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byko3y
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CuReUS, yes, I used higher concentration, because the 0.25 M carbonyl and NaOH concentration used by original researchers is some
really dillute solution, you gonna need 1 L of isopropanol to reduce 21 g of cyclopentanone, which is unacceptable, unless you are reducing some
really precious ketone. I was using 0.78 M cyclopentanone and 2.5 M NaOH concentration (exactly 10x original), but sodium hydroxide was not completely
dissolved, so the real concentration was below 2 M.
Quote: Originally posted by CuReUS  | | I don't understand why everyone is behind microwave chemistry.So many books have been written on it and there are few reactions which have not been
tried in MW. |
It's easy to declare novelity of your research just by repeating a well known reaction using
MW. Especially indian and iranian researchers love it.
Unraveling the Mysteries of Microwave Chemistry Using Silicon Carbide Reactor Technology
They do. Small amount of water can
heavily shift the equilibrium of IPA+NaOH<->NaIPA + H2O towards left side.
| Quote: Originally posted by CuReUS | | Are you saying that water should not be removed from this reaction because then you would get predominantly aldol products ? |
It might be true, it might be not though. Only practice will tell us, and my trial yielded some mediocre results.
I have a data for cyclohexanone self condensation only: the equilibrium is shifted to the monomer at room temperature and saturation with water, while
at higher tempeartures solubility of water rizes and monomer becomes the sole compound. Above 100 C ketol (dimer) is dehydrated, irreversibly yielding
an unsaturated ketone.
But in our case there's also acetone and reduction of condensation products, so the resulting mixture is very complex, and I have cyclopentanone
instead of cyclohexanone, so god only knows what are the products.
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ziqquratu
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| Quote: Originally posted by CuReUS | | I don't understand why everyone is behind microwave chemistry.So many books have been written on it and there are few reactions which have not been
tried in MW.Obviously these chemists are not fools to follow something if it was completely fake. |
It's not about being stupid, it's about it being very easy to fool yourself into thinking there's an effect when there may not actually be one. For
microwave chemistry, much of the initially reported effect seems to have resulted from poor measurement of the reaction temperature (meaning the rate
was faster than predicted for the temperature measured - but the temperature measured was lower than the actual temperature, accounting for the
increased rate!). That was compounded by the "lab" microwaves which had dodgy IR sensors, because it's *really* easy to just trust what the shiny,
expensive new machine tells you...
There can be advantages. With the lab instruments, it's an easy way to do a reaction under pressure, without needing a special autoclave setup. You
can also generally heat the reaction very quickly and more evenly than a heat bath, which can have its advantages (less hot-spots on the walls can
lead to less charring, say). You can get reactors to improve the rate of heating, too - like the SiC ones byko3y mentioned. And there do seem to be a
few examples where there might be actual microwave-specific (albeit still thermal) rate increases - if you have a solid reactant which strongly
absorbs microwaves, for example, you can get a high surface temperature (which means high temperature where the reaction is actually taking place) but
a low bulk temperature, leading to fast reaction with less thermal decomposition. But much of the hype seems to be unjustified when people have gone
back and studied it more carefully. There was a reasonably amusing (for the scientific literature) argument in Angewante Chemie back in 2012 or so
between Oliver Kappe (one of the microwave chemistry gurus, who holds that there's no microwave specific effect for reactions in homogeneous
solutions) and another author on the topic.
byko3y is right, though, that using a microwave is a good way to get a paper published!
I think the point was that water will irreversibly destroy aluminium alkoxides (because aluminium hydroxide will precipitate from solution), whilst
sodium and potassium alkoxides exist in a reversible equilibrium - which the reaction in the original reference seems to rely upon.
| Quote: Originally posted by byko3y | | sodium and potassium alkoxides catalyze aldol condensation much more than those of aluminium, thus removal of water might lead to dehydration of aldol
condensation products. |
It's possible, certainly. I don't know how big a deal it's going to be, given the low concentration of base and the *very* low concentration of
alkoxide. I'd think the hydroxide itself (which is entirely capable of promoting aldol condensation, and present in a much higher concentration) would
be the bigger concern - and taking the paper at face value, it doesn't seem to be an actual concern. But also, they seem to have taken no particular
precautions about the presence of water - perhaps you'd see a rate increase if you removed the water (due to an increase in alkoxide concentration),
but whether that's worth the hassle is another matter, too.
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zed
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Well, somewhere, I read an account of quantitative yields, reducing benzaldehydes w excess isopropyl-alcohol via heat and pressure. No catalyst.
Naturally, I can no longer find the reference.
The suggestion that aldehydes are preferentially reduced vs ketones, does suggest that once benzylalcohol has been formed, it likes to stay
benzylalcohol.
The Vespiary has a reference.
"reducing benzaldehyde with isopropyl" Type the foregoing in, and search via Google. Should come right up.
[Edited on 7-3-2017 by zed]
[Edited on 7-3-2017 by zed]
[Edited on 7-3-2017 by zed]
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byko3y
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Equilibrium constant for
IPA+NaOH<-> NaIPA+H2O, K=[NaIPA][H2O]/[IPA][NaOH] is close to 1, although i have no idea how close it is: it might be 3, it might be 0.3. For
initial 13 M IPA and 0.25 M NaOH with relatively anhydrous reagents the equilibrium is K=[NaIPA]*[NaIPA]/(15-[NaIPA])(0.25-[NaIPA]), from which we get
something like [NaIPA] = [H2O] = 0.246 M, [NaOH] = 0.004 M.
For my case I can calculate [NaIPA] = [H2O] = 2.14 M, [NaOH] = 0.36 M. I believe it was finally dissolved completely, despite the fact I was not able
to verify it due to some difficulties.
As you can see, the ratio [NaIPA]/[NaOH] in original conditions is 300-1000, while in my conditions it is 5-10. AFAIK, alkoxides are just as potent
aldol catalysts, as hydroxides. But MPV is not catalyzed by hydroxides. I believe the reason I got so much by-products is high concentration of
carbonyls, and not a high concentration of the base.
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ziqquratu
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I've never even bothered to look up those equilibria - you're somewhere around the right number (for EtOH + NaOH <--> NaOEt + H2O, K ~ 0.7). Colour me surprised, for sure!
I think my analysis holds, though, just changing the focus to the still low (but not "very" low) alkoxide concentration, and effectively neglecting
the hydroxide now.
I agree with your analysis, too - higher ketone concentration will likely favour addition/condensation for a given concentration of base, and you've
got a much higher concentration of base as well (the paper reported 0.25 M with respect to both ketone and base).
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CuReUS
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Unless someone repeats the synthesis,following the author's instructions to the letter(byko3y ) and reports their findings,there is no use talking about it.The whole point of the thread was to find out whether the
paper was correct or not.
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byko3y
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This insane level of dilution makes the method of a low practical value, not just because of the need for a large reaction vessel, but also because of
tedious workup to separate few grams of the target product from a bunch of byproducts in the excessive amount of solvent. In fact, it took me more
time to extract the pure product than to perform the actual reaction.
If I had to do the reduction, I would prefer a good old covalent Al(Oi-Pr)3 catalyst with high concentration of reagents, from which you can easily
distill acetone, and finally do a liquid-liquid extraction right from the reaction mixture without the need to concentrate it.
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