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Chapter 15

CHMB42H3 Chapter Notes - Chapter 15: Aromaticity, Sodium Cyclopentadienide, Cyclic Compound

Course Code
Lana Mikhaylichenko

of 15
Aromatic Compounds a cyclic and planar compound with an uninterrupted ring of p
orbital-bearing atoms containing an odd number of pairs of electrons.
Aliphatic Compounds a nonaromatic organic compound.
15.1 Aromatic Compounds are Unusually Stable
Benzene is a planar, cyclic compound with two cyclic clouds of delocalized electrons.
Because its electrons are delocalized, all the CC bonds in benzene have the same length,
shorter than a typical single bond but longer than a typical double bond.
Benzene is a particularly stable compound because it has an unusually large delocalization
Delocalization energy (also called resonance energy) tells us how much more stable a
compound with delocalized electrons is than it would be if all its electrons were localized.
Compounds with unusually large resonance energies, like benzene, are called aromatic
Aromatic compounds are particularly stable.
15.2 The Two Criteria for Aromaticity
To be classified as aromatic, a compound must meet both of the following criteria:
1. It must have an uninterrupted cyclic cloud of π electrons (called a π cloud) above and
below the plane of the molecule. Let's look a little more closely at what this means:
For the cloud to be cyclic, the molecule must be cyclic.
For the cloud to be uninterrupted, every atom in the ring must have a p orbital.
For the cloud to form, each p orbital must overlap with the p orbitals on either side of it.
Therefore, the molecule must be planar.
2. The π cloud must contain an odd number of pairs of π electrons.
Therefore, benzene is an aromatic compound because it is cyclic and planar, every carbon in
the ring has a p orbital, and the cloud contains three pairs of electrons.
For a compound to be aromatic, it must be cyclic and planar and have an
uninterrupted cloud of π electrons. The π cloud must contain an odd number of pairs
of π electrons.
Huckel’s Rule (4n + 2 rule) states that for a compound to be aromatic, its cloud of
electrons must contain (4n + 2) π electrons, where n is an integer (any whole number). This
is the same as saying that the electron cloud must contain an odd number of pairs of
Because 
have 1, 3, 5, 7, 9, and so on pairs of 
way of saying that an aromatic compound must have an odd number of pairs of electrons.
15.3 Applying the Criteria For Aromaticity
Monocyclic hydrocarbons with alternating single and double bonds are called annulenes.
A prefix in brackets denotes the number of carbons in the ring.
Ex of annulenes are:
Cyclobutadiene has 2 pairs of electrons, and cyclooctateraene has 4 pairs of electrons,
therefore these compounds are not aromatic because they have an even number of pairs of
electrons. Cyclooctatetraene is also not planar, it is tub-shaped.
Cyclopentadiene is not aromatic because it does not have an uninterrupted ring of p orbital
bearing atoms. One of its ring atom is sp3 hybridized, and only sp2 and sp carbons have p
The cyclopentadienly cation also is not aromatic because, although it has an uninterrupted
ring p orbital-bearing atoms, its cloud has two (an even #) pairs of electrons.
The cyclopentadienyl anion is aromatic: it has uninterrupted ring of p orbital-bearing atoms,
and the cloud contains three (an odd #) pairs of delocalized electrons.
-pair electrons from a bond in the resonance
contributors. Thus, they are electrons.
The criteria that determine whether a monocyclic hydrocarbon is aromatic can also be used
to determine whether a polycyclic hydrocarbon is aromatic. Naphthalene (5 pairs of
electrons), phenanthrene (7 pairs of electrons), and chrysene (9 pairs of electrons) are
15.4 Aromatic Heterocyclic Compounds
A heterocyclic compound is a cyclic compound in which one or more of the ring atoms is
an atom other than carbon. The atom that is not carbon is called a heteroatom.
The most common heteroatoms are N, O, and S.
Heterocyclic Compounds:
Pyridine is an aromatic heterocyclic compound. Each of the six ring atoms of pyridine is sp2
hybridized, which means that each has a p orbital, and the molecule contains three pairs of
electrons. The lone pair electrons on the nitrogen are not electrons.
The lone-pair electrons on the nitrogen atom of the pyrrole are electrons. The resonance
contributors show that lone-pair electrons form a bond, thus they are electrons. Pyrrole,
therefore, has 3 pairs of electrons and is aromatic.
Furan and thiophene are stable aromatic compounds. Both the oxygen in furan and the
sulphur in thiophene are sp2 hybridized and have one lone pair in an sp2 orbital.
Quinoline, indole, imidazole, purine, and pyrimidine are other examples of heterocyclic
aromatic compounds: