Answer :
The best reason for why nitriles do not undergo overaddition with Grignard reagents is because the metalloimine intermediate formed is not a good electrophile (option B).
Nitriles (also known as cyanides) do not undergo overaddition with Grignard reagents primarily due to the nature of the intermediate formed during the reaction. When a Grignard reagent reacts with a nitrile, it forms a metalloimine intermediate, which is a complex containing a metal-carbon-nitrogen bond.
This intermediate is not a good electrophile, meaning it does not readily accept additional nucleophiles to undergo overaddition. The carbon-nitrogen bond in the metalloimine intermediate is relatively strong, making it less reactive towards further nucleophilic attack. Therefore, overaddition does not occur, and the reaction proceeds through other pathways, such as the addition of the Grignard reagent to the nitrile carbon atom.
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