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

Step 1: Recall what a carbocation needs.
A carbocation has a carbon with an empty \(p\) orbital. It becomes more stable when nearby electrons flow into that empty orbital. So we should look for filled orbitals that can donate.
Step 2: List the donating mechanisms.
There are two ways electrons feed the empty \(p\) orbital: hyperconjugation from neighbouring filled \(\sigma\) bonds, and resonance from neighbouring filled \(\pi\) bonds. Both involve filled orbitals giving electron density.
Step 3: Rule out empty orbitals.
Empty orbitals such as \(\sigma^*\) and \(\pi^*\) have no electrons to give, so they cannot donate into the empty \(p\) orbital. Any option naming antibonding or empty orbitals is wrong.
Step 4: Apply to the given cation.
The cation in the figure is flanked by C-H or C-C sigma bonds and by a pi system, both of which are filled and can overlap with the empty \(p\) orbital.
Step 5: Pick the donors.
The stabilisation comes from filled \(\sigma\) (hyperconjugation) and filled \(\pi\) (resonance) orbitals.
Step 6: Choose the option.
That is option 2: filled \(\sigma\) and filled \(\pi\) orbitals.
\[ \boxed{\text{filled } \sigma \text{ and filled } \pi \text{ orbitals}} \]