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This fact was written in my text book. I wonder why that should be so.

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Grignard reagents are highly reactive and react with any source of proton to give hydrocarbons. Even water, alcohols, amines are sufficiently acidic to convert them to corresponding hydrocarbons.

$\ce{RMgX + H2O -> RH + Mg(OH)X}$

Where, $\ce{R}$ is a hydrocarbon.

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Aside from decomposing as described, Grignard formation will not initiate given a trace of water. There is a bulky lore of getting the things started even under carefully dried conditions. Consider an oven-dried roundbottom containing absolute ether, Mg turnings, and a small portion of the total amount of alkyl halide. It routinely sits there doing nothing.

Crush a turning to expose fresh surface. Add a small iodine crystal to corrode the Mg. Heat the thing, oh yeah, then scramble for the exotherm. Making Grignards spawns neurotic behavior like lucky lab coats, flame drying everything, always adding a drop of freshly distilled $\ce{Me3SiCl}$ (always adding a drop of old $\ce{Me3SiCl}$), saving some of the last batch to set off the next.

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  • $\begingroup$ I have engaged in much of the neurotic behaviour described above. I now just try to use organolithiums for all instances where Grignards are proposed. $\endgroup$ – Sam May 3 '14 at 16:16
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The first answer doesn't really answer the question. It merely shows, correctly, what happens to water which is introduced into a Grignard solution already prepared.

The second one hints at it. Water passifies the surface of the Mg (presumably coating it with MgO or Mg(OH)2) making it almost impossible to initiate the reaction. I have found that the techniques used in answer usually work.

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    $\begingroup$ The grignard reagent (magnesium halide) has then already formed. The passivation of the magnesium surface will not play a role. $\endgroup$ – tschoppi Mar 26 '14 at 9:24

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