Chapter 13
Conjugated Unsaturated Systems
Conjugated Unsaturated Systems
Resonance – remember that?
VERY important concept that will keep
coming up in the course
Moving around electrons in the p orbitals
(can move pi electrons or lone pairs) that
are all planar (in the same pi system)...
are all planar (in the same pi system)...
Rules of Resonance
Individual resonance structures are not a true representation
of the real structure of a molecule
A hybrid of all major resonance structures gives an indication of the true
structure
Only electrons may be moved in resonance structures, not
atoms
Onlyππππand nonbonding electrons are moved
All resonance structures must be proper Lewis structures
N
O
O O
All resonance structures must have the same number of
paired and unpaired electrons
All atoms that are part of the delocalized
π
π
π
π
-electron system
must lie in a plane or be nearly planar
Rules of Resonance cont
The energy of the actual molecule is lower than the energy
calculated for any one contributing resonance structure
Allyl cation has much lower energy than either contributing structures 4 or 5
A system with equivalent resonance structures is particularly
stable stable
The allyl cation has two equivalent resonance structures and is therefore
particularly stable
The more stable a resonance structure is, the more important it is
and the more it contributes to the hybrid
Structure 6 is a more stable tertiary carbocation and contributes more than
structure 7
What makes a GOOD resonance
structure??
The more covalent bonds the better
More octets the better
More octets the better
Keep opposite charges close together (if you have to have
charges at all)
Put negative charges on more electronegative atoms
Make carbon happy as possible!
TIP: ALWAYS check that the net charge hasn’t changed
between resonance structure, if it did, YOU MESSED UP!
This leads to...
Concept of CONJUGATION
Molecules with delocalized
π
electrons are conjugated
Generally this is seen when there is a p orbital next to a
double bond – conjugated unsaturated system
Conjugation gives higher stability!!
Remember radicals?
Which is most stable 1
o, 2
oor 3
o?
What about this? How does it compare
for stability?
Chapter 14
Nomenclature of Benzene Derivatives
Benzene is the parent name for some monosubstituted benzenes; the substituent name
is added as a prefix
often drawn as
benzene
Chapter 14 9
Nomenclature of Benzene Derivatives
For other monosubstituted benzenes, the presence of the
substituent results in a new parent name
When two substituents are present their position may be indicated
by the prefixes ortho, meta, and para (o, m and p) or by the corresponding numerical positions
When two substituents are present their position may be indicated
by the prefixes ortho, meta, and para (o, m and p) or by the corresponding numerical positions
The C6H5- group is called phenyl when it is a substituent
Phenyl is abbreviated Ph or ΦΦΦΦ
The phenylmethyl group is called a
benzyl (abbreviated Bz)
The History of Benzene
From petroleum distillate
- Michael Faraday
From benzoic acid and calcium oxide
- Eilhardt Mitscherlich
Chapter 14 15
Empirical Formula: “CH”
Molecular formula: C
6H
6The Kekule Structure for Benzene
Kekule was the first to formulate a reasonable representation of
benzene
The Kekule Structure for Benzene
The Kekule structure suggests alternating double and single
carbon-carbon bonds
Based on the Kekule structure one would expect there to be two different
1,2-dibromobenzenes but there is only one
Kekule suggested an equilibrium between these compounds to explain this
observation but it is now known no such equilibrium exists
Modern Theories of the Structure of Benzene
The Resonance Explanation of the Structure of Benzene
Structures I and ΙΙΙΙΙΙΙΙ are equal resonance contributors to the real
structure of benzene
Benzene is particularly stable because it has two equivalent and important
resonance structures
Each carbon bond is 1.39 Å, which is between the length of a
carbon-carbon single bond between sp2carbons (1.47Å) and a carbon-carbon double bond (1.33 Å)
Often the hybrid is represented by a circle in a hexagon (III)
Recall from Chapter 1
The Molecular Orbital Explanation of the Structure of Benzene
(a) benzene carbons are sp2 hybridized with p-orbitals on all 6 carbons (b) p-orbitals overlap
(c) forming a bonding molecular orbital with electron density above and
below the plane of the ring
Recall from Chapter 1
The Molecular Orbital Explanation of the Structure of Benzene
There are six ππππ molecular orbitals for benzene
“Benzene is weird”: Reactions of Benzene
Recall the reactions of alkenes:OH
OH
KMnO
4/ H
2O
25
o+ enantiomer
OH
OH
Chapter 14 23
OH
H
H
3O
+/ H
2
O
25
oOH
25
H
H
Ni / H
2Reactions of Benzene
Even though benzene is highly unsaturated it does not undergo
any of the regular reactions of alkenes such as addition or oxidation
“Benzene is weird”: Reactions of Benzene
Benzene can be induced to react with bromine if
a Lewis acid catalyst is present however the
reaction is a substitution and not an addition
Br
Br
Br
2, CCl
4dark, 25
oBr
2, CCl
4no reaction
Chapter 14 25
Benzene produces only one monobrominated
compound, which indicates that all 6
carbon-hydrogen bonds are equivalent in benzene
2 4
dark, 25
ono reaction
H
Br
Br
2
“Benzene is weird”: The Stability of Benzene
“
Benzene is weird”:
The Stability of Benzene
Benzene is much more stable than would be
expected based on calculations for
“cyclohexatriene”
A reasonable prediction for the heat of hydrogenation of
hypothetical cyclohexatriene is -360 kJ mol-1 (3 times that of
cyclohexene, -120 kJ mol-1 )
Chapter 14 27
cyclohexene, -120 kJ mol )
The experimentally determined heat of hydrogenation for
benzene is -280 mol-1, 152 kJ mol-1 more stable than
hypothetical cyclohexatriene
Aromatic compounds are found to be unusually stable,
and don’t react as one would predict
The molecular orbital diagram helped clear things up
(all the electrons went into nice stable bonding orbitals)
The verdict – Benzene is aromatic
Huckel noticed a coincidence in all these crazy stable
monocyclic rings that leads us to the definition of
aromatic....
