Chapter 16- Aromatic Compounds Flashcards
Each sp2 hybridized C in the Benzene ring has what? How are the six pi electrons oriented?
An unhybridized p orbital perpendicular to the ring that overlaps around the ring
They are delocalized over the 6 carbons
Describe the physical properties of aromtic compounds. (MP, BP, Density, Solubility)
MP- more sym than corresponding alkane, pack better into crystals, so HIGHER melting points
BP- depends on dipole moment. Ortho>meta>para for disubstituted benzenes
Density- MORE dense than nonaromatic compounds, LESS dense than water.
Solubility- generally INSOLUBLE in water
What are the resonance energies of cyclohexene, 1,3- cyclohexadiene, and “cyclohexatriene?”
Cyclohexene: -119.7 kj/mol
1,3 cyclohexadiene: -239.4 kj/mol (predicted) and
-232.0 kj mol (experimental)
Cyclohexatriene: -359.1 kj/mol (predicted) and -208.4 kj/mol (expiremental)
What are the various positions on a substitued Benzene?
Ortho- 1,2
Meta- 1,3
Para- 1,4
Why is the resonance energy difference between the predicted and expiremental calculations of cyclohexatriene so great? (150.7 kj/ mol)
“Cyclohexatriene” does not exist, the compound is called Benzene. The great energy difference between the predicted and expiremental means that Benzene is more stable than expected.
What are the three rules of aromaticity?
- Continuous cyclic cloud of pi electrons (p orbital on each carbon of the ring)
- Cyclic molecule must be planar
- 4n+2 pi electrons (huckle number)
How does deprotonating cyclopentadiene allow it to be aromatic?
When deprotonated, a pair of electrons is left in one of the sp3 orbitals, which can then rehybridize to a p orbital. The 6 electrons in the p orbitals can now resonate around the 5 carbon atoms, making the molecule aromatic.
Why are planar molecules with an uninterrupted pi electron cloud and 4n+2 pi electrons so stable?
If you look at MO diagram, all the bonding pi orbitals are filled with electrons (closed shell), which as an extremely stable arrangement.
What makes a molecule nonaromatic?
If either of the first two rules for aromaticity is violated
What makes a molecule antiaromatic?
If the first two aromaticity rules are followed, but the third rule is violated
Anitaromatic compounds always have what?
Unfilled bonding molecular orbitals
Describe the MO diagram of antiaromatic cyclobutadiene.
Two of the four electrons are in SEPARATE nonbonding mos. This diradical is reactive and unstable.
Benzyne (and alkynes in general) are good what?
Dienophiles
For a hydrocarbon, azulene has an extremely large dipole moment. Why?
It is aromatic. It can resonate to create a negative charge on the cyclopentadienyl anion and a positive charge on the tropylium cation.
What are annulenes?
Conjugated hydrocarbons
