Sciencemadness Discussion Board

My First Grignard

Meltonium - 14-2-2017 at 14:47

This is my write-up of my first attempt at a grignard reaction. Although the procedure I have written was completed, a workup is still necessary to obtain a pure product and determine a percent yield.

I will say that all of my reagents were stored on drying agents (except for acetone which could make a lower yield) and I ran out of ethyl ether because I had made it from ethanol and only obtained ~70mL. This became a problem later on.

Objective: To preform a Gringard Reaction to make 2,3-dimethylbutan-2-ol

Materials:
- Diethyl Ether
- 8g Mg turnings
- 19g Acetone
- 26g 2-chloropropane
- Hydrochloric Acid
- Iodine granuals

Procedure:

- Setup glassware so that a 250mL flat-bottom flask connects to a Claisen adapter. On the arm of the Claisen, attach a 200mm Liebig condenser with a balloon covering the top. On the central arm connect a stoppered pressure eaqualizing addition funnel.
- Add Mg turnings to the flask followed by enough ether to cover them entirely.
- Place the flask in an icebath
- Dropwise add diluted 2-chloropropane to the flask
- Add a few iodine granuals if needed.
- Use a hot water bath to heat the flask until reaction is complete
- Remove heat and cool to 0C
- Slowly add diluted acetone with rapid stirring
- Allow reaction to complete
- Add hydrochloric acid and wait for fizzing to stop
- Evaporate off the ether

Notes:
- Prior to the reaction, the glassware was baked at 300F for an hour
- The solution turned yellow after iodine was added
- The strong yellow color faded over time while the reaction was being heated
- The solution fizzed and bubbled heavily and deposited white precipitate on the walls of the flask
- The solution then turned black and appeared soupy
- The acetone caused a white precipitate to form
- Not enough ether was left to dilute the acetone so the reaction was heavily exothermic and caused the solution to form a white crust
- The mixture solidified so much that it had to be manually stirred
- When the HCl was added, a brown color appeared probably due to iodine presence
- There was a faint gasoline smell present (maybe the 2-chloropropane or product?)
- The solid was left open to air for 3 hours before all HCl could be added (I had to go to class)
- After all HCl was added, the solution became fully liquid and separated into two layers. The bottom layer was a light brown and the top layer was a dark, almost black brown. (Actually not it has a green hue)
- The bottom layer is most likely the aqueous layer.

~Obviously this isn't complete. I still need to purify the product, which there isn't much information on that I could find. Sigma-Aldrich claims a MP of -15C and a BP of 120C which both seem likely considering the structure. However, I don't know if it forms any azeotropes with anything, its solubility in water/other solvents, or what it even looks like (I am assuming a clear liquid) I guess my next step is a distillation of the top layer.

If you have any suggestions for me, I'd love to read them! If not, then my next post will be an update on my progress.


[Edited on 2-14-2017 by Meltonium]

aga - 14-2-2017 at 15:08

Sounding awesome Meltonium.

Some photos would be great, especially for those of us who have never performed a Grignard.
(just use your smartphone if you have no digital camera)

A quick question though :-
Quote: Originally posted by Meltonium  
...
Procedure:
...
- Add a few iodine granuals if needed.
...

Notes:
- Prior to the reaction, the glassware was baked at 300F for an hour
- The solution turned yellow after iodine was added

How did you know that the Grignard Reagent formation reaction was not happening, and felt it necessary to add the iodine ?

Meltonium - 14-2-2017 at 16:37

Quote: Originally posted by aga  


Quote: Originally posted by Meltonium  
...
Procedure:
...
- Add a few iodine granuals if needed.
...

Notes:
- Prior to the reaction, the glassware was baked at 300F for an hour
- The solution turned yellow after iodine was added

How did you know that the Grignard Reagent formation reaction was not happening, and felt it necessary to add the iodine ?


