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chemcam
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Quote: Originally posted by MrHomeScientist  |
(cut)...The only troublesome part of the experiment was the precipitation of a good amount of white crystals when I added the water for the first
water wash. I looked at quite a few procedures online, but nobody mentioned the generation of any solids. I managed to gravity filter it off, but it
was VERY slow and I'm sure I lost some oil in the filter paper and trapped in the crystals. Has anyone else seen such a precipitate?
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Yes! I have gotten the precipitate of white crystals and you're right I have never found a mention of this either. I think it is unreacted salicylic
acid since that is not really soluble in water. I have read also that this esterification take a long time, like refluxing for 12-18 hours depending
on the material.
[Edited on 5-8-2013 by chemcam]
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Nicodem
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| Quote: Originally posted by Hockeydemon | | I follow the instructions off some K12 New Jesery AP chemistry lab - they indicated that the stopping point was when two layers form, and I used their
ratios. Why would two layers not form? The equation is C7H6O3 + CH3OH → C8H8O3 + H2O and methyl salicylate is not soluble in water.
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I assume your reference is http://phs.prs.k12.nj.us/clee/LabEsterificationSalicylic.pdf (make sure you always properly cite references)
Note that only one equivalent of water forms and note the MW of water is 18 g/mol. That does not cause much dilution to the methanol which is commonly
in large excess (though in your case the excess was not really that large).
| Quote: | | How do you determine the appropriate catalytic amount of something? |
By experiment, by literature, by experience, by mechanistics, or by the combination of these.
Since the literature says that benzoic acids and in particular the hydroxybenzoic acids are poorer substrates to the Fischer esterification reaction
when compared to most aliphatic acids, you can assume you need more sulfuric acid (see also the Kirk-Othmer chapter on esterification). Since methanol has a low reflux temperature and the methyl acetate side product even lower, you can assume
the esterification will either take several days of reflux or the reaction temperature will need to be increased by applying autoclave conditions
(like in the procedure you tried to follow). It seems to me that this esterification is not as easy as I initially thought.
In my experience, the esterification of 2&4-hydroxybenzoic acids with methanol is a very slow reaction. I never applied it to salicylic acid, but
the few related substrates that I used required at least two or three days of reflux to achieve >70% yields (using about 1 eq. sulfuric acid or
thereabout, can't really remember). It was bad enough to gave up on using reflux and thus switched to higher temperatures via autclaves (I actually
already mentioned this here). In retrospective, my previous suggestion for using 0.3 eq. of sulfuric acid only makes sense for aliphatic acids (kind of forgot that I
already dealt with this problem in the past).
| Quote: | | I thought that TLC required special equipment to actually view the results? If I'm able to do this at home, and you know of a set of decent
instructions on how to do it please direct me to it. |
TLC is the simplest chromatographic analytical tool available to home chemists. There are several threads on the topic of making TLC plates and using
them.
| Quote: Originally posted by MrHomeScientist | My final product was 2.7g of wintergreen oil with a light tan color, which is about a 25% yield (pretty poor, really, and I'm not sure why).
The only troublesome part of the experiment was the precipitation of a good amount of white crystals when I added the water for the first water wash.
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Poor conversion is the most likely reason, which makes salicylic acid a good candidate to be those white crystals.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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Hexavalent
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| Quote: Originally posted by Nicodem |
Poor conversion is the most likely reason, which makes salicylic acid a good candidate to be those white crystals. |
Perhaps also hydrolysis of the product, if there was any additional water present in the reaction mixture? As I mentioned upthread, acid catalyses the
reversible reaction in both directions, so the main limiting factor is the amount of water present; more water pushes the equilibrium to the left,
meaning you will invariably end up with some salicylic acid crystallizing out in addition to what would be unreacted, anyway.
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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Hockeydemon
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| Quote: Originally posted by Nicodem |
TLC is the simplest chromatographic analytical tool available to home chemists. There are several threads on the topic of making TLC plates and using
them. |
Well what do you know I happen to have a TLC reagent sprayer, but I haven't the fainest idea how to use it. Though I do understand how to use TLC (in
theory). How would one use the sprayer? I couldn't find anything through google other than the sale of it.
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Nicodem
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| Quote: Originally posted by Hockeydemon | | Well what do you know I happen to have a TLC reagent sprayer, but I haven't the fainest idea how to use it. Though I do understand how to use TLC (in
theory). How would one use the sprayer? I couldn't find anything through google other than the sale of it. |
You use it just like any other sprayer. You just spray the visualization reagent on the developed plates. It is not very useful though, as the
simplest visualization reagent is a basic aqueous solution of KMnO4 (which I assume is the most available to amateurs) and works best if
you simply dip the TLC in it for a second and then heat it up. The second simplest is to use UV detection on fluoroscein stained TLC plates (254 nm UV
lamps are not prohibitively expensive, good ones can be bought for 100 EUR). For these two methods you do not need a sprayer. Some reagents are best
applied by a sprayer rather than dipping the plate, but most of these reagents are functional group specific. They are most often used as a
qualitative test in order to tentatively identify compounds in the TLC spots (for example, when one does not have any reference standard).
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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MrHomeScientist
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I did another run making wintergreen oil, and the results were much much better. I followed the exact same procedure as outlined in the link I posted
earlier, except this time I added a reflux condenser to my flask and properly refluxed the solution for three hours instead of just one. This time,
there was absolutely no solid precipitate at any stage. I ended up with 6.7 grams of slightly tan methyl salicylate oil - over double what I got the
first time! This is a 61% yield.
I think that seals it that Nicoderm was right: the solid precipitate that appeared in the water washing stage was unreacted salicylic acid / aspirin.
Here's a picture of my products - the first run is on the right, and this second trial is on the left. The first run's product is noticeably darker
and quite a bit more cloudy.
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chemcam
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| Quote: Originally posted by MrHomeScientist |
I think that seals it that Nicoderm was right: the solid precipitate that appeared in the water washing stage was unreacted salicylic acid / aspirin.
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Did you even read my post BEFORE nicodem's? I clearly was the first person to tell you about the unreacted salicylic acid.
It's okay though, listen to the mods not the new guy. 
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MrHomeScientist
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All power to the hypno-mods!
You did indeed mention it first, I just hadn't read the thread in a few days and the last response stuck with me. Apologies.
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chemcam
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Oh, its all good, MrHomeScientist, I was just giving you a hard time. 
BTW, I enjoy your videos on YouTube will you ever upload new ones?
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DraconicAcid
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For TLC detection of salicylates, perhaps you could dip/spray the plate with iron(III) chloride? Both the oil of wintergreen and salicylic acid
should give a strong colour with iron(III), due to the phenol functional group (I know salicylic acid goes very purple with iron).
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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MrHomeScientist
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| Quote: Originally posted by chemcam | Oh, its all good, MrHomeScientist, I was just giving you a hard time.
BTW, I enjoy your videos on YouTube will you ever upload new ones? |
Thanks! Yes, I'll definitely be uploading more. I tend to go through spurts of activity where I'll get 2 or 3 up in a short period then go a few
months before doing another. Now that I've got the wintergreen synthesis down, I'll make one of that. I actually tried to record it while I did it
this time, but the damn camera keeps running out of batteries. I think the indicator is broken, and it thinks brand new batteries are empty.
I've got plans for the "golden rain" demo as well, and the lead nitrate I made for it is just now crystallizing.
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