Master of the Elements
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Color of Phenylacetone
Wikipedia states that phenylacetone is colorless.
I made phenylacetone twice. The first time I made several errors but obtained a very small amount of a peach-colored oil. I distilled it twice without
vacuum. The final product was a yellow oil that smelled like apples.
I attempted to prepare phenylacetone again using a novel synthesis that I devised. Again I obtained a light orange oil. It vacuum distilled through a
long column to a yellow oil with the same characteristic apple odor.
When the Chem Player channel made phenylacetone using Friedel Crafts alkylation, the product was a yellow oil.
The Department of Chemistry channel shows phenylacetone as a colorless oil.
Is phenylacetone truly colorless? What purification method might remove yellow impurities?
The chemistry was fun, but I destroyed both samples of phenylacetone, so please do not U2U me asking to buy them.
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woelen
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You could try adding some activated charcoal. It may absorb the colored compound.
I had a similar experience with aniline. This is said to be a colorless liquid, but I only have seen brown or yellow 9or even reddish) samples. Adding
some activatd charcoal made the sample colorless, but only for a limited time. When stored in a small vial it turned yellow again. Probably it is
easily oxidized by oxygen in the air.
I also noticed that acetone and also MEK tend to turn slightly greenish/yellow on storage. Is this also due to oxidation and formation of condensed
species? The phenylacetone may also suffer from this effect.
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kmno4
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Yes, it is. Loooong time ago I distilled a sample of this compound under normal pressure (with correct b.p.) and got water-like liquid.
Unfortunately, I overlooked the very end of distillation and last few droplets of the distillate made my product slighlty yellow. It has very
pleasant, fresh odour, but first time I see "apple comparasion". Anyway, it is completely different than acetophenone and benzylacetone odour.
BTW. My acetone and MEK, although +15 years old, are coloureless.
Colours mean impure chemicals and/or bad containers.
[Edited on 30-1-2018 by kmno4]
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Mesa
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I've noticed with aryl carbonyl compounds that even tiny amounts of impurities give significant discolorations. Even the fresh sigma brand
benzaldehyde we used when learning IR-spec was a visible pale yellow color. And every time I've seen 2-phenylpropanal, it's been about the same shade
as redbull, despite MSDS saying it's "clear colourless liquid"
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zed
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Been a long time, but I may have seen a commercial product, that was basically a colorless or nearly colorless oil.
Ketones of this type, steam distilled, appear kinda lemon yellow-ish. At least, the way I remember it.
Aniline? I have never seen any, that wasn't slightly brownish-red tinted.
And, that would be, from a freshly opened bottle.
[Edited on 30-1-2018 by zed]
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Reboot
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It's been my impression that color from a simple aromatic compound comes from shifting the electron density (bond lengths?) in that aromatic ring.
For instance, strongly electron withdrawing or donating substituents tend to produce a shift in color away from colorless.
So, in that light, we might ask if the ketone of the phenylacetone is a strong enough electron withdrawing group to give the aromatic ring color. I
would say it's not, since it's just one functional group, not an insanely strong one (as far as influencing the aromatic ring goes), and there's some
distance between it and the aromatic ring. And of course, we know benzaldehyde is colorless, and that places the C=O closer to the aromatic ring.
:-)
Or maybe I'm just hallucinating all this. Opinions from more knowledgeable chemists are, as always, greatly appreciated. :-)
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woelen
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I think that the color is due to formation of condensed species. Condensed species have larger molecules and if electrons can flow around through
these larger molecules, then larger wavelengths of absorption are possible, leading to colored compounds. In some compounds, the colors can be
extremely intense and even concentrations like 0.01% or so may render an otherwise colorless compould look dark brown or red.
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Melgar
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Never tried it myself, but I have read a bunch of synthetic chemistry on the rhodium/vespiary/hive/whatever archives that ultimately ended in the
production of phenylacetone, and they always refer to it as yellow, and call it "honey" and themselves "bees". Could be MDP2P that they were talking
about (I forget now) but I think it would still apply to either.
The first step in the process of learning something is admitting that you don't know it already.
I'm givin' the spam shields max power at full warp, but they just dinna have the power! We're gonna have to evacuate to new forum software!
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LearnedAmateur
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Quote: Originally posted by Mesa  | | I've noticed with aryl carbonyl compounds that even tiny amounts of impurities give significant discolorations |
I think that may be limited to the more reactive species and/or specific routes of synthesis, depending on whether it’s an unwanted degradation or
particularly stubborn byproduct. I’ve made benzoic acid from toluene and KMnO4 (I know it’s a waste but it’s good for practice) on several
occasions and every time they’ve always come out as perfect white crystals, despite being an aryl carbonyl. I haven’t played with benzaldehydes
yet, but I was thinking of synthesising some piperonal from the small amount of piperine I have extracted - videos I’ve seen commonly end up with a
yellowish product although it is unclear what causes it.
In chemistry, sometimes the solution is the problem.
It’s been a while, but I’m not dead! Updated 7/1/2020. Shout out to Aga, we got along well.
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zed
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Piperonal? A white crystalline solid, of heavenly aroma.
Easily acquired, and fairly inexpensive.... Once upon a time.
Still is, I suppose, if you have the requisite permits.
