In organic chemistry, the factor called resonance or mesomerism describes the delocalized electrons within certain molecules where one single Lewis structure does not express the bonds. An ion or molecule with these delocalized electrons can be represented by contributing several structures which are called resonance structures.
The concept of resonance effect tells about the polarity induced in a molecule by the reaction between a lone pair of electron and a pi bond. It also occurs by the interaction of 2 pi bonds in the adjacent atoms. Resonance in simple is the molecules with multiple Lewis structures. Resonance in chemistry helps in understanding the stability of a compound along with the energy states.
Definition: The resonance effect can be defined as a chemical phenomenon that is observed in the characteristic compounds having double bonds in the organic compounds. The organic compounds contain these double bonds in the structures and usually have the overlapping of the p-orbitals on the two adjacent sides of carbon atoms.
Resonance Effect Chemistry Questions with Solutions
Q1. Resonance effect involves-
a.) Delocalisation of σ electrons.
b.) Delocalisation of 𝜋-electrons.
c.) Partial displacement of electrons.
d.) Delocalisation of 𝜋 and σ electrons.
Correct Answer– (b.) Delocalisation of 𝜋-electrons.
Q2. Which of the following exhibit +R effect?
Correct Answer– (c.) –OH
Q3. Which of the following is the correct statement about resonance–
a.) It decreases the energy of the system.
b.) The hybridisation of atoms do not change due to resonance.
c.) Resonance hybrid is more stable than any resonating structure.
d.) Resonating structures cannot be isolated at any temperature.
Correct Answer– (c.) Resonance hybrid is more stable than any resonating structure.
Q4. Choose the incorrect statement about the resonance contributing structures of a resonance hybrid?
a.) Equivalent contributing structure make the resonance very important.
b.) Contributing structures contribute to the resonance hybrid in proportion of their energies.
c.) Contributing structures represent hypothetical molecules having no real existences.
d.) Contributing structures are less stable than the resonance hybrid.
Correct Answer– (b.) Contributing structures contribute to the resonance hybrid in proportion of their energies.
Q5. Which of the following compound does not have a resonant effect?
Correct Answer– (c.) C6H5NH3
Q6. State the Positive Resonance Effect.
Answer. A positive resonance effect occurs when the groups release electrons to the other molecules by the process of delocalization. The groups are usually denoted by +R or +M. In this process, the molecular electron density increases. For example- -OH, -SH, -OR,-SR.
Q7. State the Negative Resonance Effect.
Answer. A negative resonance effect occurs when the groups withdraw the electrons from other molecules by the process of delocalization. The groups are usually denoted by -R or -M. In this process, the molecular electron density is said to decrease. For example- -NO2, C=O, -COOH, -C≡N.
Q8. State True or False.
Aromatic rings do not have resonance structures.
Due to cycling double bonds, aromatic rings have resonance structure, and all aromatic rings must have resonance, but the converse is not required.
Q9. What causes the effect of resonance?
Answer. Resonance can occur through the interaction of two π-bonds or between a π-bond and a lone pair of electrons on an adjacent atom. This effect is caused by the delocalisation of π-electrons.
Q10. What are the conditions of resonance?
Answer. The conditions of resonance are as follows-
- All atoms participating in resonance must be sp or sp2 hybridised.
- The parallel p-orbitals overlap to each other.
- Molecule should have conjugated system (parallel p-orbitals system is called conjugate system).
Q11. State the difference between resonance effect and mesomeric effect.
Answer. The main difference between resonance and the mesomeric effect is that the resonance effect describes how a molecule’s lone electron pair and bond electron pair determine its chemical structure, whereas the mesomeric effect describes how a molecule’s chemical structure is stabilized by using a functional group.
Q12. Discuss how does the stability of resonance increases?
Answer. The stability of resonance increases with:
- The number of covalent bonds.
- The number of atoms that have an octet of electrons (except hydrogen).
- Separation of the opposite charges.
- Charge dispersal.
- A negative charge on a more electronegative atom, or a positive charge on a more electropositive atom, increases the atom’s stability.
Q13. Why is the effect of resonance more dominating than hyperconjugation?
Answer. Like resonance, hyperconjugation has a stabilising effect, but resonance has a stronger effect than hyperconjugation because only p-orbitals overlap in resonance, whereas σ molecular orbitals overlap with p-orbitals in hyperconjugation.
Q14. On the basis of resonance energy, compare between aromatic, anti aromatic and non-aromatic compounds.
Answer. On the basis of resonance energy, comparison between aromatic, anti aromatic and non-aromatic compounds can be done as–
- Aromatic compounds have a very high energy.
- Anti aromatic compounds have zero resonance energy.
- Non Aromatic compounds have approximately 4 – 8 kcal/mol resonance energy like alkenes.
Q15. Give the difference between Resonance effect and Inductive effect.
Answer. The difference between inductive effect and resonance effect is that inductive effect is used for describing the transmission of electrical charges between the atoms whereas resonance effect is used for describing the transmission of electron pairs between the atoms.
Practise Questions on Resonance Effect
Q1. In which of the following molecule resonance effect is possible?
a.) Vinyl carbocation
b.) Allyl carbocation
d.) Vinyl chloride
Q2. Which of the following molecules, does not have the mesomeric effect with the benzene nucleus?
Q3. Fill in the blank.
___ electrons are displaced in mesomeric effect.
Q4. State True or False.
Resonance forms are in equilibrium with each other.
Q5. Is resonance effect temporary or permanent?
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