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Understanding Resonance: Theory & Examples

Explore the concept of resonance in chemistry, including definitions, hybrid structures, characteristics, and necessary conditions. Learn through examples like Benzene and delve into resonance energy. Enhance your understanding with valuable insights.

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Understanding Resonance: Theory & Examples

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  1. Concept of Resonance By K. A. R. S. S. PRASAD Lecturer in Chemistry Dr. V. S. Krishna Govt. Degree & PG College(A) Visakhapatnam

  2. CONTENTS • INTRODUCTION • DEFINITION OF RESONANCE • DEFINITION OF RESONANCE HYBRID • EXAMPLES

  3. CHARACTERISTIC FEATURES OF RESONANCE HYBRIDNECESSARY CONDITIONS FOR RESONANCERESONANCE ENERGY

  4. INTRODUCTIONA single structural formula of certain compounds cannot explain all its properties.So two or more structures are necessary to explain all its properties.Each structure would explain some properties of the molecule. All the structure put together explain all the properties of the molecule.

  5. DEFINITION OF RESONANCEThe phenomenon in which two or more structures can be written for a molecule but none of them can be said to represent it uniquely or completely is called resonance.The various possible structures are known as resonating structures or canonical structures

  6. For example take Benzene C6H6, it has different structures as shown below:Kekulae Structures

  7. KEKULE STRUCTURE

  8. Dewar structures

  9. Definition of Resonance HybridThe true structure of the molecule is a hypothetical structure said to be resonance hybrid of the various possible alternating structures

  10. The resonance hybrid may be defined as the actual structure of all the different possible structures that can be written for the molecules without changing the relative positions of atoms

  11. Characteristics of Resonance Hybrid More stableBond lengths different from those in the canonical formsEx: Bond length in Benzene resonance hybrid is 1.39Ao Bond length in Benzene canonical form is 1.54Ao and 1.34 AoC-C bond length 1.39 Ao (Resonance hybrid)

  12. Conditions for resonanceAll resonance structures have the same or nearly same atomic nucleiAll resonating structures have the same no of paired and unpaired electronsResonating structures must be planarNon polar structures are more stable than dipolar structures

  13. Stability decreases if there are isolated charges or increase in charge separation.If all structures have formal charges the most stable one is that in which the negative and positive charges reside on the most electronegative and electropositive atoms respectively

  14. Resonance structure with electron deficient positively charges atom has very high energy and unstableMore the number of covalent bonds in a resonance structure more is its stabilityIf the molecule has more number of resonance structures it is more stable

  15. Example:H2 C=CH-Cl : H2C- –CH=Cl+: H2C+-CH=Cl- (a) (b) (c)Stability order a > b > c(a) has no formal charges

  16. Resonance EnergyThe difference in energy between the resonance hybrid and the most stable canonical form is called resonance energyResonance energy of benzene is 36Kcal/moleLarger the number of canonical forms , greater will be the resonance energyIf resonance energy is more stability is more.

  17. Thank YouByK A R S S Prasad

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