NEET Organic Chemistry Solved Questions

One of the product formed in the following reaction is
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The correct answer is (A) Cyclohexane.

Explanation

This reaction is an acid-base reaction (specifically following the Zerewitinov reaction mechanism) rather than a nucleophilic addition or ring-coupling reaction.

1. Nature of the Grignard Reagent: Cyclohexylmagnesium bromide (C₆H₁₁MgBr) contains a highly polarized carbon-magnesium bond. The cyclohexyl ring behaves as an extremely strong base (a carbanion equivalent, C₆H₁₁^-).
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2. Nature of the Amine: Cyclohexylamine (C₆H₁₁NH₂) is a primary amine. The hydrogen atoms directly attached to the nitrogen atom are active (acidic) hydrogens.
3. The Reaction: When a Grignard reagent encounters any compound containing an active hydrogen (such as water, alcohols, carboxylic acids, or primary/secondary amines), it instantly abstracts that proton (H⁺) to form a stable hydrocarbon.

Because the carbanion grabs the proton from the amine group, the cyclohexyl ring becomes fully saturated, turning into a plain cyclohexane ring.

The following carbocation is stabilized by the interaction of the empty p orbital with
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(A) empty σ* and empty π* orbitals
(B) filled σ and filled π orbitals
(C) empty σ and empty π* orbitals
(D) empty σ* and filled π orbitals

The correct answer is (b) filled σ and filled π orbitals.

Explanation

The given compound is a 1-phenylethyl cation (a benzylic carbocation). The carbon bearing the positive charge is sp² hybridized and possesses an empty p-orbital. This empty orbital is stabilized through two distinct modes of electron delocalization:

• Resonance (with Benzene Ring): The empty p-orbital of the carbocation overlaps side-by-side with the filled π orbitals of the aromatic benzene ring. This spreads the positive charge over the ortho and para positions of the ring.
• Hyperconjugation (with Methyl Group): The adjacent methyl group (-CH3) has three C-H σ-bonds. The electrons from these filled σ orbitals delocalize into the adjacent empty p-orbital of the carbocation.

Therefore, the stabilization occurs via interaction with the filled σ (hyperconjugation) and filled π (resonance) orbitals.