What makes something aromatic aka
crazy stable?
Something is aromatic if:
1 – It has a closed ring
2- It has a conjugated system of p orbitals (every
2- It has a conjugated system of p orbitals (every
atom in the ring has a p orbital)
3- It is planar
4- Has 4N + 2 pi electrons ( n = 0, 1, 2, 3, etc)
* Huckels rule was designed for planar monocyclic rings
The Annulenes
Annulenes are monocyclic compounds with alternating double
and single bonds
Annulenes are named using a number in brackets that indicates the ring size Benzene is [6]annulene and cyclooctatetraene is [8]annulene
An annulene is aromatic if it has 4n+2ππππelectrons and a planar carbon skeleton
The [14]and [18]annulenes are aromatic (4n+2, where n= 3,4)
The [16] annulene is not aromatic
The [10]annulenes below should be aromatic but none of them
can be planar
4 is not planar because of steric interaction of the indicated hydrogens 5 and 6 are not be planar because of large angle strain in the flat molecules
Where there’s a Yin, there’s a Yang
Anti-aromatic systems have 4n
π
electrons!
These systems are particularly UNSTABLE!
•
for example, cyclo-octatetraene would have
unpaired electrons in its molecular orbital, which
Chapter 14 32
eight p atomic orbitals
E n e rg y bonding non-bonding anti-bonding
unpaired electrons in its molecular orbital, which
would be unfavourable.
•
cyclo-octatetraene is anti-aromatic!
So compounds fit into 3 categories:
1- Aromatic
2 – Anti-aromatic
2 – Anti-aromatic
3- Non-aromatic
Other Aromatic Compounds
Benzenoid Aromatic Compounds
Polycyclic benzenoid aromatic compounds have two or more
benzene rings fused together
Chapter 14 34
Heterocyclic Aromatic Compounds
Heterocyclic compounds have an element other than C in the ring Examples of aromatic heterocyclic compounds:
(Numbering always starts at the heteroatom)
Heterocyclic Aromatic Compounds
Pyridine has an sp2 hybridized nitrogen
nitrogen's p-orbital is part of the aromatic ππππsystem
the nitrogen lone pair is in an sp2orbital orthogonal to the porbitals of the ring;
these electrons are not part of the aromatic system
The lone pair on nitrogen is available to react with protons and so pyridine is
basic
N
The nitrogen in pyrrole is sp2 hybridized and the lone pair resides
in the p orbital
This p orbital contains two electrons and participates in the aromatic system The lone pair of pyrrole is part of the aromatic system and not available for
protonation; pyrrole is therefore not basic
NH
In furan and thiophene an electron pair on the heteroatom is also
in a p orbital which is part of the aromatic system
O
Chapter 14 38
Biologically important aromatic compounds
Amino acids
the lone pair electrons needed to form the aromatic systems are
shown for Trp and His, other lone pairs are not shown
NH
Phe
(phenyl ring)
Trp
(indole ring)
Chapter 14 39
+
H
3N
H
N
N
H
H
N
COO
Biologically important aromatic compounds
Nucleoside bases
lone pair electrons needed for the aromatic systems are shown purine and adenine are aromatic
guanine is pseudoaromatic
it violates the 4n+2 rule as commonly drawn, but important resonance forms are
aromatic
in practice, it behaves like other heterocyclic aromatic compounds
NH
2O
O
-Chapter 14 40
N
N
N
H
N
N
N
N
H
N
NH
2NH
N
N
H
N
O
NH
2purine
adenine
guanine
NH
+N
N
H
N
O
NH
211
π
-electrons,
not 4n+2
10
π
-electrons,
Biologically important aromatic compounds
Nucleoside bases
lone pair electrons needed for the aromatic systems are shown
pyrimidines are also pseudoaromatic
NH
O
NH
O
NH
NH
2NH
+O
-Chapter 14 41
NH
N
H
O
NH
N
H
O
NH
N
H
O
NH
+N
H
+O
-uracil
8
π
-electrons,
not 4n+2
6
π
-electrons,
4n+2
Intermolecular interactions of aromatic compounds
homocyclic (all carbon) aromatic compounds are strongly
hydrophobic
heterocyclic compounds can interact with both hydrophobic
and polar functional groups
the faces (top and bottom) of aromatic rings are
electronegative because of the large
π
π
π
π
-electron clouds
the edges are electropositive because they consist of
hydrogens
Chapter 14 42
δ
-δ- δ -δ -δ -δ
-δ+
δ+ δ+
Intermolecular interactions of aromatic compounds
aromatic rings tend to interact with each other through
edge-to-face or through staggered face-edge-to-face interactions that
bring the negatively and positively charged parts of the
molecules together
edge-to-face interactions are commonly seen in protein
structures
this is the structure of cathepsin S, showing only the Phe & Tyr side chains; the blue ones
are engaged in edge-to-face interactions
Chapter 14 43
