Question

The following carbocation is stabilized by the interaction of the empty \(p\) orbital with

• A. empty \(\sigma^*\) and empty \(\pi^*\) orbitals
• B. filled \(\sigma\) and filled \(\pi\) orbitals
• C. empty \(\sigma\) and empty \(\pi^*\) orbitals
• D. empty \(\sigma^*\) and filled \(\pi\) orbitals

Hint:

Stabilization of electron-deficient carbocations always requires electron donation from an filled orbital source into an empty destination: - \(\text{Filled Orbital (Donor)} \rightarrow \text{Empty Orbital (Acceptor)}\) - In this benzylic carbocation system, the donors are the filled \(\pi\)-orbitals of the aromatic ring (resonance) and the filled \(\sigma\)-orbitals of the \(\text{C}-\text{H}\) bonds (hyperconjugation).

Answer 1

The Correct Option is B

Concept: Carbocations are highly reactive, electron-deficient species containing a positively charged carbon atom with an

empty valence \(p\) orbital. The stability of a carbocation relies on electron-donation mechanisms that share electron density into this empty orbital. Two main intramolecular mechanisms accomplish this:

• Resonance / Delocalization: Electron density from neighboring filled \(\pi\) bonds shifts into the empty \(p\) orbital.

• Hyperconjugation: Electron density from neighboring filled \(\sigma\) bonds (typically \(\text{C}-\text{H}\) or \(\text{C}-\text{C}\) bonds) delocalizes into the empty \(p\) orbital.

Step 1: Identifying structural features of the given carbocation
The molecule illustrated is a benzylic-type carbocation containing an adjacent methyl group: \[ \text{C}_6\text{H}_5-\!\!{\overset{\oplus}{\text{C}}}\text{H}-\text{CH}_3 \] Let's analyze how the electron-deficient center interacts with both sides of the molecule:

Step 2: Analyzing interactions with the benzene ring (\(\pi\)-system)
The positively charged carbon is directly bonded to an \(sp^2\)-hybridized carbon of the aromatic benzene ring. The

filled \(\pi\) molecular orbitals of the aromatic ring align parallel to the empty \(p\) orbital of the carbocation. This allows electron density to delocalize into the empty \(p\) orbital through resonance, stabilizing the charge over the ortho and para positions of the ring.

Step 3: Analyzing interactions with the methyl group (\(\sigma\)-system)
On the other side, the carbocation is bonded directly to a \(\text{-CH}_3\) group. The

filled \(\sigma\) orbitals of the adjacent \(\text{C}-\text{H}\) bonds align correctly to donate electron density into the empty \(p\) orbital of the carbocation through hyperconjugation.

Conclusion:
The empty \(p\) orbital of the carbocation acts as an electron acceptor that receives stabilizing electron density from the surrounding

filled \(\pi\) orbitals of the benzene ring and the

filled \(\sigma\) orbitals of the adjacent \(\text{C}-\text{H}\) group. Therefore, the stabilizing interactions involve filled \(\sigma\) and filled \(\pi\) orbitals.