Resonance Effect (Bond Polarization) Homework Help, Tutoring
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Bond Polarization - Resonance Effect
As we all know each compound will have a specific structure. But, for some compounds
a single structure cannot satisfactorily explain its properties. In such cases, a
series of structures are proposed for the compound to explain the various properties.
No single structure can explain the compound completely. The best example for such
a compound is benzene. It has two types of carbon-carbon bonds. They include three
single bonds and three double bonds.
Benzene Structure
Though the bonds are different, they all show the same characteristics. The
phenomenon in which two or more structures are written to explain the properties
of a single compound is termed as resonance and the structures are termed as
resonating structures or canonical structures.
These compounds with alternating double bonds and resonating structures are said
to exert resonance effect. This effect is mainly show via π orbitals. It can best
be defined as transfer of π electrons from a multiple bond to an atom or from a
multiple bond to a single covalent bond or from an atom with lone pair of electrons
to adjacent covalent bond. For example, in case of carbonyl group, the π electrons
are transferred towards the oxygen atom all along the chain of carbon atoms.
H3C-CH=CH-CH=O
The π electrons located at the CH=CH double bond are easily transferred to the adjacent
covalent bond resulting in a small positive charge at the carbon atom and a negative
charge at oxygen atom.
H3C-CH-CH=C+H-O-
As can be seen in the above case, the transfer of electrons has taken place through
a conjugated double bond system i.e. alternating double bonds and hence, the mesomeric
or the resonance effect is also best described as conjugative effect.
Similar to inductive effect, mesomeric effect can also be either positive or negative and is
denoted as +M and -M respectively.
Positive mesomeric effect (+M) involves transfer of electrons away from the atom and the negative (-M)
mesomeric effect involves transfer of electrons towards the atom. In the above example, the transfer
of electrons is taking place towards the oxygen atom. Hence, this effect is termed as -M effect.
As said above, the transfer of electrons can also take place from a compound with lone pair of
electrons to an adjacent covalent bond. An example for this type of mesomeric effect is
C=C-C=C-NH2 H3C-CH=CH-CH=O
←--------------- ------------------→
Transfer of electrons Transfer of electrons
+ M effect
-M effect
Example of groups exerting the +M and -M effects:
+M effect: -Cl, -Br, -I, -NH2, -NR2, -OH, -OCH3.
-M effect: -NO2, C=O
From the above discussion, it can be seen that mesomeric effect seen in compounds is not
under the influence of the attacking compounds. Hence, this is also a permanent effect.
Comparison of inductive effect and mesomeric effect:
- Inductive effect is seen in saturated compounds. While the mesomeric effect is seen unsaturated compounds. Especially, those with conjugated bonds.
- Electrons of the sigma bond show inductive effect. In case of mesomeric effect, electrons of the pie bond participate.
- In case of inductive effect only partial displacement of the electrons takes place. But, in case of mesomeric effect complete displacement of the electrons takes place resulting in development full positive and negative charges.
- Inductive effect is transmitted only to a certain distance of a molecule. But, the mesomeric effect transfers throughout the length of the compound. However, there must be a conjugated double bond system throughout the molecule for the transfer of electrons.
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