How do you titrate Grignard reagents?

1 Answer
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.