testimento
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Using CO2 gas for grignard reaction instead of dry ice?
I have difficulties accessing dry ice, so could carbon dioxide be injected as a gaseous product into the grignard reagent? From 1.2kg of carbon, one
could theoretically obtain about 4.4kg of carbon dioxide gas, and an equivalence for dry ice could be calculated.
The question is, how much excess would be needed due to the misabsorption of the grignard reagent of CO2 gas? I could easily construct a device where
I would insert coal(in charcoal or coke form) and blow air with an air pump to burn the coal into CO2, lead it through a water scrubber to remove soot
and then through CaCl2 to remove moisture. For getting 4.4kg of CO2 I would need to push at least 12m3 of air(at 21% oxygen), and this sounds rather
nasty.
Are there other substitutes than carbon dioxide for grignard?
[Edited on 7-10-2013 by testimento]
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sonogashira
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Better to use pure CO2 from a fire extinguisher, rather than fumes from a camp fire!
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MichiganMadScientist
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What's wrong with using nitrogen gas, passed through a drying tube? Of course, if you live in an area where dry ice is difficult to obtain, nitrogen
gas cylinders may be an issue to...
Hopefully, I'm correctly understanding why you need the gas for the grinard reaction in the first place. Last time I performed one, I really don't
remember using any form of purging gas.
Or is the purpose of the gas actually have to do with the reaction it's self???
[Edited on 7-10-2013 by MichiganMadScientist]
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DraconicAcid
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Quote: Originally posted by MichiganMadScientist  | What's wrong with using nitrogen gas, passed through a drying tube? Of course, if you live in an area where dry ice is difficult to obtain, nitrogen
gas cylinders may be an issue to...
Hopefully, I'm correctly understanding why you need the gas for the grinard reaction in the first place. Last time I performed one, I really don't
remember using any form of purging gas.
Or is the purpose of the gas actually have to do with the reaction it's self???
[Edited on 7-10-2013 by MichiganMadScientist] |
He wants to react the Grignard reagent with carbon dioxide to get the carboxylate salt.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Crowfjord
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When using gas, you would also want to remove oxygen somehow, as it would also react with the organomagnesium halide to give hydroperoxide and/or
alcohol (see Wikipedia's Grignard reaction page).
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sonogashira
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The problem with using gas is that there is a large excess of the grignard during the 'addition'. This, coupled with the higher temperature (compared
to dry ice) causes further addition of the grignard to the intermediate, so that one will often find tertiary alcohols upon workup. I read this in a
book!
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DraconicAcid
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| Quote: Originally posted by sonogashira | | The problem with using gas is that there is a large excess of the grignard during the 'addition'. This, coupled with the higher temperature (compared
to dry ice) causes further addition of the grignard to the intermediate, so that one will often find tertiary alcohols upon workup. I read this in a
book! |
Does that book say what you would get if you reacted the Grignard with sodium carbonate? If it reacted at all (which is even less likely than the
RMgX reacting with a carboxylate), the hypothetical RCO3(3-) ion formed would easily hydrolyze to the regular carboxylate.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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DJF90
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Grignard reagents aren't strong enough nucleophiles to add to carboxylate salts. You'd need an Organolithium for that, so I don't think it'd be too
much of an issue.
Grignards also add to oxygen giving hydroperoxides, so make sure your CO2 source is oxygen free. Dumping the reagent onto a beaker of dry ice isn't
exactly scalable, and you'll get some hydrolysis by the water that condenses onto the dry ice. The best method I've seen so far (at least for a
relatively small scale) is to use a flask of dry ice as the CO2 source, which can be heated in a water bath to hasten the evolution of dry CO2 (as the
water vapor remains essentially in the remaining dry ice).
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DraconicAcid
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If you don't have access to dry ice, you would probably be better off generating your CO2 with baking soda and sulphuric acid, as long as you remember
to use a drying tube to keep water from accompanying the CO2. Purging the oxygen from that set-up will be easier than keeping out the oxygen that is
used for the fire.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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sonogashira
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The handbook of grignard reagents, no less, is where I read it. I bow to the greater knowledge of the writer:
http://books.google.co.uk/books?id=82CaxfY-uNkC&pg=PA364...
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DJF90
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The trialkyl carbinol the author speaks will originate from the collapse of the tetrahedral intermediate formed on addition of the grignard to the
carboxylate salt, followed by subsequent addition to the newly formed ketone.
After checking some literature it appears the general approach is the addition of the grignard reagent to dry ice. However, further searching reveals
references such as: http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv2...
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Magpie
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In addition to convenience (for most of us) another benefit of dry ice would be its cooling effect. Remember that your solvent is usually diethyl
ether which has a boiling point of 34.6°C.
The single most important condition for a successful synthesis is good mixing - Nicodem
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DJF90
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I appreciate your point magpie but as the reagent (CO2 gas) is introduced relatively slowly (compared to dumping the grignard into a beaker of dry
ice) the need for a great heatsink (c.f. cardice) to protect against exothermic reaction is greatly reduced.
