Today I decided to re-trace my steps on Monday and provide a more complete description of chemical bonds through the eyes of molecular orbital theory. A list of take-home lessons from this presentation:
- MO models contain new orbitals (molecular orbitals), while VB (valence bond) models do not
- MOs can be mentally (de)constructed as combinations of atomic orbitals
So far not much new stuff … the previous points had already been made in the context of the “localized MO” model presented at the start of the semester.
But now we add these points:
- MO energies depend on molecular geometry & the # of orbital nodes
- Molecular
geometry influences MO energy by changing the shape of the orbital.
This affects where electrons hang out and how they move, but we can
simplify all that by simply watching how geometry affects the way the
atomic orbital “pieces” overlap. - Constructive overlap of these pieces stabilizes an MO (bonding MO)
- Destructive overlap destabilizes an MO (antibonding MO)
- Loss of overlap as atoms move apart makes MO energy equivalent to energy of atomic orbital “pieces”
Where
this gets really interesting (disturbing) is when we look at any system
in which antibonding orbitals are occupied, e.g., He + He or F-F. The
VB model contains lots of lone pairs. The MO model, on the other hand,
contains no lone pairs, just bonding and antibonding electrons. Whoa.
Check out the PowerPoint slides.
Radical reactions.
The lecture was completed by looking at a typical radical reaction:
radical-initiated anti-Markovnikov addition of HBr to an alkene. The
mechanism involved three different types of processes: initiation, propagation, and termination.
The heat of reaction for these processes can be calculated using bond energies, DHo. The general rule is this: an exothermic reaction breaks a weaker bond (smaller DHo) and makes a stronger bond (larger DHo).
Compare this wording to the general rule for acid-base reactions: a
favorable reaction consumes a stronger acid-base pair and makes a
weaker acid-base pair.