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One of the products formed in the following reaction is: Cyclohexylmagnesium bromide + Cyclohexylamine →
Grignard reagents are powerful BASES first, nucleophiles second! Active H on -NH₂ = Alkane!
The carbon-magnesium bond in a Grignard reagent is highly polarized: C^{δ-} - Mg^{δ+}Br. The alkyl group behaves as a extremely strong Brønsted-Lowry base (conjugate base of an alkane, pKa ≈ 50).
Primary amines possess two acidic protons bonded to the electronegative nitrogen atom (-NH₂). The cyclohexyl group of the Grignard reagent abstracts a proton from the amino group of cyclohexylamine.
Structure (1) is dicyclohexylamine; Structure (2) is 2-cyclohexylcyclohexanamine; Structure (3) is bicyclohexyl; Structure (4) is a plain cyclohexyl ring (cyclohexane).
Diagnose every choice: Know exactly why the right option works and why distractors fail.
Incorrect. Nucleophilic substitution on amine nitrogen does not occur.
Incorrect. No ring substitution or electrophilic aromatic-style addition can occur on an aliphatic cyclohexane ring.
Incorrect. Bicyclohexyl would be a Wurtz-coupling product, not formed in the presence of an amine.
Exact product of the Zerevitinov acid-base reaction between a Grignard reagent and an amine.
Students see two large cyclohexyl rings and instinctively look for a bonded dimer or secondary amine (Structures 1, 2, or 3). They forget that Grignard reagents are among the STRONGEST BASES known and simply steal a proton to yield an alkane!
"Grignard reagents react with compounds containing active hydrogen atoms to form hydrocarbons."
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