The following reaction is carried out: A cyclopropane ring bearing a substituent R on one carbon and a nitro group (NO2) on the adjacent carbon reacts with ethylmagnesium iodide (\(C_2H_5MgI\)) to give A. What is the structure of A?
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Grignard reagents react with acidic alpha-hydrogens, like those next to a nitro group, as a base before they can add as a nucleophile.
Step 1: Grignard reagents such as \(C_2H_5MgI\) contain a strongly basic carbanion, \(C_2H_5^-\). Before attempting nucleophilic addition to any weakly electrophilic center, a Grignard reagent will always react first with the most acidic proton present in the substrate.
Step 2: In the cyclopropane substrate, the ring carbon bearing the \(-NO_2\) group also carries a hydrogen atom, and this hydrogen is unusually acidic since the negative charge left behind is delocalized onto the two oxygen atoms of the nitro group.
Step 3: \(C_2H_5MgI\) therefore acts purely as a base here rather than as a nucleophile, abstracting this acidic proton from the ring carbon.
Step 4: The proton combines with the ethyl carbanion of the Grignard reagent to release ethane gas, \(C_2H_6\), while the substrate is converted to its magnesium nitronate salt.
Step 5: This is the same behavior Grignard reagents show with any compound containing an active hydrogen, where the observable product is the alkane gas evolved from the Grignard reagent's own alkyl group.
