Resonance/
Mesomerism
Introduction
In 1933, Ingold
explained phenomenon and gave the name as mesomerism and intermediate structure
but later on Heisenberg gave name as resonance. The
placement of elements and electrons can differ and in reality bonds are a
blend of all these different possible structures. If you only have single
bonds, you can’t show resonance though. It only appears when there are
double or triple bonds asymmetrically placed.
Definition
In many
cases, a single Lewis structure fails to explain the bonding in a
molecule/polyatomic ion due to the presence of partial charges and fractional
bonds in it. In such cases, resonance structures are used to describe chemical
bonding. Resonance is a method of describing the delocalized electrons in some molecules where the bonding cannot be explicitly expressed by a
single Lewis structure.
Resonance is the phenomenon in which two or more Lewis
structures can be written for a molecule which involves different arrangement
of electrons but identical position of atoms. These two or more Lewis
structures are called resonating structures or contributing structures.
Contributing structures are not isomers of
the target molecule or ion since they only differ by the position or placement
of delocalized electrons. The
actual structure is weighted average of all resonance structure is called
resonance hybrid. A double headed arrow is used to indicate resonance in
different resonating structures i.e. a double
headed arrow (↔) on both ends
of the arrow between Lewis structures is used to show their inter-connectivity.
In resonance structures, the electrons
are able to move to help stabilize the molecule. This movement of the electrons
is called delocalization. In molecules or ions containing more than
two atom and having double bonds or triple bonds, there are pi-bonds. The
pi-electrons involved in pi-bonds are not localized between two atoms, but are
spread over the entire structure (or the portion of the structure) which has
only sigma-bonds. This spreading of pi-electrons is known as delocalization of
the electrons. Broken lines or dashes depict the region of delocalization of
the pi-electrons. Delocalization results in stabilizing the structure.
Therefore, the resonance hybrid is more stable than the contributing canonical
structures.
Electrons have no fixed position in
atoms, compounds and molecules but have probabilities of being found in certain
spaces (orbitals). Resonance forms illustrate areas of higher probabilities
(electron densities). This is like holding your hat in either your right hand
or your left. The term Resonance is applied when there are two or more
possibilities available. Resonance structures do not change the relative
positions of the atoms like your arms in the metaphor. The skeleton of the
Lewis Structure remains the same, only the electron locations change. Sometimes
the structure of a resonance hybrid is shown with the help of representation of
bonds by broken lines or dashes.
Resonating
Compounds
Resonance is
shown by benzene, toluene, O3, allenes (>C = C = C<), CO, CO2,
SO2, SO3, NO, NO2,
Applications
1. Resonance
gives stability to resonance hybrid.
2.The
energy of resonance hybrid is always lower than
resonating structures.
3. It explains a lot of factor which
cannot be explained by other phenomenon. E.g. nitric acid is stronger than
nitrous acid.
4. The resonance
hybrid has lower energy and hence greater stability than any of
the contributing structures.
5. Greater is the
number of canonical forms especially with nearly same energy, greater is the
stability of the molecule.
Conditions
For Resonance
Resonance
can occur when the canonical structures
(i) have the constituent atoms in the same relative positions;
(ii) have nearly the same energy;
(iii) have the same number of unpaired electrons (to allow for continuous change from one type of bond to another);
(iv)differ in the distribution of electrons around the
constituent atoms;
(v) molecules
or ions are planar.
Resonance
Energy
The
resonance hybrid is a more stable structure than any of the contributing
structures. This means that resonance hybrid has less energy than any of the
contributing structures. The difference in energy between the actual observed
energy of the resonance hybrid and the most stable of the contributing
structures is called resonance energy.
[Decrease in dipole moment also indicates
resonance].
Resonance in Ozone
Sometimes it is possible to write more
than one Lewis structure of a compound that agrees with the electronic
requirements. For example, we can draw two Lewis structures for ozone (O3)
molecule.
Both of these structures satisfy the
octet rule and have a double bond one side and single bond on the other side of
the central oxygen. According to these structures one oxygen-oxygen bond (O=O)
would be shorter than the other (O–O). However, this is not the case.
Experimentally both the oxygen-oxygen bonds are found to have the same bond
length (128 pm) which is in between the
bond lengths of a (O=O) double bond (121 pm) and
a (O–O) singe bond (148 pm) indicating partial double bond.
These experimental observations can be
explained with the help of the concept of resonance. The alternate Lewis
structures are called canonical structures. These are separated by a double
headed arrow (↔).
All canonical structures must similar positions of nuclei, similar number of
bonding and non-bonding electrons and similar energy. The actual structure is
the resonance hybrid of all the contributing canonical or resonating
structures. The structure of O3 molecule is represented as:
Resonance in Carbon dioxide
Carbon
dioxide may be represented by Lewis dot formula as
The bond
length of C=O is 1.22 Å, but the actual measured value is 1.15 Å. Further CO2 is quite stable
and does not show the characteristic reactions of the carbonyl group, as shown
by aldehydes and ketones. Without shifting, the relative positions of atoms of
CO2 can be represented by two more Lewis formulae:
In (ii) and
(iii), the two bonds between C and O are different, one being a triple bond and
the other a single bond. Both the C-O bonds in CO2 are identical.
