Mesomerism, or Resonance
Mesomerism, or Resonance
The theory
of mesomerism was developed on chemical grounds. It was found that no structural
formula could satisfactorily explain all the properties of certain compounds,
e.g., benzene. This led to the idea that such compounds exist in a state
which is some combination of two or more electronic structures, all of which
seem equally capable of describing most of the properties of the compound, but
none of describing all the properties. Ingold (1933) called this phenomenon mesomerism
(‘between the parts’, i.e., an intermediate structure). Heisenberg
(1926), from quantum mechanics, supplied a theoretical background for
mesomerism; he called it resonance, and this is the name which is widely
used.
The chief conditions for resonance are :
- The positions of the nuclei in each structure must be the same or nearly the same.
- The number of unpaired electrons in each structure must be the same.
- Each structure must have about the same internal energy, i.e., they various structures have approximately the same stability.
Let us
consider carbon dioxide as an example. The electronic structure of carbon
dioxide may be represented by at least three possible electronic arrangements
which satisfy the above conditions :
Structure (II)
and (III) are identical as a whole, since both oxygen atoms are the same. Each structure,
however, shows a given oxygen atom to be in a different state, e.g.,
the oxygen atom on the left in (II) is negative, whereas in (III) although two
(or more) of the electronic structure may be the same when the molecule is
considered as a whole, each one must be treated as a separate individual which
makes its own contribution to the resonance state. Structure (I), (II) and
(III) are called the resonating, unperturbed or canonical
structures of carbon dioxide, and carbon dioxide is said to be a resonance
hybrid of these structures, or in the mesomeric state.
It is hoped
that the following crude analogy will help the reader to grasp the concept of
resonance. Most readers will be familiar with the rotating disc experiment that
shows the composite nature of white light. When stationary, the disc is seen to
be coloured with the seven colours of the rainbow. When rotating quickly, the
disc appears to be white. The resonating
structures of a resonance hybrid may be compared to the seven colours, and the
actual state of the resonance hybrid to the ‘white’; i.e., the
resonating structures may be regarded as superimposed on one another, the final
result being one kind of molecule. In a resonance hybrid all the molecules are
the same; a resonance hybrid cannot be expressed by any single structure.
In a
resonance hybrid the molecules have, to some extent, the properties of each
resonating structure. The greater the contribution of any one structure, the
more closely does the actual state approach to that structure. At the same
time, however, a number of properties differ from those of any one structure. The
observed enthalpy of formation of carbon dioxide is greater than the calculated
value by 132.2 kJ (31.6kcal). in other words, carbon dioxide requires 132.2kJ
more energy that expected to break it up into its elements, i.e., carbon
dioxide is more stable than anticipated on the structure O=C=O. How can this be
explained? Argument based on quantum mechanics show that a resonance hybrid
would be more stable than any single resonating structure, i.e., the
internal energy of a resonance hybrid is less than that calculated for any one
of the resonating structures. The difference between the enthalpy of formation
of the actual compound, i.e., the observed value, and
that of the resonating structure which has the lowest internal energy
(obtained by calculation) is called the resonance energy. Thus the
value of the resonance energy of any resonance hybrid is not an absolute
value; it is a relative value the resonating structure containing the least
internal energy being chosen as the arbitrary standard for the resonance
hybrid. The greater the resonance energy, the greater is the stabilization. The
resonance energy is a maximum when the resonating structures have equal energy
content, and the more resonating structures there are, the greater is the
resonance energy, provided that all contributing structures have similar
stabilities.
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