columns, which is to make a slurry, was not effective. BTW, our compound does not have either acidic or basic functional groups.

1 hour ago, BabcockHall said:

Studiot, the paper that you cited looks quite useful; thank you. Aliceinwonderland, we used neutral alumina, 70-230 mesh, activity level I. It had been sitting on the shelf for a while; therefore, it might have picked up water from the atmosphere. As a practical matter, the particles settled quickly, meaning that the technique we use to pour silica columns, which is to make a slurry, was not effective. BTW, our compound does not have either acidic or basic functional groups.

My Braithwaite and Smith is rather out of date but has some stuff on alumina tlc.

I expect the latest edition will have more

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Chromatographic Methods