How do you titrate Grignard reagents?

1 Answer
anor277
Aug 21, 2016

Simply take a known volume of the Grignard, dump it into water, and then titrate to a stoichiometric endpoint.

Explanation:

From above, we drop a solution of #RMgX# into water to give #R-H# and #MgXOH#; an aqueous titration to a stoichometric endpoint follows with standardized #HCl(aq)#.

Alternatively, there are indicators that you could dissolve up in dry ether, and titrate to an endpoint with the Grignard in ethereal solution. This is a bit more of a pfaff, but is still doable. The Grignard could even be used as the titrant; in which case we would typically use a #1# #mL# syringe with #0.01*mL# graduations. Diphenyl acetic acid, #Ph_2CHCO_2H#, in ether is used as the standard. Addition of #1# #"equiv Grignard"# gives #Ph_2CHCO_2^(-)MgBr + RH#. The second equiv of Grignard gives #Ph_2C^(-)CO_2^-#, which is noticeably yellow. Other such reagents exist.

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