H3 Chemistry 9813 · Study focus: H3 Chemistry: Apply LCAO to benzene and linear polyenes
H3 Chemistry: Apply LCAO to benzene and linear polyenes
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Apply LCAO to benzene and linear polyenes
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Orientation and prerequisite retrieval
N aligned p AOs in a conjugated chain form N π molecular orbitals.
The π MOs are ordered qualitatively by increasing node count.
Definitions and exact language
Butadiene's four p AOs form four π MOs.
Four π electrons fill the two lowest butadiene π MOs in its ground state.
Six benzene p AOs form six π MOs.
Benzene's six π electrons occupy the three bonding π MOs.
Text alternative: Text alternative: Adjacent p orbitals must remain aligned for continuous conjugative overlap. 9813 requires qualitative LCAO shapes and ordering, not numerical MO coefficients.
Detailed molecular explanation
Six benzene p AOs form six π MOs.
Benzene's six π electrons occupy the three bonding π MOs.
Worked leaf-specific case
Adjacent p orbitals must remain aligned for continuous conjugative overlap.
A break at an sp3 centre interrupts the aligned p-orbital system.
- A candidate writes, “Any conjugated molecule forms exactly one π MO.” Which correction should replace it?
Open the feedback checkpoint after attempting
- N aligned p AOs in a conjugated chain form N π molecular orbitals.
Guided evidence check
The lowest conjugated π MO has no node between adjacent p centres.
The highest conjugated π MO has the greatest number of phase changes.
- Which labelled diagram, spectrum or calculation would most directly disprove “Butadiene places all four π electrons in its lowest MO.”?
Open the feedback checkpoint after attempting
- Four π electrons fill the two lowest butadiene π MOs in its ground state.
Independent transfer
Degenerate pairs may occur in cyclic benzene but not every linear polyene level is degenerate.
9813 requires qualitative LCAO shapes and ordering, not numerical MO coefficients.
- Design a specific chemical check that would expose the error in “Conjugation persists when one p orbital is rotated perpendicular to its neighbours.”
Open the feedback checkpoint after attempting
- Adjacent p orbitals must remain aligned for continuous conjugative overlap.
Misconception repair
Misconception: Every polyene MO occurs as a degenerate pair.
Repair: Degenerate pairs may occur in cyclic benzene but not every linear polyene level is degenerate.
- Write leaf-specific feedback for the misconception “Every polyene MO occurs as a degenerate pair.”
Open the feedback checkpoint after attempting
- Degenerate pairs may occur in cyclic benzene but not every linear polyene level is degenerate.
Practice, unseen assessment, re-test and next step
Complete the diagnostic questions, review the feedback, then attempt the final check.
After delayed re-test, continue to Construct and interpret diatomic MO diagrams, including HOMO and LUMO at /learning/h3-molecular-orbitals-diagrams-diatomics-lesson.html.
- Review the feedback for each diagnostic question, then attempt the final check.
Open the feedback checkpoint after attempting
- Before moving on, state this boundary: 9813 requires qualitative LCAO shapes and ordering, not numerical MO coefficients.