Chapter 1:- Structure and Reactivity 1st year Book
Organic Chemistry
(Page 7)


Sometimes more than one Lewis structure can be written for a molecule, especially if the molecule contains double or triple bonds. A simple example is ozone for which we can write the following Lowis structure :
ozone-struture-by-lewis-structure-method-resonance, ozone

This Lewis formula for ozone, however, is inconsistent with the experimentally determined mixed structure. On the basis of this structure, we would expect ozone to have two different oxygen-oxygen bone lengths, one oxygen-oxygen single bond distance of 147pm (as in HO−OH ) and the other similar to the 121pm oxygen-oxygen, double bond (as in O2 ). But in fact, both distances have been found to be the same (128pm). It is somewhat shorter than a single bond and somewhat longer than a typical oxygen-oxygen, double bond. The structure of ozone requires that the central oxygen must be identically bonded to both terminal oxygens. To deal with this circumstance such as the bonding in ozone, yet retain the Lewis formula as a useful tool for representing molecular structure, a motion resonance was developed.

According to the resonance concept when two or more Lewis structures that differ only in the distribution of electrons, can be written for a molecule no single Lewis structure is sufficient to describe its true electron distribution. The true structure is said to be the resonance hybrid of various Lewis formulae called ‘contributing structures’ or ‘resonance structures’ or ‘canonical formulae’ that can be written for the molecule. In the case of ozone, the Lewis formulae are equivalent and contribute equally to the resonance hybrid, or an average of different Lewis structures is sometimes drawn using a dashed line to represent a partial bond.
Resonance structures of ozone, Resonance hybrid

Thus, the representation of a real structure as a weighted average of two or more canonical forms is called “resonance”. Benzene has the following canonical forms contributing to the real structures.

Resonance in benzene

I and II are Kekule structures III, IV, and V are Dewar’s structures. The hybrid structure is usually represented as structure VI.

Illustration: To illustrate resonance, consider a carbonyl group (>C=0), whose all of the properties can neither satisfactorily represented by the classical formula I nor by the extreme polar structure II. It is, therefore, conceived that the true structure of >C=O group is an intermediate (or resonance hybrid) of structures I and II.

Apolar form, polar form, hybrid resonance

The resonance hybrid may best be represented by structure III in which π-electrons are drawn preferentially towards oxygen rather than carbon. If >C=O group is conjugated with the C=C type of system then the polarisation is transmitted further via the π-electrons.

To indicate the two forms contribute a resonance hybrid, a double-headed arrow of the type, (↔) is placed between the two forms. By way of analogy, a mule is a hybrid of a horse and a donkey and these are best represented as: “mule = horse  donkey” but never “mule = horse = donkey”.

crotonaldehyde resonance hybrid

Necessary Conditions for Resonance:

(1) The major contributing forms are of comparable energy.
(2) The nuclei in each of the canonical structures (or polar structures or resonance structures) must be in the same relative positions.
(3) The various resonating structures differ only in electron distribution and are obtained by a slight displacement of electrons.
(4) All the resonating structures in a set must have the same number of paired and unpaired electrons; although their positions may be different.
(5) Structures involving formal charges are the most stable when the negative and positive charges reside on the most electronegative and electropositive atoms respectively. Thus, in ketones, the ionic structure (II) is more stable than the III.

structure of electronegative, electropositive resonance

(6) More the number of covalent bonds in resonating structure more will be its stability.
(7) Most important and essential requirement for resonance is that the resonating structure must be planar or nearly planar. Thus, if planarity is reduced, resonance is diminished or inhibited.
Importance :
(1) The phenomenon of resonance explains the abnormal bond length between C−C,C=C,C=O, etc. in the compounds exhibiting resonance.
(2) The concept of resonance explains clearly the acidic character of carboxylic acids and the basic character of Lewis bases.

For example, Resonance explains why alcohol is neutral and carboxylic acids are strong acids.

The ionization of the molecules in the two cases is as follows :

alcohol resonance
The carboxylate anion obtained in the above reaction can undergo the phenomenon of resonance while the alkoxide ion does not. Moreover, the resonance stabilization is more in carboxylate ions as compared to the parent acid, this causes, carboxylic acids are stronger acids and alcohols to be neutral.
(3) The addition reaction of conjugated dienes is well explained by resonance. For example, the 1,2 and 1,4 addition of bromine on 1,3-butadiene is due to the resonance effect.
1,3-butadiene, 3,4-dibromobut-1-ene
Like the inductive effect, this effect is of two types +R and −R. It is +R when the transference of the electrons pair is away from the atom and −R when the transference of the electrons pair is towards the atom. In general :
+R effect, -R effect
Spread The Love