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Preface: I've read this question about water preventing an initiation from occurring.

I understand why it's important to slowly add the alkyl halide to the dry ether and magnesium for exotherm control. But why is the first addition called an initiation and what's its purpose?

Is it just to confirm that the conditions are ready to continue slowly adding the rest of the reagent? I know it's a free-radical reaction, maybe that's where the term "initiation" comes from. But I've seen the reaction described as "self-sustaining" after the initiation, but it's not a chain reaction. Perhaps that's referring to a predictable reaction proceeding forward instead of stalling then blowing up your glassware.

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The first step in the Grignard reaction involves transfer of an electron from the surface of the metal to the alkyl halide to form a radical anion. The radical anion then disproportionates into a free radical and halide anion. \begin{aligned} \ce{R−X + Mg &-> R−X^{•−} + Mg^{•+}}\\ \ce{R−X^{•−} &-> R^{•} + X^{−}}\\ \ce{R^{•} + Mg^{•+} &-> RMg^{+}}\\ \ce{RMg^{+} + X^{−} &-> RMgX}\\ \end{aligned}

When you take the magnesium out of the jar, the surface has a thin oxidation layer on it that prevents the initial electron transfer from occurring. We used to take one piece of magnesium and scratch it good with a knife to expose a clean, fresh magnesium surface. We would place this piece of magnesium in the flask (dry ether and the rest of the magnesium already added) and drip in a few drops of the alkyl halide. Without fail, within a minute or two, mild bubbling would be noticed (local ether boiling), which became vigorous as the initiation phase of the reaction "cleaned" the surface of other pieces of oxidized magnesium. After the vigorous bubbling started to subside we would drip in the remaining alkyl halide plus ether solution at a rate to maintain ether reflux. So the reaction does involve a free radical but the initiation phase really has more to do with removing the oxidation layer from all of the magnesium and allowing that initial electron transfer step to occur.

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  • $\begingroup$ Good answer, I'd like to add a point to be considered: The initial electron transfer is usually slightly endothermic. In the initiation process you will usually need to add some energy - the warmth of the hand might do (depending on the used halid). After the reaction started, the formation of the Grignard is exotherm, hence providing enough energy to overcome the activation energy of the electron transfer. It will then proceed until the limiting reactant is used up - hence it is called self-sustained. $\endgroup$ – Martin - マーチン Jul 14 '14 at 7:15
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    $\begingroup$ @ron That makes a lot of sense. My group has been performing Grignard reagent prep and reactions on and industrial scale, and your answer gives much needed perspective to why we have the initiation step, and how to troubleshoot it if it doesn't go right away. $\endgroup$ – tralston Jul 14 '14 at 15:57
  • $\begingroup$ @Martin That's an interesting point. Thanks for explaining the self-sustaining aspect of the initation. $\endgroup$ – tralston Jul 14 '14 at 16:05
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First, you are mixing magnesium metal with an alkyl halide. The product of this reaction is $\ce{R-Mg-I}$, where $\ce{R}$ is the alkyl group and $\ce{I}$ is the halide. It is called an initiation reaction because you first need to make the reactive compound.

Next, you add the alcohol to produce the product. Mixing an alkyl halide and an alcohol together will not yield the product you want.

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