Q.Why is it necessary to avoid even traces of moisture during the use of a Grignard reagent?

Step 1. The Grignard reagent's carbon is strongly nucleophilic/carbanion-like (R delta- - Mg delta+ - X), and RMgX is also a very strong base.

Step 2. Water, alcohols and amines all carry an 'active' (acidic-enough) hydrogen. If even a trace of any of these is present, RMgX instantly abstracts that proton, exactly as in reaction 10 of its synthetic uses: RMgX + H-OH gives R-H + Mg(OH)X.

Step 3. This side reaction converts the valuable, versatile Grignard reagent into the simple, synthetically useless alkane R-H -- and it happens far faster than the reagent's intended reaction with the actual target substrate (a carbonyl compound, CO2, etc.), so even a small amount of moisture can quietly consume most of the reagent before the real reaction ever gets a chance. …

Connect the reagent's strong basicity/nucleophilicity directly to its own 'Preparation of Alkanes' reaction (RMgX + active-hydrogen compound gives R-H) -- m …

• Thinking moisture merely 'dilutes' or 'slows' the Grignard reagent -- it actually reacts with and CONSUMES the reagent outright (converting it to …