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Author: Subject: Grignard Fail.
SmellNoEvil
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sad.gif posted on 10-3-2016 at 17:29
Grignard Fail.


I tried a Grignard reaction to make tert-butanol from acetone and methylmagnesium iodide. Needless to say,it failed.


First, 30ml of clear methyl iodide was added to a 3-neck flask with a condenser attached and equipped with a stir bar. Next,35ml of ether and 11.7 grams of magnesium was added,the mixture bubbled aggressively,and I had to replenish the ether two times. One the bubbling stopped, I added 35ml of acetone to the reaction flask. After 22 minutes of stirring, I disassembled the setup and then poured the liquid-which was somewhat dark in color- into a separate flask, with a few washings with hydrochloric acid. The flask with the liquid was then connected to a condenser for simple distillation. Everything coming over below 80C was discarded.

The final yield of tert-butanol was around 7 grams.
Theoretical yield: 35 grams.


It seems that I utterly botched the synthesis. Is there any way to improve the amount of tert-butanol product in future runs?

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[*] posted on 10-3-2016 at 17:37


1) IIRC Iodides are known to give poor yields in Grignard. The tendency with them is to form alkanes, in this case Ethane.

2) You need to work on your work-up. Washing a substrate containing tert-Butanol with HCl is not a good idea. Even dilute HCl reacts with it to form Tert-Butyl Chloride.

gsd
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[*] posted on 10-3-2016 at 18:03


Quote: Originally posted by gsd  
1) IIRC Iodides are known to give poor yields in Grignard. The tendency with them is to form alkanes, in this case Ethane.

2) You need to work on your work-up. Washing a substrate containing tert-Butanol with HCl is not a good idea. Even dilute HCl reacts with it to form Tert-Butyl Chloride.

gsd


1) In that case, what is the solubility of methyl bromide in ether? I used methyl iodide due to the fact that methyl bromide is a gas at room temperature,so I thought that most of the MeBr would be lost once the methylmagnesium bromide started to form.

2) Will sulfuric acid be a better substitute for hydrochloric acid?
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[*] posted on 10-3-2016 at 18:17


I think you threw out most of your product in washings. Also, do you have any verification that your product is clean/dry tert-butanol?

Your procedure seems very sloppy. Was the glassware flame or oven dried beforehand?

Why is the halide in the flask first? It should be added to the magnesium as a solution in ether and slowly. Was it dried with anything (if you made it, to remove methanol) (or was it reagent)? If it was dried with sulfuric acid, was it distilled or otherwise treated to remove traces of sulfuric acid? Why did you use an entire half of a mole on an untested procedure?


There is not anywhere near enough ether in your procedure. You should be using 150-200ml. Was the ether dried with anything beforehand? You state that you had to replenish the ether. Were you not using a condenser at all? Methyl iodide boils at 42C. Ether boils at 35C. If the ether boiled off, so did most of the halide.

You use slightly less than an equivalent of magnesium. This is probably okay since you'll lose some halide to Wurtz coupling, but if there isn't any at all left at the end of the grignard formation you probably should have a bit more.

Was the acetone distilled prior to use? Was it dried with anything? Addition of RMgX to a carbonyl is strongly exothermic. It should be added slowly.


Next time, look up a procedure beforehand and follow it. Here is one I found by googling the words "methylmagnesium iodide orgsyn." http://www.orgsyn.org/demo.aspx?prep=CV6P0442 The procedure is in Part B. The nitrogen can be omitted without much reduction in yield. If using THF, you need nitrogen/argon.

t-BuOH is both reactive with HCl as mentioned above as well as miscible with water. It forms a low-boiling azeotrope (79.9C) containing 88.3% tBuOH and 11.7% H2O (the wikipedia azeotrope table lists this). I don't see any drying steps included in your procedure and ether cannot be counted to exclude most of the water as I would expect the Et2O-tBuOH-acetone mixture to have considerably elevated solubility for water. Unless you used a very efficient fractionating column, you probably discarded some in the forerun as well along with unreacted acetone. Acetone and tBuOH (azeotrope) are only 23C apart. That is a very difficult separation.

Your stillpot residue may also have a considerable amount of water carried over with the ether beyond the azeotropic quantity.

If this is en-route to a catalyst for making potassium, I would suggest EtMgBr next time to make tert-amyl alcohol which has reduced solubility in water and elevated boiling point relative to the other contents of the stillpot. Even better, use MEK instead of acetone and make 3-methyl-3-pentanol. Or cyclohexanone.

[Edited on 11-3-2016 by UC235]
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[*] posted on 10-3-2016 at 18:33


MeI is toxic as hell. Try it with MeBr which is available as a mineral analysis tool. Organic iodides don't make Grignards very easily as was mentioned.



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[*] posted on 10-3-2016 at 22:08


As long as you work dry and clean, iodides shouldn't be that much of a problem. Maybe use a small crystal of I2 to get the reaction started if it doesn't by itself.

