BUILDING Formaldehyde

• Oxygen p orbitals are lower energy than the CH2 p orbital.
• MO’s analogous to key orbitals in ethylene are formed including both the σ and π orbitals of the double bond.
• However, Rule 9 predicts polarization in all of the orbitals.
• Rule 9: When two orbitals interact, the lower energy orbital mixes into itself the higher energy one in a bonding way, while the higher energy orbital mixes into itself the lower energy orbital in an antibonding way.

• In the case of the π and π* orbitals, the oxygen p orbital is lower in energy than the CH2 p.
• In the case of the π and π* orbitals, the oxygen p orbital is lower in energy than the CH2 p.
• The lower energy π MO that is formed i s polarized towards the O, and the higher energy π* orbital is polarized towards the C. Corroborated by ab initial models.
• 12 ē valence electrons between O and CH2.
• LUMO is the π* [p – py] MO
• HOMO is [π(CH2) – px] MO
• MOT does not always lead to simple correspondence with
classical views.
• The MO’s of O-containing molecules predict the existence of lone pairs of ē.
• The MO diagram for this prototype carbonyl has significant
ramifications for predicting and rationalizing reactivity patterns.
• Nucleophiles will preferentially interact with the LUMO at the
atom/group with the larger coefficient. (in this case C)
• Polarization of the HOMO towards O has implications for reactivity as well. (protonation at O not C)
• The simple QMOT model of a carbonyl (formaldehyde) is completely compatible with more conventional (VBT) bonding
models.
• Group orbitals for an olefin would be those derived for
ethylene.
• Group orbitals for an aldehyde or ketone would be those derived
for formaldehyde.