Walsh Cyclopropane Molecular Orbitals

Walsh Cyclopropane is a common topic in upper-level organic chemistry courses, particularly those taught by chemists who (like me) are fascinated by interesting ways of describing bonds.

Cyclopropane is usually described with so-called "banana bonds," which are sp³-hybridized (as shown below left), but A. D. Walsh, in 1947, invented a different way of describing bonding in the molecule. Walsh's model explains several ways in which cyclopropane differs from other saturated hydrocarbons. For example,

• Cyclopropane undergoes addition reactions.
• Cyclopropane shows properties and reactivity consistent with π-type conjugation with neighboring carbocations and radicals.
• The C-H force constants in cyclopropane are consistent with Walsh's assumption of sp² hybridization.

Introduction Pertinent references Derivation of the Walsh Cyclopropane MOs Rigorous derivation of the MOs Orbital Energy Diagram Conclusions