Sciencemadness Discussion Board

Grignard work up

Xylitol - 30-9-2016 at 17:08

I am going to be doing a grignard reaction for the first time (and I'm a novice).

I am planning on using H2SO4 for the workup. It only donates one H per mol, right? So I should be using an equimolar amount to the alkoxide?

If I don't understand correctly, how much should I use?

Also, I am producing a tertiary alcohol and I read an article saying that you must use NH4Cl because the alcohol can be dehydrated to an alkene. However, I've seen other synthesis of a very similar alcohol and they used H2SO4.

To be clear, the synthesis that stated NH4Cl was not the alcohol I'm making either.

The alcohol I am producing is 3-methyl-3-pentanol.

[Edited on 1-10-2016 by Xylitol]

DraconicAcid - 30-9-2016 at 17:14

Why use such a strong acid? And sulphuric acid will donate two protons.

Xylitol - 1-10-2016 at 01:43

Thanks for the reply. The only two acids I have in my lab now are HCl and sulfuric. Do you think it's a problem?

So I need to use 0.5 mol of sulfuric acid for every mol of alkoxide?

[Edited on 1-10-2016 by Xylitol]

Texium - 1-10-2016 at 06:16

Just dump your reaction mixture into an excess of cold, dilute (~.5 M) sulfuric acid. You don't need a stoichiometric amount, and you'll probably need more anyway because you'll likely have some excess magnesium floating around in there. Swirl it around and once it's done reacting check the pH. If it's still basic, just add more acid until it is acidic.

DraconicAcid - 1-10-2016 at 16:42

Stress on the cold and dilute.

Xylitol - 16-10-2016 at 19:47

I completed the reaction last week and here are the results:

After oven drying the glassware and drying solvents with MgSO4...

I set up a flat bottom flask with reflux condenser and stirbar and began the reaction with 0.25 mol for the grignard.
The reaction began within 5 minutes of slowly adding the bromoethane. It took about an hour for the grignard to appear complete - no bubbles. At that point I added the ketone 'dropwise'. The reaction was vigorous, but stayed localized to the drops. At one point the entire volume began to react and the reaction had to be put in an ice bath to prevent boiling over. After the reaction, I let the beaker sit for 12 hours and a black precipitate formed on the bottom leaving the ether on top perfectly clear. There were also some large clear crystals on the bottom (not the product). I decanted the ether and added dilute 0.5M sulfuric acid. I didn't have PH strips that worked in the solution, so I just added until the ether stopped boiling. I then added to a sep funnel with some additional water and saved the ether layer. I distilled off the ether and was left with about 2g of yellow liquid. I increased the temp (and had to add foil) because nothing was coming over. I eventually got the liquid to come over at the exact boiling point of 122C (correct for 3-methyl-3-pentanol).

I compared the resulting clear liquid with a sample of high grade 3-methyl-3-pentanol. Even though the boiling point is the same, the smell is a bit different. I am convinced it is pretty pure, but I didn't do any further analysis.


Everything went pretty much how I expected, but I'd like some clarification:

Why 0.5M sulfuric acid? Should I use more/less? Is the water alone good enough to protonate the alkoxide?
After distilling the pentanol, I was left with a brown residue. Is this the bromine?
What is the minor impurity that is producing the smell and how to remove it? (distilling didn't seem to improve) - it's not the ketone

I'd also appreciate any comments on my procedure; I am new to this.

Xylitol - 16-10-2016 at 20:00

I also forgot to mention: How to I test the PH of the ether or do I need to separate it first and check PH of aqueous layer? Are there special PH strips for this?

DraconicAcid - 16-10-2016 at 20:30

Quote: Originally posted by Xylitol  
I also forgot to mention: How to I test the PH of the ether or do I need to separate it first and check PH of aqueous layer? Are there special PH strips for this?


You cannot test the pH of a non-aqueous solution. In ionizing solutions, pH has a different meaning, and in non-ionizing solutions (such as ether) it has no meaning at all.

Amos - 17-10-2016 at 08:58

Your brown residue is most likely a number of side products or impurities from your initial reagents. The washing step doesn't need a specific amount of sulfuric acid because the alkoxide will be fully protonated by water present, forming an alcohol and a metal hydroxide. Acid would be good for removing magnesium hydroxide or metal left over, though. If you want to remove funky smelling stuff from your product, I'd recommend running it through a GC/MS that can identify what compounds are also present. If that option isn't available, re-distillation or passing nitrogen or air through your compound are some other possibilities.

Xylitol - 17-10-2016 at 23:39
Quote:

The washing step doesn't need a specific amount of sulfuric acid because the alkoxide will be fully protonated by water present, forming an alcohol and a metal hydroxide. Acid would be good for removing magnesium hydroxide or metal left over, though.


I don't see the point of the acid. If there is any magnesium hydroxide left over after the reaction, wouldn't it be removed by the distillation process?

So is it possible to just do the work up with water?

Texium - 18-10-2016 at 05:32

You're going to put it in a separatory funnel afterwards, so unless you want to filter it first it's best to have the magnesium in solution. Having those extra ions in solution will also help push more of your product into the ether layer, as it is appreciably soluble in water.