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bfesser
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You'll evaporate off more of the ether if you use CO<sub>2</sub>(g) rather than CO<sub>2</sub>(s). This would be particularly
problematic on a small scale.
Honestly, <strong>testimento</strong>, if you can't obtain something as simple as dry ice, then perhaps you shouldn't be attempting a
Grignard—particularly on such a large scale.
[Edited on 8.10.13 by bfesser]
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Magpie
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Yes, DJF90, of course the rate of gas sparging can be controlled to fit the situation, ie, scale, heat transfer, etc.
I, as likely many of you, have made benzoic acid using dry ice as the carbonyl. As I recall the bubbling gas comes rolling out of the reaction vessel
at a pretty good rate. I looked up the mole ratio of dry ice/Gringard reagent specified by two lab procedures, ie, one by Kriz et al and one by
Brewster et al. Kriz used a mole ratio of 10.8 (in 19mL ether) whereas Brewster used a mole ratio of 5.4 (in 180mL ether). This should give the OP a
rough idea of the efficiency of the utilization of gaseous CO2, ie, it's around 10-20% at these small scales.
The single most important condition for a successful synthesis is good mixing - Nicodem
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testimento
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I understand your concern, beefie, but as I told you before, I live in a very controlled, small country where you gotta order everything aboard or do
it yourself. We don't have even the basic chemicals available, for ex. sulfuric and hydrochloric acid just simply doesn't exist in any other place
than large scale industry. So getting even basic stuff for me is extremely difficult, glassware is not (it's a wonder) banned (YET), but there is a
round zero suppliers. I know only one single supply of dry ice with waiting periods of 2 weeks, price at least 10 bucks a kg + delivery and handling
fee and one must register into the company to order it or any other products they sell(some gases).
And that's where I see the issue using gaseous CO2. One solution could be to use long bubbling time and extremely cold cold baths with mechanical
refrigeration(literally, putting the reaction vessel inside a freezer with full power at -35C at glycol filled reflux column and cold bath and bubble
the noted amount through in 2-4 hours.
Damn, it'd be even considerable idea to make dry ice oneself, a small compressor isn't quite expensive...
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WPK0129
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I like sonagashira's original response: use a fire extinguisher. Don't forget that PV = NRT means that the carbon dioxide coming out of the fire
extinguisher will cool as the pressure falls. I can't speak to the % yield of this process, but I have seen dry ice condense on surfaces continually
blasted with a fire extinguisher for long enough.
Granted, it'd have to be a big one or maybe multiple extinguishers. . .
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DraconicAcid
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Apparently, carbon dioxide fire extinguishers are out of fashion; dry powder is much more common. Which is a pain in the [expletive], because
cleaning up ammonium phosphate after a fire is nowhere near as easy as letting the CO2 sublime.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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Magpie
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Another possible source of gaseous CO2 is paintball suppliers - that's where I bought my nice little black cylinder of compressed CO2. Maybe not so
possible for testimento. 
Just today I saw a huge truck of compressed CO2 rolling down the city streets to supply the local restaurants for use in making carbonated soft
drinks. But this is not a likely source for individuals.
The single most important condition for a successful synthesis is good mixing - Nicodem
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gsd
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I have tried to use CO2 fire extinguisher as a source of CO2 gas in lab - Not for grignard but for a steady purge required in an analytical procedure.
I fitted a needle valve with proper upstream fittings and a hose nipple at discharge side. I observed that after opening the main valve, within few
seconds the needle valve used to get chocked due to formation of dry ice.
The solution to this problem was rather expensive. I had to buy a proper CO2 gas regulator which comes with thermostatically controlled electrical
heating element. This contraption works like a charm. However I had to spent equivalent of 50 $ on regulator as against about 2 $ for needle valve.
gsd
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Morgan
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The other day I bought a 4 ounce bottle of CO2 made by Cuisinart from Home Depot just because I wanted the aluminum bottle for some experiments. They
make a larger 16 ounce bottle. Just another source of CO2. You'd have to buy a regulator or design something of course. It's free shipping if you have
it sent to the store. They stopped making that size in paintball tanks.
http://www.youtube.com/watch?v=SCfMmcL8rbM
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Magpie
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I would love to see that woman doing a Grignard with a Cuisinart demo. 
Here's a 24 oz CO2 cylinder for sale:
http://www.amazon.com/gp/product/B007XKFQCM/ref=s9_dnav_bw_g...
The single most important condition for a successful synthesis is good mixing - Nicodem
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Morgan
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The thought crossed my mind when I posted the clip, somehow the "Grignard Cuisinart Machine" had a ring to it. I've bought several of the paintball
tanks at a few sporting goods stores but they are sold new and empty. One store also filled them for you.
Today at some Habitat for Humanity store they had two fat CO2 tanks for sale. They were about 8 inches in diameter and around two feet tall.
And some other sizes.
http://www.beveragefactory.com/draftbeer/tanks/co2/C5.shtml
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