Taaie-Neuskoek - 16-9-2005 at 14:40
Today I did a small experiment to see whether acetic anhydride would be able to dehydrate sulphuric acid, but I observed a reaction between the two...
I added 30ml Ac2O into is a 50ml erlenmeyer, and added 10ml of H2SO4 (drain opener, it saids 97% on the bottle, but no artificial colouring
whatsoever).
I didn't expect something to happen, as both should be fairly anhydrous, so mixing shouldn't cause anything to heat up.
I was wrong. The mixture heated up very quickly, and went from the original colourless to orange and red. The solution started to boil in a couple of
seconds after adding the acid, I grabbed my gas mask and ran out. I put on my mask, and looked carefully around the corner what the hell was going on.
The solution finally stopped boiling, by then it looked black, but upon shaking it is revealed that it is actually very dark red. Now it is cooled
down, the liquid is rather viscous, and is not sticking to glass, but sliding away from it, rather strange...
Now my lab stinks of vinegar 
working with Ac2O is always a pleasure... 
Does anyone know what the hell had happened here? If even some SO3 had formed, it has probably distilled off by the heat... I saw some vapours, but
have no iea whether they were SO3 or not, I was wearing a full-face gas mask...
(Forgive me if there was already a thread about this, I did use the search, but it was hard to find anything with the google search, Polverone's
sig. contains the words acetic anhydride, and the searching machine doesn't make any difference between sig's and posts. The board's
search didn't get any hits.)
[Edited on 16-9-2005 by Taaie-Neuskoek]
ADP - 18-9-2005 at 15:30
Hmm, I think it's an interesting reaction. I wish I knew what happened. One would think that the anhydride could pull water out of H2SO4 at
reflux when it separates into H2O and SO3, However maybe it reacted with the buffers in the drain cleaner. I'm not sure of their makeup
chemoleo - 18-9-2005 at 15:39
I was wondering earlier but then forgot - wouldnt a mixed anhydride be possible, at the extent of heating up for it being a favourable reaction?
I.e. CH3CO-O-SO2-O-OC-CH3
This might also explain the more hydrophobic character (not sticking to glass).
Alternatively - did you notice gas evolution, i.e. CO2 due to decarboxylation?
What I find confusing is the colour - normally that makes me think of a polymeric (even unsaturated) product.
Did you analyse it further?
What happens if you add water to it?
Does the colour disappear? Etc Etc. Would be interesting to follow this up, preferably though with H2SO4 of known quality.
woelen - 18-9-2005 at 23:39
I would not expect too much from this reaction. I have similar observations with the combination H2SO4/acetone, both chemicals lab-grade, so no
coloring agents from drain cleaner. With MEK the reaction is even more pronounced. I had similar observations with colorless denatured alcohol,
although in that case the liquid did not became syruppy.
To my opinion the combination conc. H2SO4 / organic compound often leads to useless dark red/brown/black crap. I think that the oxidizing properties
of the acid, combined with its dehydrating properties easily leads to polymerization and charring of most organic compounds. I certainly do not expect
this reaction to be of any use for making SO3.
This reaction also strongly reminds me at the reactions of aqueous solutions of polyphenols in dil. NaOH. Those liquids also form dark red/brown
polymerized species, which cannot be easily characterized. The color red/brown/black to my experience seems to be very common when smaller molecules
condense to larger species with a high carbon-content, but this latter thing is just my personal observation and has not scientifically been
established.
chemoleo - 19-9-2005 at 05:16
Actually the reaction between H2SO4 and acetone has been described, have a look in the DPPP thread.
I think the consensus was that a variety of products, inc. mesitylene and mesityl oxide, and other condensations products are formed.
Here however we have a double bond from the CO facilitating this, something not quite the case with acetic anhydride.
But then.... yes I agree, most likely they are condensation and dehydration products.
As I said, it would be interesting to see what happened if H2O was added, and whether i.,e. the colour would disappear.
Taaie-Neuskoek - 19-9-2005 at 08:12
I've trown the stuff away, as I needed the erlenmeyer and it was annoying me with the red/brown stains it left behind.
When I added the stuff to a large amount of water it mixed well after some stirring, and formed a brownish color. The solution was (doh) heavier than
water, so it needed stirring to mix.
If I can find some time, I'll try to do this again in smaller and better defined amounts, with H2SO4 of known quality.
I did add later some H2SO4 to GAA, but that didn't do anything other than heating up a little bit. no coloring, or whatever...
woelen - 19-9-2005 at 13:25
The fact that the liquid does not loose its color on dilution with water indeed is strong evidence that this is a condensation product (or better: a
badly characterized mix of many condensation products). Such mixes of condensation products hardly are interesting, because they are not single
compounds, but a very complicated load of 'crap'.
kmno4 - 28-9-2005 at 00:06
H2SO4 reacts with Ac2O in this way:
H2SO4+2(CH3CO)2O -> CH3CO2SO2CH2COOH + 2CH3COOH.
woelen - 28-9-2005 at 04:09
So, this compound CH3CO2SO2CH2COOH is responsible for the brown color? This compound also is stable in water?
This looks like an interesting compound. Is the structure of this compound as follows?
H3C-C(O)-O-S(O)(O)-CH2-C(O)OH.
With S(O)(O) I mean S with two O's bonded on it as side-chain and the C of the CH2 group connected directly to the S.
Can you give an on-line reference to this reaction?
12AX7 - 28-9-2005 at 12:04
That would be uhhh, acetyl sulfonate acetate or so?
Tim
kmno4 - 2-10-2005 at 10:24
That equation is taken from :
"Reaction of organic compounds" a textbook for the advanced students
,
Reynold C.Fuson
1962, copyright by Wiley&Sons (I have authorized translation of this publication)
It is a little old, but a very good guide-book through OS and OR.