o Besides the bottom and top MO, the MO’s come in degenerate pairs
o Instead of a node at each antibonding interaction you have pairs of antibonding interactions that lie along a nodal plane. As you increase the energy of a MO, you add a nodal plane, not one antibonding interaction (as before)
- The benzene MO
o All the electrons are very comfortable, because they are in low lying, bonding orbitals
- The cyclobutadiene MO
o The last two electrons have to get isolated into two nonbonding orbitals. It’s like having a diradical. If you thought plain old radicals were unstable, this is WORSE.
- What’s the official definition of aromatic
o It needs to be cyclic, with conjugation, no sp3 hybridization, a continuous ring of p orbitals (planar), and spreading the electrons around the ring must lower the energy
o Sometimes, spreading the electrons around raises the energy, as in cyclobutadiene. That’s called antiaromatic
o Sometimes, you have a ring that you might think is aromatic, but it doesn’t meet the criteria. That’s called nonaromatic.
- Huckel’s Rule
o If a ring has a continuous p system, you can use this rule
o 4N + 2 = aromatic
o 4N = antiaromatic
- Aromatic ions
o You can apply Huckel’s rule to ions, and those that are aromatic will be very stable and unreactive.
- Nitrogen
o Pyridine has a lone pair of electrons; the reactions that involve these electrons do not affect the stability of the ring and you don’t count those electrons for Huckel’s rule
o Pyrrole has a nitrogen with lone pairs that interact with the ring. That makes it a weaker base, because taking a proton in means disrupting the stable ring structure
o Pyrimidine has two basic nitrogens (6 membered ring)
o Imidazole has one basic, one non-basic nitrogen (5 membered ring)
o Purine is a fusion of pyrimidine and imidazole
- Furan and Thiophene
o Are basically the same as pyrrole, but with a lone pair instead of the H sticking out
- Bigger aromatic chains
o These can react similarly to normal alkenes
- UV specs of aromatics
o There is a characteristic benzenoid band around 254
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