Been kinda hard to casually buy, for the last 45 years or so.
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Master of the Elements
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That is a good idea.
| Quote: Originally posted by woelen |
I also noticed that acetone and also MEK tend to turn slightly greenish/yellow on storage. Is this also due to oxidation and formation of condensed
species? The phenylacetone may also suffer from this effect. |
My acetone has remained colorless for years in its factory bottle.
| Quote: Originally posted by kmno4 |
Yes, it is. Loooong time ago I distilled a sample of this compound under normal pressure (with correct b.p.) and got water-like liquid.
Unfortunately, I overlooked the very end of distillation and last few droplets of the distillate made my product slighlty yellow. It has very
pleasant, fresh odour, but first time I see "apple comparasion". Anyway, it is completely different than acetophenone and benzylacetone odour.
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Thank you for your description and service to science! My distillate looked colorless at first. The final product was light yellow. Every drop of
distillate was nearly colorless. I did not see the color changes.
| Quote: Originally posted by Melgar | | Never tried it myself, but I have read a bunch of synthetic chemistry on the rhodium/vespiary/hive/whatever archives that ultimately ended in the
production of phenylacetone, and they always refer to it as yellow, and call it "honey" and themselves "bees". Could be MDP2P that they were talking
about (I forget now) but I think it would still apply to either. |
I remember the Hive. I always wondered what happened to rhodium. I think "honey" was MDMA.
MDP2P is colorless. It has a sweet, pleasant smell that reminds me of birch beer more than safrole.
Phenyl acetic acid smells exactly like honey.
This Hive post mentioned that P2P smells like applies. The crude product from my second attempt left behind a very high boiling oil that smelled like
burnt flowers when distilled. I think burnt smells are usually tars.
| Quote: Originally posted by Mesa | | I've noticed with aryl carbonyl compounds that even tiny amounts of impurities give significant discolorations. Even the fresh sigma brand
benzaldehyde we used when learning IR-spec was a visible pale yellow color. And every time I've seen 2-phenylpropanal, it's been about the same shade
as redbull, despite MSDS saying it's "clear colourless liquid" |
Stabilized benzaldehyde will stay colorless for many months. Pure, it will turn yellow.
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Reboot
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| Quote: | | MDP2P is colorless. |
Ah! I've seen accounts of it having a yellow or even slightly greenish tint, but it would make sense for the pure product to be colorless. Was the
sample you saw purified by simple distillation or something more exotic (if known)?
If memory serves, "honey" was used to refer broadly to (liquid) drug freebases. Mostly MDMA but also other amphetamines and hash oil.
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Master of the Elements
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| Quote: Originally posted by Reboot | | Quote: | | MDP2P is colorless. |
Ah! I've seen accounts of it having a yellow or even slightly greenish tint, but it would make sense for the pure product to be colorless. Was the
sample you saw purified by simple distillation or something more exotic (if known)?
If memory serves, "honey" was used to refer broadly to (liquid) drug freebases. Mostly MDMA but also other amphetamines and hash oil.
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I saw it a few times, decades ago. I vacuum distilled some through a 40 cm column. The product was colorless. A friend vacuum distilled some through a
20 cm Vigreux column filled with beads while I watched. He had the same result, a colorless, fragrant oil
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Ozone
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Color is a tricky thing--it only takes a tiny bit of something brown to make a large batch of something yellow. There are probably some
parts-per-million of condensation "tar" in it (as Woelen said). I'd try treating it with a pinch of activated decolorizing carbon at around 70°C
(typical adsorption isotherm for activated carbon), cooling to below the isotherm, say 45°C, and filtering through diatomaceous earth (Celite 454 is
a solid choice) prior to isolation.
Your call,
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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Panache
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Colour is a very misleading indicator of purity, simply because coloured compounds can be highly coloured even at very low percentages.
Consider dyestuffs.
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aga
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Some things are colourless in certain conditions, yet have a pronounced colour in others.
e.g. good old phenolphthalein.
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nleslie321
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my aniline is colourless but i store it in the dark and have not had mine too long 3 or 4 months max also its sigma branded. i will see if it turns
colour after some time.
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zed
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Did you open it? Letting O2 into the bottle seems to lead to discoloration.
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wildfyr
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The discoloration of aniline is due to aniline N-oxide formation, so indeed depriving it of oxygen is the key to purity. It must have strong color,
because just about every sample of aniline (or dimethylaniline) I've ever seen had at least a yellowish tint (going all the way to deep orange). It
also seldom caused any issues with reactions I used it in.
https://en.wikipedia.org/wiki/Amine_oxide
Frankly, information on aniline N-oxide it is rather thin considering aniline is in the cabinet of most organic chemists, and this N-oxide such a
common impurity. I didn't find the boiling point in a quick google survey. No one seems to fret about that impurity much.
For phenylacetone, that benzylic position seems rather reactive, perhaps it reacts with oxygen or other trace materials to give a colored product.
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UC235
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Aniline, allowed to stand over KOH flakes and vacuum distilled is a refractive, but colorless liquid. Phenylactone is susceptible to self-condensation
to enones at the highly activated benzylic position, but I would expect freshly prepared, vacuum distilled material to be colorless.
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unionised
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Thanks for confirming that you don't deal with the pure material.
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