Grignard reagents must be prepared and used under strictly anhydrous conditions because water

1. Aprotonates the carbon of $\mathrm{R}-\mathrm{MgX}$ and converts it into the alkane $\mathrm{R}-\mathrm{H}$
2. Bmakes diethyl ether immiscible with the alkyl halide
3. Cconverts the alkyl halide $\mathrm{R}-\mathrm{X}$ into an alkene by elimination
4. Doxidises magnesium metal to $\mathrm{MgO}$ so that it cannot react with $\mathrm{R}-\mathrm{X}$