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Author: Subject: Unexpected ruby red colour
LearnedAmateur
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[*] posted on 12-3-2018 at 14:37
Unexpected ruby red colour


I wanted to see what would happen if I added sulphuric acid to anhydrous isopropanol. The acid I use is a brown colour, impure drain cleaner which contains trace organics and possibly a small amount of iron from containers used in shipping. I’ve done this before when synthesising isopropyl nitrite, usually I’ll dilute the iPrOH first to 50% with water then add the acid, and it always turns clear with an ever so slight brown colour. What happened surprised me though, it immediately turned a deep red colour and I’m really not sure why. The temperature quickly increased and there was a very pungent, petrol like odour which I’ve identified as propene, formed by dehydration, maybe some diisopropyl ether too but there wouldn’t be more than trace considering that there wasn’t a separate layer when I added water. Anyone ever had this happen before, and additionally, does anybody know what is responsible? Some kind of complex?

0A06008D-2CC8-4D0F-92C8-34F8EC5DF7B5.jpeg - 1.4MB




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Tsjerk
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[*] posted on 13-3-2018 at 01:53


Oxidation of IPA to acetone by the hot sulfuric acid I guess. Acetone forms red polymers with sulfuric acid.
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LearnedAmateur
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[*] posted on 13-3-2018 at 02:20


That seems like the most likely explanation, but the thing is, it happened instantly - as soon as the acid hit the isopropanol it went red whereas I would have thought that polymerisation would take minutes to days depending on conditions. Also, I couldn’t detect any acetone odour, if it were present in significant quantities then I would’ve thought that it would smell like nail polish remover, unless the propene overpowered it since I couldn’t even detect the sweetish isopropanol.



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clearly_not_atara
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[*] posted on 13-3-2018 at 02:53


If Fe3+ was present (as Fe(SO4H)3) that could facilitate oxidation I think.



[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 13-3-2018 at 03:14


I've had similar experiences with conc. H2SO4 and pure ethanol. Mixing these also led to red/brown colors, but the change of color was not immediate. Maybe the same kind of reaction occurs, but with iso-propanol the reaction may be much faster?



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LearnedAmateur
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[*] posted on 13-3-2018 at 05:22


I wouldn’t know whether any iron would be in the 2+ or 3+ state since their colours are easily masked by the brown colour, but I have read that it is a contaminant of drain cleaners for the aforementioned reason, still strange that it happened so quickly though.

I think polymerisation is still on the table, I mean if the ethanol is being oxidised to acetaldehyde then that can undergo (potentially rapid) polymerisation like all sterically unhindered aldehydes. Not entirely sure whether acetone or acetaldehyde would polymerise faster, but assuming that is the case, I would’ve thought the latter would be quicker due to higher reactivity? I’m sure we’ll get to the bottom of it eventually.




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[*] posted on 13-3-2018 at 08:50


Polymerization sounds like the most likely source. There could be iron but I doubt there is enough iron to cause that much of a color change. Your solution looks exactly like a mixture of Acetaldehyde and dioxane that I had after I added KOH to polymerize the Acetaldehyde. There might be some impurities in the sulfuric acid that act as a catalyst in the polymerization. Did it get any darker over time or did it turn it's final color almost instantly?
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[*] posted on 13-3-2018 at 09:58


The colour didn’t change, not from what I could see but it appeared to get ever so slightly more opaque over the course of a few minutes. I added in some sodium nitrite solution to make the most of it and the red turned into a lighter brown (seemed to be a reversal but it was a more intense and yellower brown than usual for those concentrations), but the weird thing was that the isopropyl nitrite didn’t separate out, even when left overnight whereas it pretty much immediately does on every other occasion. I’ll do it again in a test tube and report back here if/when changes occur.

Okay so I just added sulphuric acid to isopropanol, for some reason the reaction was unrepeatable. Brown colour was diluted as it usually does, but the reaction generated propene gas evidenced by smell and evolution of bubbles. The temperature shot up to around 80 degrees C. In this case and the one above, freshly cleaned glassware was used with minimal water present

Test 2: adding the sulphuric acid first. This time a layer of alcohol was deposited on top and the tube rapidly swirled to mix the two, to the same outcome. More isopropyl alcohol was added shortly thereafter and strangely I was able to reproduce the colouration, even though I literally just did the exact same thing (that’s chemistry for you I guess). After 10 minutes, no changes were observed.

Pictured: test 2 after mixing, showing the iPrOH diluted sulphuric acid. Will leave it for an hour to see if anything interesting occurs, after then I will add some sodium nitrite to get the colour ‘reversal’ again.

After one hour, no further changes were noted. Addition of sodium nitrite gave a chunky, gelatinous precipitate but no bubbling and a tiny amount of NO2 (by smell, no red-brown gas observed)

4F667844-74B9-4AE6-9BDA-52EC5C234A20.jpeg - 1.5MB

[Edited on 13-3-2018 by LearnedAmateur]

318EDC57-FE09-4623-A9C7-D1E0BE8BDBFA.jpeg - 1.8MB




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Dr.Bob
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[*] posted on 14-3-2018 at 10:33


Perhaps your isopropanol was dehydrated to propene which polymerized via the impurities. Hard to say what impuries might catalyze dehydration or polymerization. Not likely a good food coloring agent...
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[*] posted on 14-3-2018 at 13:43


OMG..... another useless topic about playing impure chemicals.
This is not chemistry, but pure amateurishness.




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[*] posted on 15-3-2018 at 02:36


Probably a mixture of acetone polymers and polypropene then, I’ve tried looking in the past but couldn’t find any information on other additives like corrosion inhibitors and such.

Kmno4: not all of us have access to pure chemicals nor the means of producing them from impure products. Drain cleaner works fine for most of my H2SO4 needs, this is just an outlier which produced an unexpected result and I just wanted to know if anyone could explain it so that it can be avoided in future so it doesn’t get in the way of my reactions like it obviously has done in this case - can’t see why you have a problem with that unless you just want to ignite some sort of flame war which is unnecessary and detrimental to progress. Plus, we’re on an amateur chemistry forum, so of course it’s amateurish, and it is chemistry because there’s obviously a chemical reaction going on leading to a qualitative result (adding A to B leads to result C).

Seriously, if you don’t have any constructive feedback to add then don’t say anything because your reply is more useless than my topic - it has happened to others and we are trying to root out the issue. If my post is that bad then surely this would’ve been moved to detritus, no? Don’t take this the wrong way, but the only issue here is your negative opinion which is just clogging up a legitimate and perfectly normal discussion.

[Edited on 15-3-2018 by LearnedAmateur]




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[*] posted on 15-3-2018 at 12:17


Quote: Originally posted by kmno4  
OMG..... another useless topic about playing impure chemicals.
This is not chemistry, but pure amateurishness.
I agree with LearnedAmateur. You have posted similar comments like this in the past and the kind of attitude you exhibit in your posts is the best way to put off (mostly young) enthusiast people who take their first steps in chemistry (or more general, in science).

If you really think that amateur chemists are doing useless things with junk instead of chemicals, then try to find another forum, where only people, affiliated to a well-equipped lab with reagent grade chemicals only, are participating. But I am quite sure that on such forums you will not find the enthusiasm and wonder you can read about over here.




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[*] posted on 15-3-2018 at 18:25


I agree that kmno4 was being unnecessarily rude, but this definitely does belong in Beginnings, not Organic Chemistry.



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