Use molar sieves and distill your CH3I prior to use. Leaving the glass in an oven at 150 degrees for an hour orso worked fine for me as I didn't have a gas flame at the time. Also put a CaCl2 tube on top of your condensor.
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[*] posted on 11-3-2016 at 04:10


Quote: Originally posted by chemrox  
MeI is toxic as hell. Try it with MeBr which is available as a mineral analysis tool. Organic iodides don't make Grignards very easily as was mentioned.


MeMgI is one of the easiest to prepare, and the bromide is not exactly nontoxic.

Quote: Originally posted by UC235  
If this is en-route to a catalyst for making potassium, I would suggest EtMgBr next time to make tert-amyl alcohol which has reduced solubility in water and elevated boiling point relative to the other contents of the stillpot. Even better, use MEK instead of acetone and make 3-methyl-3-pentanol. Or cyclohexanone.


I thought MEK gives t-amyl alcohol? I'm using both acetone and MEK with MeMgI soon.




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thumbup.gif posted on 11-3-2016 at 16:12
Found something useful...


I looked up "prepchem tert butanol", and the first result was:

http://www.prepchem.com/synthesis-of-tert-butyl-alcohol/

The procedure calls for using barium oxide as the drying agent for the T-Butanol, I think calcium chloride will be a suitable alternative though.



Tsjerk was right, I didn't dry the reagents much.

I'm planning a re-run sometime next week. Is there anything else that I should know before retrying the procedure besides better drying technique and placing a CaCl2 tube on top of the condenser?

[Edited on 12-3-2016 by SmellNoEvil]
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[*] posted on 11-3-2016 at 17:50


Calcium chloride is NOT a suitable drying agent for t-butanol. I will form a complex and will not serve to dry the solution. If you don't have any barium oxide, I would use magnesium sulfate. To run Grignards, you need to make sure that the glassware is flame or oven dried immediately prior to use, use a drying tube, use dried solvents and reactants, and I would recommend a crystal of iodine or a few drops of dry and ethanol free chloroform or carbon tetrachloride to activate the magnesium. Much of this was mentioned above. Also, be sure that you run ice water through your condenser, it was definitely much too hot if you lost all of your ether the first time you tried this.



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[*] posted on 11-3-2016 at 18:17


Quote: Originally posted by SmellNoEvil  
I think calcium chloride will be a suitable alternative though.


Calcium chloride is handy when you want an alcohol (among other things) removed as well. We have come some way since 1919, we have molecular sieves now, and Perrin, Armarego et al.'s Purification of Laboratory Chemicals.

There are general recommendations for the Grignard to be aware of, that you are unaware of, such as very specific volumes of ether, which should total like 500 ml at that scale. The acetone itself should be added in ether, and so on.

My scans Cumming, Vanino, and Cheronis all have the butyl alcohol from acetone, but no cookbook has been found that gives amyl alcohol from MEK, presumably because the old preparation from amylene is much simpler, and modern books do triphenylmethanol and not much else.

PS An example of general recommendations, from the Organikum:

Preparation of the Grignard compounds
In a 1-litre three-necked flask with a dropping funnel, stirrer, and reflux condenser with calcium chloride tube, 0.5 mole of magnesium turnings is covered with 50 ml of absolute ether and treated with about 1/20 of a total of 0.5 mole of alkyl or aryl halide with stirring. The start of the reaction is shown by the occurrence of a slight turbidity and a rise in the temperature of the ether. If the reaction should not start, 0.5 ml of bromine or a few drops of carbon tetrachloride are added to the reaction mixture, which is gently heated. After the reaction has started, the remaining alkyl or aryl halide, dissolved in 125 ml of absolute ether, is added dropwise with further stirring in such a way that the ether boils gently. If the reaction becomes too vigorous, the flask is cooled with water. Towards the end of the addition, the mixture is boiled gently on the water bath until almost all the magnesium has dissolved (about 30 min.)

Reaction of Grignard compounds with aldehydes and ketones
0.4 mole of the carbonyl compound (but 0.2 mole of an ester, why?) in the same volume of absolute ether is stirred dropwise into the solution of the Grignard reagent from 0.5 mole of halide. After the addition is complete, the mixture is heated on the water bath with stirring for another 2 hr, and it is then cooled, hydrolysed by the addition of 50 g of crushed ice, and subsequently treated with just sufficient 50% concentrated hydrochloric acid-water to dissolve the precipitate that has been formed. In the preparation of tertiary alcohols, dehydration may occur even under these conditions. In such cases the hydrochloric acid is replaced by 50% aqueous ammonium chloride. The ethereal layer is separated off, the aqueous phase is extracted twice more with ether, and the combined extracts are washed with saturated bisulphite solution, bicarbonate, and a little water. After drying over sodium sulphate, the ether is distilled off and the residue is fractionated or recrystallized.

[Edited on 12-3-2016 by S.C. Wack]




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[*] posted on 12-3-2016 at 16:45


SM Evil.

I am surprised you got a yield at all. Considering that your ran the procedure Bass Ackwards.

Dr. Wack's procedure is much more orthodox and likely to produce a much better yield.

"Grignard successes and failures"

Got a heading of that title, in the Organic Chemistry Thread. Worth a look.
Take some pointers from the veterans.

[Edited on 13-3-2016 by zed]

[Edited on 13-3-2016 by zed]
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