Resonance
Resonance or Mesomerism
Let us begin this section with an example of benzene :
The formula of benzene is C6H6 and its structure has alternating C−C single and C=C double bonds. Benzene can be represented by the following two structures :
Resonance structures of Benzene
The actual bond lengths of C-C single and C=C double bonds are 1.54 Å and 1.34 Å respectively. However, all the six carbon-carbon bond lengths in benzene are equal (1.39 Å). This implies that any two adjacent carbon atoms in benzene are neither joined by a pure single bond nor by a pure double bond. This leads to the following conclusion :
The behaviour of some molecules cannot be explained by a single Lewis structure. In such cases the molecule can be represented by two or more electronic structures one of which can explain most of the properties but none of them can explain all the properties of the molecule. The real structure of the molecule is an approximate intermediate between all these electronic structures. Such molecules are said to exhibit resonance or mesomerism. The various Lewis structures of these molecules are known as resonance structures or canonical structures and the real structure is called resonance hybrid.
Resonance hybrid of benzene
Actual molecule of benzene is usually represented by this structure.
Types of Resonance Effect
There are two types of resonance effects :
- Positive resonance effect (+R Effect or +M Effect)
- Negative resonance effect (-R Effect or -M Effect)
Positive Resonance Effect
In positive resonance, the transfer of electrons is away from an atom or substituent group attached to the conjugated system ( conjugated systems contain alternate σ and π bonds ).
Negative Resonance Effect
In negative resonance, the transfer of electrons is towards the atom or substituent group attached to the conjugated system.
Example : CH2CHCN
Transfer of electron towards CN
Conditions For Resonance
- The molecule can be represented by more than one correct lewis structures.
- All resonance structures should differ only in the positions of electrons and not in the positions of atoms or nuclei. Study the
following example to understand what that means :
Question : Is the following pair of structures constitutes resonance structures?
Answer : Since they differ in the position of atoms, they are not resonance structures. They are, in fact, tautomers.
- All structures should have equal number of valance and unpaired electrons.
Major and Minor Contributors in Resonance
Of all resonance structures, the structure which is more stable resembles the actual molecule more than the others. Stability of resonance structures can be understood with the help of the following points :
- Identical resonance structures have equal energy; therefore, they contribute equally towards the resonance hybrid.
- Structure that has maximum number of covalent bonds is the major contributor.
- Structures that involve separation of positive and negative charge are less stable because of their higher energy.
- When atoms of different electronegativity are involved, structures with negative charge on more electronegative atoms and positive
charge on more positive atoms contribute more towards resonance hybrid. Hence, the stability, and thus contribution,
of the above resonance structures follows the order :
- Structures that involve distribution (delocalisation) of positive charge are significant contributors regardless of the fact whether they are
electronegative or electropositive.
true/false
The following pair of structures is an example of resonance :
true/false
The following pair of structures is an example of resonance :
Which one of the following resonance structures is the most stable and which one is the least?