Prior to doing the Grignard, I researched many videos of it online. Every time when the reaction started, there would be bubbling and the solution would become cloudy. Before I added the iodine, this was not happening. The reaction could have been taking place, but I didn't notice any of the telltale signs. Me adding the iodine was a product of these observations and my impatience. (Also, unfortunately, I forgot to take pictures for this writeup. It only occurred to me after I had already added the HCl and at that point I think it would be useless more or less)



On another note, I preformed the distillation of the top layer. Distillate started to come over at 58C and the stillhead temp continued to rise slowly until ~73C then it dropped off suddenly and distillate stopped. This was because there was hardly any liquid left in the flask to boil off. Only some black crud was left. I do not really know what this distillate is. It is clear, with maybe some cloudyness, but because of it's low boiling point, I do not think it is the 2,3 dimethylbutan-2-ol

j_sum1 - 14-2-2017 at 18:36

Good stuff Meltonium. I am yet to attempt a Grignard. It sounds like you made a good first stab at it.
I like the sig - appropriate.

AvBaeyer - 14-2-2017 at 20:24

Meltonium,

Good report! You picked a very tough set of reactants to introduce yourself to a Grignard reaction. But first I should say your attempt to induce Grignard formation by the addition of iodine was the correct thing to do. Just keep in mind that this consumes magnesium.

The formation of Grignard reagents from alkyl chlorides is often difficult and the use of secondary chlorides, in this case 2-chloropropane, is even more difficult. Moreover, the nucleophilic reactivity of secondary Grignards is often not very good. Assuming that you did in fact form the Grignard reagent you wanted, it is highly likely the the major reaction with acetone would be to form the acetone enolate, (which would go on to form all kinds of gunk) and not undergo the desired addition to the ketone. It is well established that isopropyl magneium halides act as deprotonating bases or transmetalation reagents in organic synthesis.

Perhaps a better experiment would to be to make ethyl bromide as its Grignard reagent which would readily add to acetone. In any event I thought you made a good attempt with a difficult set of reagents.

AvB

aga - 15-2-2017 at 06:38

Quote: Originally posted by Meltonium  
... bubbling and the solution would become cloudy. Before I added the iodine, this was not happening.

Every time i read about a Grignard, iodine is added.

Your account is the first time i've seen a clear description of "how to tell if it's working".

Thanks for the info !

clearly_not_atara - 15-2-2017 at 15:45

2-chloropropane is one of the tougher alkyl Grignards to form, so nice job. It's not too bad though: as you've noticed, it works; it's the aryl Grignards that are a real pain.
Quote:
- - The solution fizzed and bubbled heavily and deposited white precipitate on the walls of the flask


Sounds like oxygen crashed the party. MgO is a white powder insoluble in any solvents. Grignards react readily with oxygen to form peroxides, which themselves react with Grignards to form ethers and MgO. Unfortunately the result can be what's known as a "complex mixture". For your next run consider a nitrogen atmosphere.
Quote:
Your account is the first time i've seen a clear description of "how to tell if it's working".


the formation of a Grignard reagent, i.e. RX + Mg >> RMgX is a redox process and it is exothermic. So "something noticeable" should happen. this paper describes the process:

http://www.sciencedirect.com/science/article/pii/S0010854504...

[Edited on 15-2-2017 by clearly_not_atara]

aga - 15-2-2017 at 16:04

Meltonium just did it, so his account is priceless.

Anyone can google and find references to things.

A First-Hand account of what it looks like is worth millions of googled references by people who have never done it.

Those who have ever done a Grignard could just say what they saw - no google required.

Not to put too fine a point on it ; say what You observed during this particular reaction, relating to the need for a grain of iodine, or shut up.

I have nothing to say about it, as i have never done it.

Tsjerk - 15-2-2017 at 23:49

Quote: Originally posted by aga  

I have nothing to say about it, as i have never done it.


And still you do....

Don't you think the auteurs of that article linked to are first hand sources? Except that, it has a whole lot more information anyone here could provide. Nice write up as it is from Meltonium no one can beat that article.

[Edited on 16-2-2017 by Tsjerk]