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Chapter 14 Study Guide
1825 Michael Faraday discovers benzene.
1834 Mitscherlich synthesizes benzene from benzoic acid.
The new compound has an index of hydrogen deficiency(I.H.D.) of 4. IE. 6 carbon
structure should have 14 hydrogens(C n H2n+2 ). Benzene only has 6 hydrogens. Therefore,
it is missing 8 hydrogens or IHD = 4. Each index is equal to a double bond or ring.
Therefore this is a highly unsaturated compound.
This led to classification of all organics as aliphatic or aromatic.
Kekule def. of aromatic—
Know structures page 624—
Ortho/para/meta—only refer to disubstituted benzenes.
If the two substituents are next to each other it is ortho.
If the two substituents are separated by one carbon it is meta.
If the two substituents are opposite each other it is para.
If both of the substituents are methyl groups the molecule is called xylene.
Nomenclatue—substituents are listed alphabetically. If more than two substituents then
they are numbered to have the lowest possible number.
If benzene is a substituent it is called phenyl.
The benzyl group is a benzene with a CH2 attached.
Benzene is non-reactive to ordinary alkene chemical reactions.
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More Nomenclature notes
CH3
OH
Toluene
NH2
Phenol
SO3H
Aniline
Benzenesulfonic acid
O
CH3
COOH
Benzoic Acid
o-xylene
O
Acetophenone
Anisole
m-xylene
p-xylene
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Kekule structure of benzene
Benzene only reacts by substitution not addition.
Search for other nonbenzene aromatics led to cyclooctatetraene(COT).
COT reacts by addition not substitution. Therefore it is not aromatic.
Comparison of ∆H of benzene to its predicted ∆H leads to discovery of resonance
stabilization energy.
Modern structure of benzene
Benzene is a planar molecule with a high degree of delocalization as per MOT.
Modern theory of Aromaticity—Huckel’s rule(4N+2)
To be aromatic, a compound must be planar, cyclic with alternating single and double
bonds and must follow Huckel’s rule(i.e. have 2, 6, 10, 14, 18…pi electrons).
By MOT, all pi electrons must be in bonding orbitals and not anti-bonding orbitals(p.
633).
Annulenes—
[10] annulene is not aromatic because it can’t be planar.
By NMR, if all aromatic hydrogens show as one peak then they are equivalent and
aromatic.
See p. 636 also.
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Other Aromatics
Ions
Cyclopentadienyl(636)
and tropylium ions.
empty p-orbital due to cation
Aromatic, antiaromatic, nonaromatic
∆H comparison of cyclic to straight chain.
Benzenoid/Nonbenzenoid Aromatic compounds
Page 640-643
Heterocyclic aromatic compounds
Page 644-645
Sunscreen
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Other Aromatic Compounds
Benzenoid
pyrene C16H10
Benzo[a]pyrene C20H12
Dibenzo[a,l]pyrene C24H14
8 double bonds = 16 pi electrons?
Internal double bond (red) does
not participate. Only 14 pi electrons.
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Br2/CCl4
Br
Br
[14]Annulene
Nonbenzenoid Aromatic Compounds
5 perimeter double bonds = 10 pi electrons
Huckel # = aromatic
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pyridine
N
pyrrole
furan
N
O
thiophene
S
H
Pyridine has 3
Pyrrole, furan and thiophene all have 2 double bonds
double bonds and
and 4 pi electrons. Therefore one lone pair is needed to
6 pi electrons so the
achieve aromaticity.
lone pair does not
participate in aromaticity
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Chapter 14 and 15 Practice
1. Assuming the following are planar, tell if they are aromatic are not. Also
explain your answer.
2 single bonds
destroys
aromaticity.
B
A
C
Ring A is aromatic. It is cyclic,
planar, has 14 Π electrons and alternating
double and single bonds.
Ring B is aromatic. It is cyclic,
planar, has 10 Π electrons, and
because of + charge has alternating
double and single bonds.
Ring C is not aromatic.
Although planar, cyclic
and 18 Π electrons it
does not have alternating
double and single bonds.
2. Give a spectroscopic values for the following.
H
H NMR ___6-9.5_ _
Major UV peaks ____205 and 250-275_
C NMR __100-170_
Major IR peaks above 1200
_______3030 and 4 peaks from 1450-1600___________________
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HOMEWORK
Determine if the following are aromatic, explain your answers
Reason:
Reason:
Reason:
Reason:
Reason:
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HOMEWORK--KEY
Aromatic
Aromatic
Nonaromatic
Reason:
Reason:
Reason:
Negative anion
contributes to
pi-electrons. 6 pielectrons = aromatic
sp2 hybridized carbocation
Allows for delocalization
of pi-electrons
Not cyclic
Aromatic
Not Aromatic
Reason:
Reason:
18 pi electrons. 2 internal double
bonds do not contribute to aromaticity.
20 pi electrons is not a Huckel number.