polymerizer87
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DMAc + SO2---> ? help?
I'm working with SO2Cl2 in a radical chlorination of Pyrazolo[2,3-a]imidazolidine (below a picture, but more besides the point of the question I'm
asking)
1) The solvent matrix in this reaction is EtOAc/DMAc
2) There is residual HCl in solution from previous steps
3) During SO2Cl2 addition there is no off gas
4) Follow SO2Cl2 addition NaOH is used to remove any over chlorinated Pyrazolo[2,3-a]imidazolidine.
5) FT-IR confirmed Sodium sulfite/sulfate salts in aqueous phase cut.
if this is in fact true than DMAc can be used as a scrubber for SO2 gas, let me know what you think...
[Edited on 6-8-2012 by polymerizer87]
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Nicodem
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Why do you invoke such an absurd SO2+DMAc adduct instead of invoking the most obvious solution, that of having SO2 in DMAc
solution? Don't you think that a polar compound like SO2 must be quite soluble in a polar solvent such as DMAc?
Have you ever heard of Occam's razor? It is a very simple device that is able to shave away all the superfluous explanations. For this reason it is a
very useful tool in science. I use it more often than I use a normal razor.
By the way, your scheme suggests a radical mechanism, but the application of the Occam's razor suggests that the mechanism is just the plain old
electrophilic aromatic substitution. To claim that a radical mechanism in a solvent like DMAc could lead to aromatic chlorination, without affecting
the so many more easily abstractable hydrogens, you would really have to have convincing evidence. We don't see such evidence, so the only thing we
are left is to use the Occam's razor and shave away the radicals.
…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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polymerizer87
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| Quote: |
Why do you invoke such an absurd SO2+DMAc adduct instead of invoking the most obvious solution, that of having SO2 in DMAc
solution? Don't you think that a polar compound like SO2 must be quite soluble in a polar solvent such as DMAc?
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Initially I did not invoke this idea, SO2 saturated in a solution would be the most obvious answer. In general acids do not off gas in
"basic" solvents such as DMAc, but there is an equivalent of Cl from the SO2Cl2 addition and an equivalent of HCl from prior
steps, as well as two amine functionality.
| Quote: |
To claim that a radical mechanism in a solvent like DMAc could lead to aromatic chlorination, without affecting the so many more easily abstractable
hydrogens, you would really have to have convincing evidence.
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Radical mechanisms in DMAc do not occur, which is what I had thought.
Although when (catalytic amounts) Br2 serving as a free radical initiator was bubbled into solution, chlorination had already occurred
(monitored by HPLC), without the addition of SO2Cl2, peak area and area percents were too high to be considered brominated.
One more thing, without getting into too much detail on the structure i'm working with R1 is a t-butyl group
I have not thrown out EAS by any means, as I truely think that there is more than one thing going on in this reaction.
[Edited on 6-8-2012 by polymerizer87]
[Edited on 6-8-2012 by polymerizer87]
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Ephesian
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| Quote: |
Why do you invoke such an absurd SO2+DMAc adduct instead of invoking the most obvious solution, that of having SO2 in DMAc solution? Don't you think
that a polar compound like SO2 must be quite soluble in a polar solvent such as DMAc?
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Nicodem-
Considering DMAc will resonate between the C-O and C-N bond its reasonable to say that this could promote some simple Lewis acid and base chemistry.
I dont know your reaction conditions but by simply calculating formal charge for DMAc you will find its dominant contributory resonance structure.
However, I did find this reference showing that DMF will form an isolatable salt with SOCl2 to form a similar adduct as you depicted in your scheme.
[Edited on 1-10-2012 by Ephesian]
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Nicodem
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Quote: Originally posted by Ephesian  | | However, I did find this reference showing that DMF will form an isolatable salt with SOCl2 to form a similar adduct as you depicted in your scheme.
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That product of the nucleophilic addition of DMF on SOCl2 is not really that similar (BTW, that compound rapidly decomposes to the
Vilsmeier reagent and SO2). Just because a structure is isoelectronic, it does not mean much when the thermodynamic aspects are so far
apart. Isoelectronicity only indicates a possible mechanistic pathway, but it does not determine the actual feasibility of a reaction. At the end, it
is the thermodynamics that determine the equilibrium constant of a reaction. In the case of the addition of DMAc on SO2, the reaction is
thermodynamically very unfavorable, while in the reaction of DMAc with SOCl2 it is favorable.
Just check the structures - you don't need to calculate anything - the rough thermodynamic aspects are obvious.
…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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