Effects of Heteroatoms - Formaldehyde

• Rule 5: Molecules with similar structures will have qualitatively similar
MO’s, with the major difference being the number of valence ē
occupying the common MO system.
• Essentially true, but situation is frequently more complicated.
• Formaldehyde and ethylene are isoelectroni; same number of
valence ē and the same types of valence orbitals.
• Can expect formaldehyde and ethylene to have similar MO’s, with some perturbations introduced by the O of formaldehyde.

• The primary consequence of introducing heteroatoms into
hydrocarbon systems is to alter orbital energies:
• Rule 12: More electronegative elements have lower energy atomic
orbitals.
• In generating mixing diagrams, valence state ionization energies
provide a convenient guideline for orbital energies. The higher
the ionization energy the harder it is to an electron... energy of
the orbital is lower.
• Electronegative elements have relatively low lying-atomic orbitals.
• Thus, must consider second order perturbation rules for orbital
mixing