CHAPTER 15: AROMATICITY • REACTIONS OF BENZENE
Aromatic Compounds – a cyclic and planar compound with an uninterrupted ring of p
orbital-bearing atoms containing an odd number of pairs of π 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 π electrons.
Because its π electrons are delocalized, all the C–C 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 π cloud to be cyclic, the molecule must be cyclic.
For the π cloud to be uninterrupted, every atom in the ring must have a p orbital.
For the π 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 π cloud contains three pairs of π 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 π
Because there are two electrons in a pair, Huckel’s rule requires that an aromatic compound
have 1, 3, 5, 7, 9, and so on pairs of π electrons. Thus, Huckel’s rule is just a mathematical
way of saying that an aromatic compound must have an odd number of pairs of π 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 π electrons, and cyclooctateraene has 4 pairs of π electrons,
therefore these compounds are not aromatic because they have an even number of pairs of π
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 sp hybridized, and only sp and sp carbons have p
The cyclopentadienly cation also is not aromatic because, although it has an uninterrupted
ring p orbital-bearing atoms, its π cloud has two (an even #) pairs of π electrons.
The cyclopentadienyl anion is aromatic: it has uninterrupted ring of p orbital-bearing atoms,
and the π cloud contains three (an odd #) pairs of delocalized π electrons.
The cyclopentadienyl anion’s lone-pair electrons from a π bond in the resonance
contributors. Thus, they are π 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 π
electrons), phenanthrene (7 pairs of π electrons), and chrysene (9 pairs of π electrons) are
aromatic. 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.
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 π
electrons. The lone pair electrons on the nitrogen are not π electrons.
The lone-pair electrons on the nitrogen atom of the pyrrole are π electrons. The resonance
contributors show that lone-pair electrons form a π bond, thus they are π electrons. Pyrrole,
therefore, has 3 pairs of π electrons and is aromatic.
Furan and thiophene are sta2le aromatic compounds. Both the oxygen in f2ran and the
sulphur in thiophene are sp hybridized and have one lone pair in an sp orbital.
Quinoline, indole, imidazole, purine, and pyrimidine are other examples of heterocyclic
aromatic compounds: 15.5 Some Chemical Consequences of Aromaticity
The strength of an acid is determined by the stability of its conjugate base: the more stable
its conjugate base, the stronger is the acid.
Alkyl halides tend to be relatively nonpolar covalent compounds – they are soluble in
nonpolar solvents and insoluble in water.
An aromatic compound is more stable than an analogous cyclic compound with localized
An antiaromatic compound is less stable than an analogous cyclic compound with
Aromaticity is characterized by stability, whereas an