Aromaticity
of electron density within the aromatic molecule’s overall structure. The single bonds are
sigma (σ) bonds formed with electrons positioned ”in line” between the carbon atoms’ nuclei.
Double bonds consist of one ”in line”σbond and another non-linearly arranged bond -- aπ-
bond. Theπ-bonds are formed from the overlap of atomic p-orbitals simultaneously above
and below the plane of the ring formed by the ”in line”σ-bonds.
Since they are out of the plane of the atoms,πorbitals can interact with each other freely,
and thereby they become delocalized. This means that, instead of being tied to one partic-
ular atom of carbon, each electron can be shared by all the carbon atoms in an aromatic
ring. Thus, there are not enough electrons to form double bonds on all the carbon atoms,
but the ”extra” electrons strengthen all of the bonds of the ring equally.
71.3 Characteristics
An aromatic compound contains a set of covalently-bound atoms with specific characteris-
tics:
- The molecule has to be cyclic
- A delocalized conjugated pi system, most commonly an arrangement of alternating
single and double bonds (can sometimes include triple bonds if the geometry of the
molecule permits) - Coplanar structure, with all the contributing atoms in the same plane
- A number of pi delocalized electrons that is even, but not a multiple of 4. (This is
known as Hückel’s (4n+2)Π rule, where,n= 0,1,2,3 and so on. Permissible numbers
ofπelectrons include 2, 6, 10, 14, and so on) - Special reactivity in organic reactions such as electrophilic aromatic substitution and
nucleophilic aromatic substitution
Whereas benzene is aromatic (6 electrons, from 3 double bonds), cyclobutadiene is not,
since the number of πdelocalized electrons is 4, which is not satisfied by any n integer
value. The cyclobutadienide (2−) ion, however, is aromatic (6 electrons). An atom in an
aromatic system can have other electrons that are not part of the system, and are therefore
ignored for the 4n + 2 rule. In furan, the oxygen atom is sp^2 hybridized. One lone pair is
in theπsystem and the other in the plane of the ring (analogous to C-H bond on the other
positions). There are 6πelectrons, so furan is aromatic.
Aromatic molecules typically display enhanced chemical stability, compared to similar
non-aromatic molecules. The circulating (that is, delocalized)πelectrons in an aromatic
molecule generate significant local magnetic fields that can be detected by NMR techniques.
NMR experiments show that protons on the aromatic ring are shifted substantially further
down-field than those on aliphatic carbons. Planar monocyclic molecules containing 4nπ
electrons are called anti-aromatic and are, in general, destabilized. Molecules that could
be anti-aromatic will tend to alter their electronic or conformational structure to avoid this
situation, thereby becoming merely non-aromatic.
Aromatic molecules are able to interact with each other in so-calledπ-πstacking: theπ
systems form two parallel rings overlap in a ”face-to-face” orientation. Aromatic molecules
are also able to interact with each other in an ”edge-to-face” orientation: the slight positive