[KritikalMass]

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*

[orgopete]

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.

[movies]

Agreed.

The pK_a 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.

[KritikalMass]

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!

[movies]

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

[nj_bartel]

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

[KritikalMass]

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

Used a heat gun.

[nj_bartel]

Oh, well that's much better than what we did   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.