RESONANCE AND IT'S APPLICATION

Rohini Narwal semester 1

1. Samrat prithviraj chauhan government college Ajmer Resonance and its applications Submitted by Rohini Narwal M.Sc. Chemistry Semester 1 st 2020-2021 Department Of Chemistry

2. Table of content

3. “What in a Name” Said Shakespeare What will be the name? Say Organic Chemists Resonance and mesomerism are terms used for explaining the delocalization of electrons in a conjugated system, organic chemists used the two terms interchangeably. The study of delocalization of electron based on quantum mechanical approach led to introduction of the term resonance [HEISENBERG 1926,PAULING] The study of delocalization of electrons as explained by overlap of p-orbital of conjugated system led to the introduction of term mesomerism [INGOLD,1934]

4. WHAT IS RESONANCE? The representation of a real structure as weighted average of two or more canonical forms is called resonance . Two or more structures of the same molecule or an ion with same geometry with the same number of paired electrons but differing in the pairing arrangement of these electrons are termed as Resonance Structures . The actual structures of the molecules is not represented by anyone of the resonance structure but is considered to be hybrid of all these structure s is called Resonance Hybrid .

5. Resonating structure of benzene Kekule structures 39% contributor (F.A. Kekule) Dewar structures 7.3% contributor (J. Dewar) Resonance hybrid Delocalised 6p-orbitals

6. Rules of Resonance All the atoms taking part in resonance i.e. the atoms that are a part of delocalised system must lie in a plane or be nearly planar. Only pi electrons movement are allowed. The number of unpaired electrons in each of the structure must be same. All resonance structures do not contribute equally to hybrid. All canonical structures must be written according to the Lewis method showing bonds, lone pair of electron and formal charges where required. Cyclopropene cation Picryl iodide

7. Stabilities of Resonance structures Structures with greater number of covalent bonds contribute more towards the resonance hybrid. Resonance structures involving charge separation are less stable than non-polar structures. Structure with a maximum of octets are preferred. Structures with isolated unlike charges have less contribution than those in which they are close to each other. Major contributor Minor contributor Major contribution Minor contributor Major contribution Minor contributor Major contributor Minor contributor Phenol 1,3-butadiene Carboxylic acid diazomethane

8. Resonance energy Resonance energy is the difference between the actual energy of a molecule(i.e. Resonance hybrid) and the energy calculated for the most stable of the resonating structure i.e. having minimum energy among the resonating structures. Resonance energy of benzene

9. Resonance effect The generation of centres of low and high electron density in conjugated system as a result of resonance or mesomerism is called resonance effect(R-effect)or mesomeric effect (M-effect). TYPES OF R(or M) EFFECT 1.Electron –withdrawing resonance effect(-R effect) 2.Electron-donating resonance effect(+R effect) Electron Donating Groups Electron withdrawing groups

10. Applications of resonance effect Low reactivity of vinyl and aryl halides towards nucleophilic substitution High reactivity of allyl and benzyl halides Due to the resonance the carbon chlorine bond acquires some double bond character, thus binding halogen to carbon atom cannot be easily displaced in a substitution reaction. Both allyl and benzyl halides undergo ionization to give resonance stabilized allyl and benzyl cations .

11. Comparison of relative base strength of Ethylamine and Acetamide Difference in C-N bond length for o and p-nitro groups in Picryl iodide Ethylamine strong base Acetamide weaker base The large iodide atom forces o-nitro groups out of the plane of benzene ring and has no effect on p-nitro group which remains in the plane of the ring. Therefore ,resonance operates in benzene ring involving p-nitro group only(a=145pm,b=135).

12. Effect on the dipole moment Resonance may increase or decrease the magnitude of this resultant dipole moment depending upon the nature of the polar bonds present in the molecule . 1)Lowering of dipole moment 2)Increasing the dipole moment Dipole moment of vinyl chloride is much lower than ethyl chloride Dipole moment of p-nitro aniline is higher than that of nitrobenzene

13. References Organic Chemistry by S.M.MUKHERJI Organic Reactions and Their Mechanism by P.S.KALSI Organic chemistry by ANUP PATHAK ANUPA SAHA Chemistry by JERRY MARCH Organic chemistry by BHUPINDER MEHTA MANJU MEHTA Books Name:

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