Why does the glassware have to be dry in a grignard reaction?
Because water is the natural enemy of the Grignard reagent.
Grignard reagents (and organometallic reagents in general) are usually formed with the strict exclusion of water. A Grignard reagent is strongly nucleophilic and will react with water rapidly and irreversibly:
Note that sometimes we can exploit this reactivity if we wanted to put a deuterium label (
This would be much cheaper than buying the labelled reagent from a commercial supplier.
I am sure that there are similar answers on Socratic, but it's always hard to find the one you need.