- Heats of hydrogenation
o Conjugated systems release less energy; they are more stable than isolated dienes
o Cumulated systems release more energy; they are less stable than dienes
- Building molecular orbitals
o # of MO’s = # of AO’s
o Each drawing (no nodes, 1 node, etc.) represents an orbital; each drawing can only hold 2 electrons
o You fill the MO’s with π electrons (2 per double bond, 4 per triple bond)
- Kinetic and Thermodynamic
o The 1,2 addition is kinetically favored because the bromine attacks a secondary cation rather than a primary
o When you add heat, the 1,2 reaction starts to reverse, and you start to get an equilibrium between the 1,2 and 1,4 but the 1,4 predominates because it’s lower in energy
- Allylic Radicals
o They are soooo stable. Even more stable than a tertiary radical. So they will dominate radical reactions, like with bromine.
o Using NBS means you have free Br radicals to play with
- Non-bonding MO’s
o Only odd numbers of MO’s have a non-bonding MO in the middle
o Electrons in a non-bonding MO are as if they are in an isolated p orbital
- SN2’s go faster when they’re allylic
o That’s because the transition state is more stable, which is because the nucleophile-C-X line overlaps with the double bond
- Diels-Alder
o Dienophiles have –W groups and dienes have –D groups
o The mechanism is concerted, so groups have to maintain their configuration
o Secondary overlap leads to endo rule
o Use charge-separated resonance forms to predict orientation of D-A product
- HOMO/LUMO
o The diene contributes the electrons held most weakly (HOMO), because they require the lowest activation energy
o The dienophile takes the electrons into the lowest energy orbital available (LUMO)
o Symmetry allowed means that the HOMO and LUMO match up
- UV
o Wavelengths are 200-400nm, energy of 70-140 kcal/mol
o Absorbed energy equals π-π* transition energy
o The longer the conjugated chain, the higher wavelength absorbed
o A = log (Ir/Is) = εcl
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