Grignard reaction
Grignard reactions will not work if water is present; water causes the reagent to rapidly decompose.  So, most Grignard reactions occur in solvents such as anhydrous diethyl ether or tetrahydrofuran (THF), because the oxygen in these solvents stabilizes the magnesium reagent.  The reagent may also react with oxygen present in the atmosphere. This will insert an oxygen atom between the carbon base and the magnesium halide group. Usually, this side-reaction may be limited by the volatile solvent vapors displacing air above the reaction mixture. However, chemists may perform the reactions in nitrogen or argon atmospheres. In small scale reactions, the solvent vapours do not have enough space to protect the magnesium from oxygen.