In the preparation of a Grignard reagent (), why must all reagents and reaction vessels be kept rigorously anhydrous (free of water)?
Answer and explanation
Correct answer: D. Water rapidly protonates the highly basic Grignard carbanion to produce an alkane
- Step 1: The carbon-magnesium bond in a Grignard reagent is highly polar and strongly carbanionic: .
- Step 2: The carbanionic alkyl group acts as an exceptionally powerful Brønsted base ( of conjugate alkane ).
- Step 3: Any trace of water immediately protonates the reagent in an exothermic acid-base reaction: , destroying the organometallic reagent.
Answer
Water rapidly protonates the highly basic Grignard carbanion to produce an alkane.
Why not the others: Passivation of magnesium is prevented by initiating with iodine crystals; Water does not convert alkyl halide into ether; Water does not polymerize diethyl ether.
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