C16: Aromatic Compounds Flashcards
benzene carbon-carbon bonds are _____ than typical C-C bonds, and ____ than typical ______
shorter
longer
C=C
bonds of a benzene ring are ______
all the same length
1.397 Angstroms
aromatic compounds are _____ containing some number of ____ bonds and having an unusually large ____
cyclic
conjugated double
resonance energy
cyclohexene + KMnO4 –>??
benzene + KMnO4 —>??
cis diol (gylcol) no reaction
cyclohexene + Br2 in CCl4 –>??
benzene + Br2 in CCL4 —>??
trans 1,2 bromocyclohexane
no reaction *** however adding FeBr3 catalyst will give bromocyclohexene
cyclohexene + H2, Pt –>??
benzene + H2, Pt —>??
cyclohexane
no reaction *** however adding heat and pressure will give cyclohexane
resonance energy
the difference between predicted and experimental energies
benzene resonance energy
-208 (151 kJ lower than predicted)
annulenes
cyclic hydrocarbons with alternating single and double bonds
cyclobutadiene
benzene
cyclooctatetraene
cyclodecapentaene
[4]annulene
[6]annulene
[8]annulene
[10]annulene
benzene first MO
donut, no nodes
benzene pi2 ___ pi3, both are ____ (bonding or antibonding)
pi2
pi3
= , bonding
vertical node splits in half
horizontal node splits in half
benzene pi4 ___ pi5, both are ____ (bonding or antibonding)
pi4
pi5
=, anti-bonding
vertical and horizontal nodes cut into quarters
diagonal x nodes
benzene pi5 ___ pi6
=/=
all antibonding
3 crisscrossing nodes
cyclobutadiene pi1___pi2
pi1
pi2
=/=
all bonding
vertical node, split in half
cyclobutadiene pi2___pi3
pi2
pi3
=
vertical node, split in half
horizontal node, split in half
both non-binding
cyclobutadiene pi3___pi4
pi3
pi4
=/=
horizontal node, split in half
horizontal and vertical node, split in quarters
aromatic compounds: 1 2 3 4
1 structure is cyclic containing some # conjugated pi bonds
2 each atom in ring must have unhybridized p orbital
3 unhybridized p orbital must overlap/structure must be planar
4 delocalization of pi electron over entire ring must lower electronic energy
anti-aromatic compound:
meets one of three criteria of aromatic
benzene
cyclobutadiene
cyclohexa-1,3-diene
aromatic
antiaromatic
nonaromatic
Huckel’s Rule: to qualify as a aromatic or antiaromatic a ____ compound must have a ______ of _____ p orbitals usually in a ____ conformation
cyclic
continuous ring of overlapping
planar
Huckels Rule:
[1] How many pi electrons in the cyclic system?
= to 4N+2 –> ???
= to 4N –> ???
aromatic
antiaromatic
cyclooctatetraene
aromatic, anti-aromatic, nonaromatic, why?
nonaromatic, not planar
[12]annulene, [16]annulene, [20]annulene
aromatic, anti-aromatic, nonaromatic, why?
nonaromatic, nonplanar
[10]annulene all-cis, two-trans, napthalene
aromatic, anti-aromatic, nonaromatic, why?
nonaromatic, nonaromatic, aromatic
nonplanar-ring strain
[14]annulene, [18]annulene
aromatic, anti-aromatic, nonaromatic, why?
aromatic
cyclopentadiene cation
aromatic, anti-aromatic, nonaromatic, why?
nonaromatic,
empty p orbital at (+)
cyclopentadiene anion
aromatic, anti-aromatic, nonaromatic, why?
aromatic
two electrons in p orbital
cyclopentene + ______ –> cyclopentadiene anion
-OC(CH3)3
potassium tert-butoxide
cycloheptarienyl cation is also known as _____?
and is aromatic, anti-aromatic, nonaromatic, why?
tropylium ion,
aromatic
cycloheptarienyl anion is also known as _____?
and is aromatic, anti-aromatic, nonaromatic, why?
tropylium ion,
antiaromatic
formation of tropylium cation
cyclohep-2,4,6 trienenol + H+ +H20
formation of tropylium anion
cycloheptriene + B:
cyclooctatetraene + 2K —>????
aromatic dianion
2pi electron aromatic
cyclopropenyl cation
4pi electron antiaromatic
cyclobutadiene, cyclopropenyl anion, cyclopentadienyl cation
6pi electron aromatic
benzene, cyclopentadienyl anion, cycloheptatrienyl cation
heterocyclic 6pi electron aromatic
pyridine (benzene with N in place of C, 3 double bonds still)
pyrrole (cyclopentane with NH in place of C)
furan (cyclopentane with O in place of C)
8pi electron antiaromatic
cyclooctatetraene, cycloheptatrienyl anion, cyclononatetraenyl, pentalene
10pi electron aromatic
napthalene, azulene, cyclononatetraenyl anion, cyclooctatetraenyl dianion
heterocylic 10 pi aromatic
indole (benzene + pyrrole)
quinoline (benzene + pyridine)
12pi electron antiaromatic
[12]annulene, heptalene
pyrrole
aromatic 5 membered ring wiht one nitrogen atom and 2 double bonds
pyrimidine
two basic N in a six membered ring, aromatic
imidazole
one basic and one protonated N in an aromatic 5 membered ring
purine
an imidazole ring fused to a pyrimidine ring
furan
aromatic five membered ring with oxygen instead of nitrogen in place of carbon
thiophene
furan with sulfur instead of oxygen
as the number of fused aromatic rings ____ the resonance energy per ring _____ and the compound becomes _____ reactive
increase, decrease, more
fused rings often attacked at __ and __ position
9- and 10- position
_____ of ____ extends through the crystal as ____ molecule, ____ carry current
lattice, diamond, giant, unable to
graphite is _____ stable than diamond and a _____ conductor along the ____ but not _____
more, parallel to the layers perpendicular to the layers
aromatic compounds are identified in IR by
1600cm-1 stretch C=C
aromatic compounds are identified in HNMR by
delta of 7 or 8
aromatic compounds are identified in C13NMR by
delta of 120 to 150
mass spec of benzene usually splits with a m/z of
91 from benzyl cation
alkyl benzene frequently give tropylium ion at m/z of 91
UV of aromatic
1
2
3
1 lamda = 184 (undetectable)
2 lamda = 204 moderate band
3 lamda = 254 low-intensity
adding groups _____ lamda absorption wavelength
increases
