What is the mechanism of formation of Grignard reagents? I couldn't find much in my book or on the Internet. This page says something which I am unable to understand. Why are the $2$ electrons in the $\ce{Mg}$ separated? Shouldn't they be together?
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asked May 15, 2015 at 21:18
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$\begingroup$ It is not entirely known. $\endgroup$– JoriCommented May 16, 2015 at 23:18
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3$\begingroup$ So why would the university website say that? Don't most things in organic chemistry have a probable mechanism rather than an actual one? $\endgroup$– Karan SinghCommented May 17, 2015 at 18:36
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$\begingroup$ It is rather simplest version, certain simplification for students, but if you have problem even with that... Actually pure griniards aren't monomers and have multicenter bonds so... $\endgroup$– MithoronCommented Aug 16, 2015 at 12:12
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1 Answer
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Looking at the mechanism:
We see that these are not double headed arrows, but single headed arrows. This indicates the movement of single electrons, rather than electron pairs as is common in most mechanisms.
Mechanisms that involve movements of single electrons tend to form free-radicals.
The lewis structure of Mg has the electrons seperated in order to make it clearer to read.
In step 1, the shared electron pair in $\ce{R-X}$ has been split to form two radicals: $\ce{R}{\small{•}}$ and $\ce{X}{\small{•}}$. However the $\ce{X}{\small{•}}$ radical is further attacked by an electron, this time coming from the $\ce{Mg}$. Thus forming a free-radical magnesium halide: $\ce{X^-Mg^+}{\small{•}}$.
After this, the two free-radicals undergo a termination step to form the Grignard reagent.
