Ethane

• As molecules get bigger constructing the molecular orbitals
becomes more challenging.
• Insights into bonding of larger molecules can be attained by
combining fragments with well defined MO’s... through orbital
mixing.
• In this manner, ethane can be constructed from MO’s of two
pyramidal CH3 groups.

• Only consider the first-order mixings:
• σ(CH3) and π(CH3) orbitals are primarily C-H bonding - do not
change much with mixing in forming C-C bond.
• σ(out) is directed away from hydrogens and towards the C-C bond
being formed... they overlap well and result in strong mixing
interaction. Significant lowering of energy of the σ(out)+ σ(out) MO.
• Each CH3 brings 7 valence ē (combine for 14 ē). MO filling
lowest to highest in energy.
• The highest occupied molecular orbital (HOMO) consists
of a degenerate pair of orbitals, π(CH3)-π(CH3).
• σ(out)+ σ(out) = C-C σ bonding translates well for alkane fragments
in general.