1) An unavoidable side reaction of alkyl halides with active metals which lowers

Question

1) An unavoidable side reaction of alkyl halides with active metals which lowers the yield of Grignard reagents is called coupling. 2RX + Mg ------> R-R + MgX2 Although the mechanism of the coupling process is not well understood, it is known that the rate appears to depend on the square of the concentration of the halide. With this in mind, explain the reason for the sequence of addition of the ether solution employed at the beginning of the formation of the Grignard reagent in the experimental procedure above.

Explanation / Answer

_____Competing reactions desired: RX + Mg ------------> RMgX:______rate ~ [RX] side__: RX + RMgX --------> R-R + MgX2; rate ~ [RX]^2 desired reaction favored by keeping the [RX] low so that it reacts with Mg and not RMgX **************************************… added later: see: http://en.wikipedia.org/wiki/Grignard_re… Via the Schlenk equilibrium, Grignard reagents form varying amounts of diorganomagnesium compounds (R = organic group, X = halide): 2 RMgX ----> R2Mg + MgX2 coupling

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