Why the Grignard reagent should be prepared under anhydrous conditions?
In Grignard reagent, the carbon-magnesium bond is highly polar, with carbon pulling electrons from electropositive magnesium. The magnesium halogen bond is essentially ionic in nature. Thus, Grignard reagent is a very versatile or a very reactive reagent which can react with any source of proton to give hydrocarbons. If even traces water are present in the reaction, then they will react with the reagent producing the corresponding hydrocarbon thus making the reagent ineffective.
RMgX + H2O → R-H + Mg(OH)X
That is why it is advised that Grignard reagent should be prepared under anhydrous conditions.