ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: sam0001 on June 07, 2012, 08:40:25 pm
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Analysis of caffeine, aspirin and paracetomol, using tlc, silica gel for the stationsry phase, and butyl ethanoate for the mobile phase. The question is, why couldn't water be used as the mobile phase?
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http://en.wikipedia.org/wiki/Caffeine
http://en.wikipedia.org/wiki/Paracetamol
By looking at all the molecules they contain a benzene ring and no really polar parts. This would make them non polar thus very insoluble in water. This is why we use butyl ethanoate as it can dissolve these nonpolar substances due to being a non polar solvent. The rule is likes dissolves likes. TLC would not work if the components do not dissolve in the mobile phase.
I hope im right
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http://en.wikipedia.org/wiki/Caffeine
http://en.wikipedia.org/wiki/Paracetamol
By looking at all the molecules they contain a benzene ring and no really polar parts. This would make them non polar thus very insoluble in water. This is why we use butyl ethanoate as it can dissolve these nonpolar substances due to being a non polar solvent. The rule is likes dissolves likes. TLC would not work if the components do not dissolve in the mobile phase.
I hope im right
Right. For VCE, this will surely be good enough.
The actual answer though, goes something along the lines of Paracetamol/Aspirin/Caffeine all have similar structures and polarity. In a medium of high polarity (water), these won't separate out sufficiently, as highly polar solvents are good at separating out components with large differences in polarities, but cannot differentiate between components with slightly different polarities. Therefore, peaks for Paracetamol/Aspirin/Caffeine won't necessarily be resolved, and we may not be able to get any quantitative data from peak areas due to overlaps.
Using a solvent of lower polarity (such as methanol, ethanol, etc) will allow a good separation of low-polarity contents, as low polarity solvents are more sensitive to small differences in polarities. However, the problem is that low polarity solvents cannot dissolve highly polar and ionic contents, so it's not always suitable.
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wouldnt the water actually hydrolyze the aspirin though? thats what i wrote!
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You need some kind of catalyst to hydrolyse an ester.
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Valid point! Thanks.