Naming Aromatic Compounds
(Benzene as the Parent)
•
•
If the the alkyl chain is smaller than the ring use the
ring as the parent
Monosubstituted benzenes named the same way
as other hydrocarbons using –benzene as the
parent name
Aromatic Compounds
CH3
CH3
CH3
CH3
H3C
CH3
CH3
CH3
Common
Name
toluene
benzene
ortho-xylene
methylbenzene
IUPAC
CH3
meta-xylene
1,2-dimethylbenzene
cumene
para-xylene
1,3-dimethylbenzene
1,4-dimethylbenzene
isopropylbenzene
Other aromatic compounds:
OH
Cl
phenol
chlorobenzene
CH2OH
O
NH2
OMe
aniline
anisole
H
O
OH
SO3H
NO2
benzene
sulfonic acid
O
nitrobenzene
CH3
biphenyl
benzylalcohol benzaldehyde
or
phenylmethanol
benzoic acid
acetophenone
HC
CH2
styrene
More Aromatic Compounds
Polycyclic Aromatic Compounds
6
5
8
1
7
6
5
1
2
2
3
3
4
4
naphthalene
9
10
7
4
8
7
3
6
2
8
9
5
anthracene
1
10
phenanthrene
2,4,5,6,10-pentamethylphenanthrene
Heterocyclic Aromatic Compounds
O
H
N
N
furan
pyrrole
N
N
pyridine
N
pyrimidine
N
H
indole
Naming Aromatic Compounds (Disubstituted Benzenes)
•
Relative positions on a benzene ring
• ipso- carbon 1
• ortho- (o) on adjacent carbons (1,2)
• meta- (m) separated by one carbon (1,3)
• para- (p) separated by two carbons (1,4)
N
H
N
N
purine
Naming Aromatic Compounds (Benzenes with More Substituents)
•
•
•
Choose numbers for the lowest possible values
List groups alphabetically (hyphenated between)
numbers
Common names, such as toluene can serve as root
name (as in TNT)
Naming Aromatic Compounds (Benzene as a Substituent)
•
•
If the the alkyl chain is larger than the ring use it as
the parent
When a benzene ring is a substituent, the term
phenyl is used (for C6H5–)
•
•
You may also see “Ph” or “φ” or “C6H5”
Remember that benzyl refers to “C6H5CH2–”
Structure of Benzene
H
H
H
H
H
H
•
All its C-C bonds are the same length (1.39 Å)
•
•
•
between length of normal single and double bonds
Electron density in all six C-C bonds is identical
Structure is planar
Stability of Benzene (Heats of Hydrogenation)
H2, Pd
H2, Pd
H2, Ni
175 °C
180 atm
H2, Pd
ΔH°exp = – 28.6 kcal/mol
ΔH°est = – 57.2 kcal/mol
ΔH°exp = – 55.5 kcal/mol
= 1.7 kcal resonance energy
ΔH°est = – 85.8 kcal/mol
ΔH°exp = – 49.8 kcal/mol
= 36 kcal resonance energy
ΔH°est = – 85.8 kcal/mol
ΔH°exp = – 80.6 kcal/mol
= 5.2 kcal resonance energy
Aromaticity and the 4n + 2 Rule
1 monocyclic cmp
(i.e. don't look at anthracene, etc)
2 All atoms in the ring must have a p-orbital (sp2 atoms)
3 The p-orbitals must overlap...the cmp must be flat and the overlap
unbroken
4 π-cloud must contain 4n+2 electrons (Hückel's Rule)
n = 0,1,2,3,4,5....
Therefore, compounds with 2,6,10,14,18 electrons will be aromatic
ALL the rules must be followed to be aromatic!
Huge energy advantage to being aromatic!
H
H H
H
H
sp3
H
Antiaromatic Compounds
1 monocyclic cmp
(i.e. don't look at anthracene, etc)
2 All atoms in the ring must have a p-orbital (sp2 atoms)
3 The p-orbitals must overlap...the cmp must be flat and the overlap
unbroken
4 π-cloud must contain 4n electrons
n = 0,1,2,3,4,5....
Therefore, compounds with 4,8,12,16,20 electrons will be antiaromatic
ALL the rules must be followed to be antiaromatic!
Basically, nature avoids...use to explain why stuff doesn't happen
H
H
X
H
H
H
H
Aromatic Ions
Aromatic Heterocycles
•
A Heterocycle
is a cyclic
compound that
contains an
atom or atoms
other than
carbon
More Aromatic Heterocycles
O
Furan
Thiophene
Special Cases Pay Attention!
O
O
N
CH3
X
N
H3C
H
N
N
N
N
H
H
Special Compounds With 4n π Electrons
4π electrons
antiaromatic
Exists @ 4 K
What is Aromaticity?
MO Description of Benzene
Frost Diagrams
Inscribe the ring in a
concentric circle with
one of the points down
Draw energy levels at
each of the
intersections between
the circle and the ring
Cut circle in half
Fill the levels with π
electrons starting at the
bottom and working
your way up
1.
2.
3.
4.
6πe
non-bond MO's
bonding MO's
Why 4n+2?
•
Aromatic
NB
•
NB
Antiaromatic or Nonaromatic
NB
NB