RESONANCE-Definition, Structures, Conditions, Conjugation, Types of Conjugation, Practice Problems & FAQs

A teacher asked a group of students, "Have you seen rhinos?"

Then the teacher shows the image of the rhinoceros to the kids for the first time. When the kid is at home, he tries to describe to his family about the rhinoceros and he says that the animal was a combination of a unicorn (a fairytale horse with one horn above its nose) and a dinosaur. So, we can say that these two animals carry the distinctive features of the rhino, so the rhino can be considered a hybrid of the two. The most important thing here is that neither the unicorn nor the dinosaur exists in real life but the rhino does; resonance structures do not exist but are merely used to describe the actual molecule- the resonance hybrid which exists in nature.

The same exists for resonance, where more than one structure is used to show the all properties of a molecule.

Whenever we hear the term resonance, a picture of benzene comes to mind, which has alternate pi bonds in conjugation. This often leads to a misconception among students that Kekule, who gave the structure of benzene, also discovered resonance.

In organic chemistry, the theory of resonance was broadly formulated from the period of 1927–to 1933 by using the quantum mechanical theory of singlet-triplet states of helium atoms, which was developed by Heisenberg in 1926. The theory contains lots of misunderstandings or incompleteness of knowledge.

Then a detailed theory on resonance was developed by Linus Pauling.

Table of content:

Resonance
Resonating structures
Resonance hybrid
Conditions for resonance
Rules for writing resonating structures
Conjugation
Different types of conjugation
Recommended Video
Practice Problems
Frequently asked questions

Resonance

Resonance is a hypothetical concept in which several different structures of any molecule are used to describe bonding and properties of that molecule instead of one Lewis structure.

Resonating structures

For a molecule, various Lewis structures can be drawn collectively; all the structures are known as resonating structures that describe the delocalization of electrons in a molecule.

Example: Resonating structures of CO₃^-2

Resonance hybrid

It is a hybrid structure of all the resonating structures possible for a molecule which explains all the properties of a molecule. It is the most stable and real structure and has the lowest energy.
Resonance hybrid of Carbonate ion.

Conditions for resonance

1. The delocalization of high-energy electrons( electrons or lone pair of electrons) takes place.
2. The atoms which are involved in conjugation must be in the same plane.
3. There should be at least three continuous parallel p-orbitals(or d-orbitals) on adjacent atoms.
4. The parallel p-orbitals or d-orbitals can be half-filled, fully filled, or vacant.

Rules for drawing resonating structures

An arrow is always drawn from the high-energy electrons( lone pair or the negative charge or electrons) towards the conjugated system or to the -X = Y (-C = O, -N = O) bond or the vacant p-orbital or d-orbital.
Position of atoms should not be changed among the resonating structures, which is not allowed in resonance.

Rules to compare the stability of resonating structures

1. The resonance hybrid should be more stable than any of its canonical forms.

2. The structure having more covalent bonds is a more stable resonating structure.

3. Neutral resonating structures are more stable than charged resonating structures.

4. Charged resonating structures with complete octets are more stable than charged resonating structures with an incomplete octet.

5. A structure having a -ve charge on more electronegative atoms and a +ve charge on less electronegative atoms is more stable.

6. Stability increases by the separation of like charges and decreases by the separation of unlike charges.

Conjugation

When an atom with a positive charge, multiple bond, negative charge, vacant p-orbital, or lone pair is bound to an unsaturated system, it exhibits conjugation, which results in simultaneous overlapping of p-orbitals with multiple parallelly placed p-orbitals.

Different types of Conjugation

1. bond- bond conjugation

Here, the conjugation occurs between two or more two alternate pi bonds.
In the given organic molecule, conjugation takes place between two C=C bonds.

2. bond - vacant orbital conjugation

It happens when a pi bond is present adjacent to the atom having a vacant orbital. The pi bond-positive charge conjugation is seen in the case of propenyl carbocation shown in the figure given below.

3. bond - unshared pair of electrons(lone pair) conjugation

In the given organic molecule, conjugation takes place between the C=C bond and the lone
pair on the nitrogen atom of the amino group present adjacent to the C=C bond.

4. bond - free radical conjugation

Here, the conjugation is between the pi bond present adjacent to the atom that has an odd electron (free radical).
In the given organic molecule, conjugation takes place between the C=C bond and a single
electron on the carbon atom adjacent to the C=C bond.

5. Lone pair - vacant orbital conjugation

Conjugation happens between a lone pair and a vacant orbital (C with a positive charge has one empty p-orbital).

6. bond -ve charge conjugation

A pi bond present adjacent to the atom that has a negative charge shows conjugation. The pi bond-negative charge conjugation is seen in the case of propenyl carbanion shown in the figure given below.

Practice Problems

Q 1. Which of the following is not a pair of resonating structures?

a. I and II
b. I and IV
c. I and III
d. II and IV

Answer: B)

Position of atoms is changed in pairs I and IV, which is not allowed in resonance.

Q 2. The correct order of stability of the resonating structure is :

a. II > I
b. I > II
c. I II
d. Cannot comment

Answer: B)

The structure having more covalent bonds is a more stable resonating structure.

The structure I is more stable than structure II because Structure I has more covalent bonds as well as no charge as compared to structure II.

Q 3. Which statement is correct regarding the stability of resonance structures?

a. Structure B is less stable than structure A
b. Structure B is more stable than structure A
c. Both structures are equally stable
d. Stability cannot be compared

Answer: A)

If the resonating structure is polar, Check for the separation of charges.

Unlike charges, separation increases the stability but separation of like charges decreases the stability.

Structure B is less stable than Structure A, In structure A, unlike charges, are closer to each other whereas unlike charges are farther in Structure B.

So, Structure B is less stable than structure A.

Q 4. Which one among the following resonating structures is not correct?

Answer: A)

Since all the given structures are polar, but like charges are closer to each other in structure A, so it is not correct.

Frequently asked questions

Q 1. What is the need for resonance?
Answer: When one structure cannot explain all the properties of a compound, then we introduce the concept of resonance to explain all the properties by a single structure(resonance hybrid) which is a combination of all the resonating structures.

Q 2. What are the different applications of resonance?
Answer: There are various applications of resonance. It helps to determine the stability of organic compounds, and intermediates, to explains acidic strength, basic strength, and rate of reaction.

Q 3. Do resonating structures maintain equilibrium with one another?
Answer: No, resonating structures do not maintain equilibrium but remain in hybrid form.

Q 4. What is the difference between the resonance effect and the mesomeric effect?
Answer: The resonance effect is the delocalization of electrons in a molecule but the mesomeric effect is the electron-donating or withdrawing nature of a substituent due to resonance.

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