1. Figure Number: UN
Title: Amine Inversion
Caption: Orbital depiction of the inversion (umbrella flip) of configuration about nitrogen which occurs in an amine.
Notes: The transition state for the inversion of configuration which occurs in an amine changes the hybridization about nitrogen. The bonding orbitals about the nitrogen in this transition state are generated by nitrogen contributing sp2 atomic orbitals rather than sp3 orbitals, and the lone pair is held in a p atomic orbital rather than in an sp3 atomic orbital.

2. Figure Number: UN
Title: Phase-Transfer Catalysis
Caption: Schematic diagram depicting a phase-transfer catalysis reaction.
Notes: The phase-transfer catalyst shuttles back and forth between the phases carrying the nucleophile from the aqueous phase to the organic phase to react with the electrophile, and then carrying the leaving group from the organic phase back to the aqueous phase where it is released.

3. Figure Number: UN
Title: Pyrrole and Furan
Caption: Orbital structures of pyrrole and furan.
Notes: Pyrrole and furan are aromatic because a lone pair of electrons on their heteroatoms delocalizes from a p orbital on the heteroatom into the ring, creating an aromatic complement of six pi ring electrons.

4. Figure Number: UN
Title: Pyrrolidine and Pyrrole
Caption: Electrostatic potential maps of pyrrolidine and pyrrole.
Notes: The nitrogen atom in pyrrolidine is red because the lone pair electrons in pyrrolidine are localized on the nitrogen. Since pyrrole is aromatic and its nitrogen's lone pair is delocalized, a buildup of electron density on nitrogen is not observed in pyrrole's potential map.

5. Figure Number: UN
Title: Pyrrole, Furan, and Thiophene
Caption: Electrostatic potential maps of pyrrole, furan, and thiophene.
Notes: From the potential maps it can be seen that the pyrrole ring is more electron rich than the furan ring, which is in turn more electron rich than the thiophene ring. Pyrrole's nitrogen donates its lone pair into the ring more readily than furan's oxygen due to electronegativity differences, and furan's oxygen donates electrons into the ring more readily than thiophene's sulfur due to poor overlap between sulfur and carbon in the thiophene pi system. All three of these heterocyclic compounds react with electrophiles better than benzene because their rings have higher electron densities than benzene's ring.

6. Figure Number: UN
Title: Pyridine
Caption: Orbital structure of pyridine.
Notes: The lone-pair electrons on pyridine's nitrogen are held in a p orbital perpendicular to the pi system, so they do not delocalize into the ring, and are available for bonding to electrophiles.

7. Figure Number: UN
Title: Benzene and Pyridine
Caption: Electrostatic potential maps of benzene and pyridine.
Notes: Since pyridine does not donate its lone-pair electrons into the ring, the pyridine ring does not have more electron density than the benzene ring. In fact, as the potential maps show, pyridine has less electron density in the ring than benzene because nitrogen is more electronegative than carbon, and pyridine's nitrogen sucks some of the electron density out of the ring. Benzene is more reactive toward electrophiles than pyridine.

8. Figure Number: UN
Title: Imidizole
Caption: Orbital structure of imidizole.
Notes: The lone pair on the imidizole nitrogen attached to hydrogen is not basic or nucleophilic because it is delocalized into the ring, whereas the lone pair on the nitrogen which is not attached to hydrogen is nucleophilic and basic because it does not delocalize into the ring.

9. Figure Number: 21-TB01
Title: Table The pKa values of several Nitrogen Heterocycles
Caption:
Notes: