C16: Aromatic Compounds

Question 1

benzene carbon-carbon bonds are _____ than typical C-C bonds, and ____ than typical ______

Answer 1

shorter
longer
C=C

Question 2

bonds of a benzene ring are ______

Answer 2

all the same length

1.397 Angstroms

Question 3

aromatic compounds are _____ containing some number of ____ bonds and having an unusually large ____

Answer 3

cyclic
conjugated double
resonance energy

Question 4

cyclohexene + KMnO4 –>??

benzene + KMnO4 —>??

Answer 4

cis diol (gylcol)
no reaction

Question 5

cyclohexene + Br2 in CCl4 –>??

benzene + Br2 in CCL4 —>??

Answer 5

trans 1,2 bromocyclohexane

no reaction *** however adding FeBr3 catalyst will give bromocyclohexene

Question 6

cyclohexene + H2, Pt –>??

benzene + H2, Pt —>??

Answer 6

cyclohexane

no reaction *** however adding heat and pressure will give cyclohexane

Question 7

resonance energy

Answer 7

the difference between predicted and experimental energies

Question 8

benzene resonance energy

Answer 8

-208 (151 kJ lower than predicted)

Question 9

annulenes

Answer 9

cyclic hydrocarbons with alternating single and double bonds

Question 10

cyclobutadiene
benzene
cyclooctatetraene
cyclodecapentaene

Answer 10

[4]annulene
[6]annulene
[8]annulene
[10]annulene

Question 11

benzene first MO

Answer 11

donut, no nodes

Question 12

benzene pi2 ___ pi3, both are ____ (bonding or antibonding)
pi2
pi3

Answer 12

= , bonding
vertical node splits in half
horizontal node splits in half

Question 13

benzene pi4 ___ pi5, both are ____ (bonding or antibonding)
pi4
pi5

Answer 13

=, anti-bonding
vertical and horizontal nodes cut into quarters
diagonal x nodes

Question 14

benzene pi5 ___ pi6

Answer 14

=/=
all antibonding
3 crisscrossing nodes

Question 15

cyclobutadiene pi1___pi2
pi1
pi2

Answer 15

=/=
all bonding
vertical node, split in half

Question 16

cyclobutadiene pi2___pi3
pi2
pi3

Answer 16

=
vertical node, split in half
horizontal node, split in half
both non-binding

Question 17

cyclobutadiene pi3___pi4
pi3
pi4

Answer 17

=/=
horizontal node, split in half
horizontal and vertical node, split in quarters

Question 18

aromatic compounds:
1
2
3
4

Answer 18

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

Question 19

anti-aromatic compound:

Answer 19

meets one of three criteria of aromatic

Question 20

benzene
cyclobutadiene
cyclohexa-1,3-diene

Answer 20

aromatic
antiaromatic
nonaromatic

Question 21

Huckel’s Rule: to qualify as a aromatic or antiaromatic a ____ compound must have a ______ of _____ p orbitals usually in a ____ conformation

Answer 21

cyclic
continuous ring of overlapping
planar

Question 22

Huckels Rule:
[1] How many pi electrons in the cyclic system?
= to 4N+2 –> ???
= to 4N –> ???

Answer 22

aromatic

antiaromatic

Question 23

cyclooctatetraene

aromatic, anti-aromatic, nonaromatic, why?

Answer 23

nonaromatic, not planar

Question 24

[12]annulene, [16]annulene, [20]annulene

aromatic, anti-aromatic, nonaromatic, why?

Answer 24

nonaromatic, nonplanar

Question 25

[10]annulene all-cis, two-trans, napthalene | aromatic, anti-aromatic, nonaromatic, why?

Answer 25

nonaromatic, nonaromatic, aromatic | nonplanar-ring strain

Question 26

[14]annulene, [18]annulene | aromatic, anti-aromatic, nonaromatic, why?

Answer 26

aromatic

Question 27

cyclopentadiene cation | aromatic, anti-aromatic, nonaromatic, why?

Answer 27

nonaromatic, | empty p orbital at (+)

Question 28

cyclopentadiene anion | aromatic, anti-aromatic, nonaromatic, why?

Answer 28

aromatic | two electrons in p orbital

Question 29

cyclopentene + ______ --> cyclopentadiene anion

Answer 29

-OC(CH3)3 | potassium tert-butoxide

Question 30

cycloheptarienyl cation is also known as _____? | and is aromatic, anti-aromatic, nonaromatic, why?

Answer 30

tropylium ion, | aromatic

Question 31

cycloheptarienyl anion is also known as _____? | and is aromatic, anti-aromatic, nonaromatic, why?

Answer 31

tropylium ion, | antiaromatic

Question 32

formation of tropylium cation

Answer 32

cyclohep-2,4,6 trienenol + H+ +H20

Question 33

formation of tropylium anion

Answer 33

cycloheptriene + B:

Question 34

cyclooctatetraene + 2K --->????

Answer 34

aromatic dianion

Question 35

2pi electron aromatic

Answer 35

cyclopropenyl cation

Question 36

4pi electron antiaromatic

Answer 36

cyclobutadiene, cyclopropenyl anion, cyclopentadienyl cation

Question 37

6pi electron aromatic

Answer 37

benzene, cyclopentadienyl anion, cycloheptatrienyl cation

Question 38

heterocyclic 6pi electron aromatic

Answer 38

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)

Question 39

8pi electron antiaromatic

Answer 39

cyclooctatetraene, cycloheptatrienyl anion, cyclononatetraenyl, pentalene

Question 40

10pi electron aromatic

Answer 40

napthalene, azulene, cyclononatetraenyl anion, cyclooctatetraenyl dianion

Question 41

heterocylic 10 pi aromatic

Answer 41

indole (benzene + pyrrole) | quinoline (benzene + pyridine)

Question 42

12pi electron antiaromatic

Answer 42

[12]annulene, heptalene

Question 43

pyrrole

Answer 43

aromatic 5 membered ring wiht one nitrogen atom and 2 double bonds

Question 44

pyrimidine

Answer 44

two basic N in a six membered ring, aromatic

Question 45

imidazole

Answer 45

one basic and one protonated N in an aromatic 5 membered ring

Question 46

purine

Answer 46

an imidazole ring fused to a pyrimidine ring

Question 47

furan

Answer 47

aromatic five membered ring with oxygen instead of nitrogen in place of carbon

Question 48

thiophene

Answer 48

furan with sulfur instead of oxygen

Question 49

as the number of fused aromatic rings ____ the resonance energy per ring _____ and the compound becomes _____ reactive

Answer 49

increase, decrease, more

Question 50

fused rings often attacked at __ and __ position

Answer 50

9- and 10- position

Question 51

_____ of ____ extends through the crystal as ____ molecule, ____ carry current

Answer 51

lattice, diamond, giant, unable to

Question 52

graphite is _____ stable than diamond and a _____ conductor along the ____ but not _____

Answer 52

more, parallel to the layers perpendicular to the layers

Question 53

aromatic compounds are identified in IR by

Answer 53

1600cm-1 stretch C=C

Question 54

aromatic compounds are identified in HNMR by

Answer 54

delta of 7 or 8

Question 55

aromatic compounds are identified in C13NMR by

Answer 55

delta of 120 to 150

Question 56

mass spec of benzene usually splits with a m/z of

Answer 56

91 from benzyl cation | alkyl benzene frequently give tropylium ion at m/z of 91

Question 57

UV of aromatic 1 2 3

Answer 57

1 lamda = 184 (undetectable) 2 lamda = 204 moderate band 3 lamda = 254 low-intensity

Question 58

adding groups _____ lamda absorption wavelength

Answer 58

increases