It is now obvious that none of these structures actually represents CO2.
To explain this difficulty the concept of resonance was introduced, according
to which CO2 cannot be accurately depicted by any Lewis formula. The
actual structure of CO2 is a resonance hybrid of the three
structures:
These
different structures are called the canonical or contributing structures. The
actual structure of CO2 is different from the canonical structures
and although it is closely related to them, the actual structure cannot be
represented on paper using the accepted symbols. All the molecules of CO2
have the same structure. Usually, a double-headed arrow is used between the
canonical structures.
Resonance in Carbonate ion
According to experimental findings, all
carbon to oxygen bonds in CO32−
are equivalent. Hence, it is inadequate to represent CO32− ion by a single Lewis structure having two
single bonds and one double bond.
Therefore, carbonate ion is described as a resonance hybrid of the
following structures:
The different resonance structures of the carbonate ion (CO32−) are illustrated above. The delocalization of
electrons is described via fractional bonds (which are denoted by dotted lines)
and fractional charges in a resonance hybrid.
Resonance in sulphur dioxide
S – O bond length = 156 pm
S = O bond length = 128 pm
S – O bond length in SO2 =
143 pm
Resonance in sulphur trioxide
Resonance in nitrogen dioxide
Resonance Structures of NO2– Ion
In the nitrite ion, the bond lengths of
both nitrogen-oxygen bonds are equal. The Lewis dot structures of
NO2–highlight a difference in the bond order of the two
N-O bonds. The resonance hybrid of this polyatomic ion, obtained from its
different resonance structures, can be used to explain the equal bond lengths,
as illustrated below.
Resonance Structures of NO3– Ion
Nitrogen is the central atom in a
nitrate ion. It is singly bonded to two oxygen atoms and doubly bonded to one
oxygen atom. The oxygen atoms that are singly bonded to the nitrogen hold a
charge of -1 (in order to satisfy the octet configuration).
The central nitrogen atom has a charge of +1 and the overall charge on the
nitrate ion is -1. The three possible resonance structures of NO3– are
illustrated below.
Resonance in dinitrogen oxide
(N2O)
Three canonical structures of dinitrogen
oxide are given below:
Resonance in Benzene
benzene is considered to be a resonance
hybrid of two Kekule’s Structures.
The benzene molecule is stabilized by
resonance, the pi-electrons are delocalized around the ring structure. This
delocalization causes each carbon-carbon bond to have a bond order of 1.5, implying
that they are stronger than regular C-C sigma bonds. In the resonance hybrid of
benzene, the delocalization of pi electrons is described with the help of a
circle inside the hexagonal ring.
In benzene, Kekule’s first suggested two cyclohexatriene Kekule’s structures that have been taken together, they constitute the general structure as contributing structures. The hexagon replaces three double bonds in the hybrid structure represents six electrons in a collection of three molecular orbitals with a nodal plane in the molecule plane.
Resonance is a mental exercise and
method within the Valence Bond Theory of
bonding that describes the delocalization of electrons within molecules. It
compares and contrasts two or more possible Lewis structures that can represent
a particular molecule. Resonance structures are used when one Lewis structure
for a single molecule cannot fully describe the bonding that takes place
between neighboring atoms relative to the empirical data for the actual bond
lengths between those atoms. The net sum of valid resonance structures is
defined as a resonance hybrid, which represents the overall delocalization of
electrons within the molecule. A molecule that has several resonance structures
is more stable than one with fewer. Some resonance structures are more
favorable than others.
Rules for
writing resonance structures
The
following rules are useful in deciding on the relative importance of resonance
structures in contributing to the actual structure of a molecule.
1. A resonating structure must be Lewis
structure (All resonance structures must follow the rules of writing Lewis Structures)
2. All resonance
structures must possess the same bond connections and position of atoms. This
feature distinguishes resonance structures from constitutionally isomeric
structures. Resonance structures possess the same number, type and positions of
atoms, but different positions of valence electrons; isomers possess the same
number and type of atoms, but differ in the way the atoms are connected.
3. The position of atom nuclei be same.
(The skeleton of the structure cannot be changed (only the electrons move).
4. The
arrangement of electrons must be different
5. Resonance structures should have the
same number of electrons, do not add or subtract any electrons. (Check the
number of electrons by simply counting them).)
6. The
energy of resonating structure must be greater than that of resonance hybrid.
7. The
hybridization of the structure must stay the same.
8. Resonance
structures must also have the same number of lone pairs.
9. There are no
exceptions to the rule that H never exceeds a duet and that C, N, O and F never
exceed the octet.
10. All other factors
being comparable, isovalent resonance is more important than heterovalent
resonance.
11. Structures which create separation of
charge are less important than structures which do not.
12. Heterovalent
resonance is only important when formal negative charge is placed on atoms of
greater electronegativity and formal positive charge is placed on atoms of
lower electron negativity.
13 Effective resonance
between resonance structures requires the atoms involved in the resonance to be
planar.
14 Resonance
stabilization is most effective when the contributing structures are all
equivalent. When the latter is the case, the resonance stabilization is most
effective the greater the number of equivalent structures.
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