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Author: Subject: TLC help - which solvent system - benzalacetone derivate
deep_dreamer
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[*] posted on 6-9-2018 at 10:56
TLC help - which solvent system - benzalacetone derivate


Hello,
I am currently trying to synthesize 4-(4'-hydroxy-3'-methoxyphenyl)-3-butene-2-one as a little project to get into organic chemistry. My first trial (which is not yet over, and waits to be taken care of) was based on a procedure I found for benzalacetone, adapted for vanillin as a starting compound and also adapted for the molar amounts I wanted to use.

After that, I realized that there is actually a paper which describes *exactly* what I want (0.1 mol starting aldehyde) for various substrates (including vanillin, in 95% yield!).

An Efficient Method for the Selective Iodination of a,b-Unsaturated Ketones
DOI: 10.1055/s-0028-1083200
Their general procedure for the condensation product is as follows:
To the mixture of aldehyde (0.1 mmol) [sic! but they obviously mean 0.1 mol!], acetone (20 mL, excess), and H2O (40 mL) was slowly added 5% aq NaOH soln (8 mL) from a dropping funnel at 40 °C. After disappearance of the reactant (TLC), acetone was removed under reduced pressure and the resi- due was poured into EtOAc (50 mL), the mixture was extracted with EtOAc (3 × 50 mL), and the combined organic layers were dried (anhyd Na2SO4). Removal of the solvent and purification of the res- idue by column chromatography or recrystallization gave the target products.

For vanillin, they use 1.2x as much NaOH.

Now, I don't know for how long I'll have to let this react and never did TLC: How would one go about finding out a suitable solvent system? I don't want you to be my replacement for a text book, but as a newbie I am feeling a little bit lost. Any book recommendations or some input?

Thanks in advance,
deep_dreamer

[Edited on 6-9-2018 by deep_dreamer]
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Sigmatropic
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[*] posted on 6-9-2018 at 12:20


There are books on tlc but finding out emperically is much easier and faster. For apolar compounds such as these I would start at 10 percent ethyl acetate in heptane, pentane, petroleum ether or whatever aliphatic solvent you can find and move up or down in the percentage of ethyl acetate depending on whether the compounds are at baseline or in the solvent front. If you find yourself having to use 1 percent switch to something like diethyl ether or MTBE instead of ethyl acetate. If you have to use 100 percent ethyl acetate consider adding an alcohol or go to DCM-methanol, DCM-acetone, see attached picture. Please feel free to substitute chloroform with DCM and benzene with toluene for toxicological reasons.

Now for specific cases additives (usually between 0.1 and 10% v/v) are used to avoid streaking (acetic or trifluoro acetic acid for compounds bearing carboxylic acids and triethylamine or ammonia for compounds bearing an amine).

This won't be necessary for your compound and I suggest starting at 10 % ethyl acetate in petroleum ether.

solventstrength.gif - 122kB
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byko3y
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[*] posted on 6-9-2018 at 16:58


There's very scarce description of the picture. What is the equal solvent strength for? LC, TLC, recrystallization? It might be for chromatography, but authors used some specific substrates to derrive the values. I was unable to find the source.
However, I've found another research with more details: http://www.rsc.org/suppdata/gc/c2/c2gc36064k/c2gc36064k.pdf
They calculated average Rf values for multiple substrates, mostly esters, amides, nitriles, (although arithmetic mean for Rf is meaningless), but they also show Rf values for individual compounds they've tested.
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Sigmatropic
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[*] posted on 6-9-2018 at 21:17


I did not know of that table! Good find, I will definitely try it out.

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deep_dreamer
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[*] posted on 6-9-2018 at 21:46


Thanks a lot, Sigmatropic and byko3y. This is really helping me out a lot. Let's see how it works out practically today - I'll come back to this topic, hopefully with some Rf values!
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DJF90
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[*] posted on 7-9-2018 at 00:38


Thats an equieluotropic table, so it allows you to find different systems of similar solvent strength/polarity for TLC. There is a more recent and "green" table published a big pharma (internal) handbook/databook if I'm not mistaken
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deep_dreamer
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[*] posted on 8-9-2018 at 09:35


Okay, I'm currently burning through my TLC plates as if they were free and never got a decent result. I'm using a reference point (vanillin, the starting aldehyde), a co-spot (double spotted with the reference solution and the diluted reaction solution) and the pure diluted reaction spot. When it was still at the beginning of the reaction, I could still interpret a 'long' spot into it, as if two substances were too close to each other.

Now, 7:00h into the reaction, this has disappeared - however, I still can't find any difference between co-spot and pure reaction spot (although there should obviously be one). I am using 3:2 heptane:EtOAc which results in a Rf of around 0.3-0.4.

Any advice at all?
I believe that the vanillin and the condensation product spots hover on each other in the TLC. Should I take a entirely different solvent system with similar Rf?

Thanks in advance.

[Edited on 8-9-2018 by deep_dreamer]
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DJF90
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[*] posted on 8-9-2018 at 13:57


Assuming your technique is right (quite difficult to ascertain without a picture), perhaps 9:1 DCM:EtOAc will give a visible separation. How are you visualising your plates?
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deep_dreamer
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[*] posted on 6-10-2018 at 01:35


Although I kind of gave up on the TLC on this one, I still want to comment. I hope this is not seen as a kind of necro-posting.

Quote: Originally posted by DJF90  
Assuming your technique is right (quite difficult to ascertain without a picture), perhaps 9:1 DCM:EtOAc will give a visible separation. How are you visualising your plates?

My technique was definitely flawed, but as it was shown to other people who had some (limited) experience on TLC, not completely garbage..

I believe the spots were too big in size, although the volume used was not too much. About 2.5 µL per spot, I used 10 µL ring caps for it (is this okay, or should I use even thinner capillaries for spotting?).

Sadly, I couldn't try the proposed DCM:EtOAc solvent system, because I didn't have any DCM available.

UV was used for visualization, as I have a lot of unused UV-active plates to my disposal.

Quote: Originally posted by deep_dreamer  
For vanillin, they use 1.2x as much NaOH.

Now to correct my error. My reading abilities seem terrible. Although it clearly stated 1.2 eq, I interpreted it as a factor of 1.2.. Somehow, I was too stupid to realize the phenolic nature of vanillin!

If I have the time (which I don't have ATM), I will surely repeat the experiment and come up with better reaction times and yield - and hopefully a successful TLC during reaction.

I still have a lot to learn, that is for sure :)

[Edited on 6-10-2018 by deep_dreamer]
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