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Author Topic: pKa of Phenyl Magnesium Bromide (Grignard reagent)  (Read 7379 times)

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KritikalMass

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pKa of Phenyl Magnesium Bromide (Grignard reagent)
« on: February 17, 2010, 07:55:03 AM »

Hello folks:

I was wondering if anyone has a table of pKa values which includes phenyl magnesium bromide? I have scoured the net and the closest I've found was some info on propyl magnesium bromide which was listed as having a pKa of ~50 so am thinking that phenyl magnesium bromide would have a similar pKa.

I need the info to see if this Grignard would be a strong enough base to abstract the hydrogen from the carbonyl in 4-chlorobenzaldehyde (pKa 3.98) in order to generate benzene as a side-product. I do believe that it would.

Additionally, I believe the chlorine is present in the 4-chlorobenzaldhyde in order to make the carbonyl more electrophilic, so if anyone cares to share their thoughts on that as well it would be appreciated.

Thanks in advance for your *delete me*
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orgopete

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Re: pKa of Phenyl Magnesium Bromide (Grignard reagent)
« Reply #1 on: February 17, 2010, 08:17:07 AM »

I usually find pKa values in Hans Reich's website (http://www.chem.wisc.edu/areas/organic/index-chem.htm or directly http://research.chem.psu.edu/brpgroup/pKa_compilation.pdf) or Evans (http://www2.lsdiv.harvard.edu/labs/evans/pdf/evans_pKa_table.pdf). However, I think you are quoting the acid, not the pKa of an aldehyde. Furthermore, as much as you may think this is a good idea, the atoms or molecules do not read my mind so I am going to expect they will not follow your wishes either. Therefore, I predict the Grignard reagent will simply add to the carbonyl group.

Sometimes, posters might offer a solution to your actual problem if you reveal it rather than your proposed solution to a problem we can only imagine.
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Re: pKa of Phenyl Magnesium Bromide (Grignard reagent)
« Reply #2 on: February 18, 2010, 03:41:51 AM »

Agreed.

The pKa of benzene is around 35 so that part would work for your deprotonation, in principle.  However, the addition to the carbonyl will be much fast so I wouldn't expect any enolate reactivity.  I would recommend you use something like sodium hydride to achieve the deprotonation.
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KritikalMass

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Re: pKa of Phenyl Magnesium Bromide (Grignard reagent)
« Reply #3 on: February 18, 2010, 01:03:17 PM »

Thank you both for your replies. The experiment was to synthesize 4-chlorobenzhydrol from 4-chlorobenzaldehyde using phenyl magnesium bromide as the Grignard. A note on the lab sheet mentioned that the crude rxn mixture may contain unreacted bromobenzene (from which the Grignard was prepared w/ Mg), unreacted 4-chlorobenzaldehyde, as well as small amounts of benzene, so I was trying to figure out if the benzene was coming from the Grignard deprotonating the 4-chlorobenzaldhyde, but in light of what you gentlemen have said I assume that the benzene is coming from trace amounts of water.

Thanks again for your replies!
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Re: pKa of Phenyl Magnesium Bromide (Grignard reagent)
« Reply #4 on: February 19, 2010, 05:34:38 AM »

Oh, in that case it certainly could come from deprotonation of the ketone, but I agree that trace water is probably more likely.
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nj_bartel

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Re: pKa of Phenyl Magnesium Bromide (Grignard reagent)
« Reply #5 on: February 19, 2010, 10:32:01 AM »

Especially if it's in organic lab.  You probably didn't even oven/flame dry.
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KritikalMass

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Re: pKa of Phenyl Magnesium Bromide (Grignard reagent)
« Reply #6 on: February 19, 2010, 12:57:11 PM »

Especially if it's in organic lab.  You probably didn't even oven/flame dry.
Used a heat gun.
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nj_bartel

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Re: pKa of Phenyl Magnesium Bromide (Grignard reagent)
« Reply #7 on: February 19, 2010, 04:28:25 PM »

Oh, well that's much better than what we did :P  However, heat guns are surprisingly not enough.  I've tried several times to get NMR tubes clean of acetone with them only to have that big singlet pop at up 2.1.
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