Ch.16 - Aromatic Compounds Flashcards
What year was benzene isolated?
1825
What year was benzene synthesized?
1834
What year was the Kekule structure proposed?
1866
Benzene is actually a resonance hybrid between the two _____ structures.
Kekule
what is the bond order of benzene?
1.5
The 6 pi electrons of benzene are ______ over the 6 carbons?
delocalized
Each sp2 hybridized C in the benzene ring has an ________ p orbital perpendicular to the ring that overlaps around the ring.
unhybridized
When Br adds to benzene, what catalyst is needed?
FeBr3
What is the observed heat of hydrogenation of benzene?
-208 kj/mol
What is the predicted heat of hydrogenation of benzene?
-359 kj/mol
What is the resonance energy?
the difference between the predicted and the observed value
What are annulenes?
hydrocarbons with alternating single and double bonds
What is so reactive that it dimerizes before it can be isolated?
cyclobutadiene
Cyclooctatetraene adds ____ readily to the double bonds.
Br2
What are the 4 MO rules for benzene?
1) 6 overlapping p orbitals must form 6 molecular orbitals (MOs)
2) 3 will be bonding, 3 antibonding
3) the lowest-energy MO will have all bonding interactions and no nodes
4) as the energy of the MO increases, the number of nodes increases
The first MO of benzene is what?
entirely bonding with no nodes
What is the energy of the first MO of benzene like?
low energy
The intermediate levels are ________.
degenerate (equal in energy) with 2 orbitals at each energy level
π2 and π3 have how many nodal planes?
1
how many nodal planes do all-antibonding π6* have?
3
Each pair of adjacent p orbitals is out of phase and interacts _________
destructively
What are the aromatic requirements?
1) the structure must be cyclic w/ conjugated pi bonds
2) each atom must be sp2 or sp (not 2 Hs on one carbon atom)
3) structure must be planar for effective overlap to occur
4) delocalization of the pi electrons over the ring must lower the electronic energy
nonaromatic compounds
do not have a continuous ring of overlapping p orbitals and may be nonplanar
antiaromatic compounds
cyclic and conjugated with overlapping p orbitals around the ring, but electron delocalization increases its electronic energy
Once the aromatic criteria are met, _______ rule applies
Huckel’s
What is Huckel’s rule?
- if the # of pi electrons is (4N+2), the system is aromatic
- if the number of pi electrons is (4N), the system is antiaromatic
Is [4]Annulene (cyclobutadiene) aromatic, antiaromatic, or nonaromatic?
antiaromatic
Is [8]Annulene (cyclooctatetraene) aromatic, antiaromatic, or nonaromatic?
nonaromatic because it’s not planar
Is [10]Annulene aromatic, antiaromatic, or nonaromatic?
aromatic (isomers are nonplanar)
larger 4N annulenes are not ________ because they are flexible enough to become nonplanar.
antiaromatic
How many orbitals are filled with an aromatic compound?
all are filled
How many orbitals are filled with an antiaromatic compound?
there are 2 degererate (half-filled) orbitals
Is the cyclopentadienyl cation aromatic, antiaromatic, or nonaromatic?
antiaromatic (4 pi electrons)
Is the cyclopentadienyl anion aromatic, antiaromatic, or nonaromatic?
aromatic (6 pi electrons)
Deprotonation will allow what?
the overlap of all the p orbitals in the molecule
Which is more stable, benzene or cyclopentadiene?
benzene
The cycloheptatrienyl cation is commonly known as what?
the tropylium ion
What does cyclooctatetraene react with to form an aromatic dianion?
K (potassium)
what is a heterocyclic compound?
cyclic compound in which one or more of the ring atoms is not carbon
What is a heteroatom?
the atom that replaces a carbon in a heterocyclic compound
Pyridine is ____, with a pair of nonbonding electrons available to abstract a proton.
basic
The protonated pyridine (the pyridinium ion) is still _________.
aromatic
Is pyrrole aromatic, antiaromatic, or nonaromatic?
aromatic (has 6 pi electrons….the lone pair on nitrogen is delocalized)
Napthalene is the simplest ____ aromatic hydrocarbon.
fused
fused rings share what?
2 atoms and the bond between them
How are larger polynuclear aromatic compounds formed?
in combustion
Many larger polynuclear aromatic hydrocarbons are what?
carcinogenic
amorphous definition
small particles of graphite
what are come amorphic compounds?
charcoal, soot, coal, carbon black
Diamond is a lattice of what?
tetrahedral carbon atoms
Graphite structure
layers of fused aromatic rings
What is graphene?
a single layer of graphite
What is an electrical insulator?
diamond
Diamond is one giant molecule that is a _____ carbon.
tetrahedral
What length are the sigma bonds in diamond?
1.54 A
What is the molecular geometry of graphite?
planar
What are the sigma bond lengths?
1.415 A
What force is between the graphite layers?
only van der Waals forces
What conducts electrical current parallel to layers?
graphite
fullerenes are what?
5 and 6 membered rings arranged to form a “soccer ball” structure
nanotubes are what?
half of a C60 sphere fused to a cylinder of fused aromatic rings
What is the best conductor of heat at room temperature?
graphene
What is ortho?
1,2-disubstituted
What is meta?
1,3- disubstituted
What is para?
1,4-disubstituted
What is the melting point property of aromatic compounds?
more symmetrical than corresponding alkane + pack better into crystals ——> higher melting points
What do the boiling points of aromatic compounds depend on?
dipole moment
What is the order of bp for disubstituted benzenes?
ortho > meta > para
What is the density of aromatic compounds like?
1) more dense than nonaromatics
2) less dense than water
What is the solubility of aromatic compounds?
insoluble in water
The C=C stretch absorption is at _______ cm-1
1600
The sp2 C-H stretch is just above ______ cm-1
3000
The 1HNMR for H’s on an aromatic ring is from ______-_______
7 - 8
The 13C NMR is at _____-_______ for an aromatic ring.
120 -150
