[leaftye]

Yesterday I tried several different ways of figuring out the last part of the reaction, checked half a dozen books, dozens of websites, and I can't quite find out for sure how to draw the mechanism for the magnesium salt of triphenylmethanol reacts with water and dilute HCl to become triphenylmethanol. 

I've attached a pdf with two ways I think the mechanism should work.  I think the first one is better.  My problem area is on the second page.

Thank you for any help with getting this figured out.

[nj_bartel]

Try drawing the grignard compound as a salt, an ionic compound, and see if that makes it any easier for you.

[leaftye]

I don't follow.  I still have the problem of the dilute HCl neutralizing any OH.  I can accept that the magnesium salt of triphenylmethanol gets protonated, but I don't see how the MgBr salt would get a OH in an acidic solution.

[leaftye]

Here's part of what I just sent my instructor via email:

I would think that H2O would form the methanol just fine and result in OH.  But since the solution is acidic the OH would simply reprotonate back into H2O wouldn't it?  Is MgBr so strong that it'd just pull the proton right back off of water?

Or could it be the solution becomes quite basic since we only added about 15 mL of dilute 1.4M HCl (10.7 mmol) compared to a theoretical 24.7 mmol of alcohol.  The magnesium salt of triphenylmethanol would have to pull protons off of water long after all the hydroniums were exhausted, so then I could see OH being available for MgBr to attach to.

I can see the latter scenario happening, but unfortunately we didn't check the pH of the solution to confirm this.

[RBF]

The magnesium will be in the form of a 'mixed salt' (MgBrCl).  Your 'H+' in the mechansim implies the presence of Cl- as the counter ion. 
This will be part of the aqueous phase, along with Mg+2 and Br- ions.  As mentioned in an earlier reply, it is more helpful to write the magnesium containing intermediates as ionic.

[leaftye]

Yes, yes, that makes sense now.  My old chem teacher was showing me the same thing this morning.  Having the single magnesium product shown in the lab book didn't make sense to me, which is what had been driving me crazy.  Thanks you two!