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Class 11  Β»  Chemistry  Β»  Organic Chemistry Basic Principles

Resonance and Mesomeric Effect - UNSOLVED PRACTICE SET

Class 11

Chapter: Organic Chemistry Basic Principles | Topic: Resonance and Mesomeric Effect

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Class 11

RESONANCE AND MESOMERIC EFFECT - UNSOLVED PRACTICE SET

Topic: Resonance and Mesomeric Effect

Time: 40 mins | Marks: 30 | Difficulty: Medium

Multiple Choice Questions

Q1. Resonance involves the delocalisation of:

  1. Only Οƒ electrons
  2. Only Ο€ electrons or lone pairs
  3. Only bonding electrons
  4. All electrons equally

Q2. Which of the following molecules does NOT exhibit resonance?

  1. Benzene
  2. Ethene
  3. Carbon dioxide
  4. Methane

Q3. The mesomeric effect (+M or –M) is:

  1. A permanent effect operating through Οƒ bonds
  2. A permanent effect operating through Ο€ bonds or lone pairs
  3. A temporary effect operating only during reactions
  4. Observed only in saturated compounds

Q4. A group that donates electrons through resonance (+M effect) is:

  1. –NOβ‚‚
  2. –CN
  3. –OH
  4. –COOH

Q5. The resonance structures of benzene show that:

  1. Benzene has alternating single and double bonds
  2. All C–C bonds in benzene are identical with bond length intermediate between single and double bonds
  3. Benzene has three single bonds and three double bonds
  4. Benzene is a cyclohexatriene

Q6. Which of the following is the most stable resonance structure of the phenoxide ion (C₆Hβ‚…O⁻)?

  1. One with negative charge on the carbon atom
  2. One with negative charge on the oxygen atom
  3. One with positive charge on the oxygen atom
  4. One with no charge separation

Short Answer Questions

Q7. What is resonance? Explain with the example of the carbonate ion (CO₃²⁻). How many equivalent resonance structures does it have?

Q8. Differentiate between the inductive effect and the mesomeric effect with respect to:

(a) Nature of electrons involved

(b) Distance of operation

(c) Permanence

Q9. Draw the resonance structures of:

(a) Ozone (O₃)

(b) Nitrate ion (NO₃⁻)

Q10. Explain why the C–O bond length in phenol is shorter than in methanol, using the concept of resonance.

Q11. Your teacher shows you that aniline is more basic than nitrobenzene but less basic than cyclohexylamine. Explain these observations using the mesomeric effect.

Q12. Why is the carboxylate ion (RCOO⁻) more stable than the carboxylic acid (RCOOH)? Explain using resonance.

Long Answer Questions

Q13. Discuss resonance in detail:

(a) Definition and conditions for resonance

(b) Rules for drawing resonance structures

(c) Resonance hybrid and resonance energy

(d) Resonance in benzene, carbonate ion, and nitrate ion

(e) Difference between resonance and tautomerism

Q14. Explain the mesomeric effect (+M and –M effects) with examples:

(a) +M effect groups (–OH, –OR, –NHβ‚‚, –Cl) β€” how they donate electrons through resonance

(b) –M effect groups (–NOβ‚‚, –CN, –COOH, –CHO) β€” how they withdraw electrons through resonance

(c) How the mesomeric effect influences the reactivity of aromatic compounds (electrophilic substitution)

(d) The combined effect of inductive and mesomeric effects in substituted benzenes

Q15. Resonance and the mesomeric effect are central to understanding colour, stability, and reactivity in organic molecules. Discuss:

(a) Why azo dyes (used in Indian textiles) are coloured due to extended resonance

(b) How resonance explains the extra stability of benzene (resonance energy = 150.4 kJ/mol)

(c) Why paracetamol (a common medicine in India) is less toxic than other analgesics due to resonance-stabilised metabolism

(d) The role of resonance in the design of conductive polymers for flexible electronics

Numerical / Application-Based Problems

Q16. The resonance energy of benzene is 150.4 kJ/mol. The experimental heat of hydrogenation of benzene is -208 kJ/mol, while the calculated heat of hydrogenation for cyclohexatriene (hypothetical) is -358.4 kJ/mol.

(a) Calculate the difference between the calculated and experimental heat of hydrogenation.

(b) Show that this difference equals the resonance energy of benzene.

(c) Explain what this value tells us about the stability of benzene compared to a hypothetical cyclohexatriene.

Q17. The bond lengths in benzene are all 139 pm, while a typical C–C single bond is 154 pm and a C=C double bond is 134 pm.

(a) Calculate the average of the single and double bond lengths.

(b) Compare this average with the observed bond length in benzene.

(c) Explain why the bond length in benzene is closer to a double bond than a single bond, despite having resonance.

Q18. A pharmaceutical company in Ahmedabad is developing a new dye for Indian textiles. The dye molecule has the following resonance structures:

Structure I: Positive charge on N, negative charge on O

Structure II: Neutral molecule

Structure III: Negative charge on N, positive charge on O

(a) Identify which structure contributes most to the resonance hybrid and explain why.

(b) If Structure I has an energy of 200 kJ/mol, Structure II has 180 kJ/mol, and Structure III has 220 kJ/mol, predict the relative contribution of each structure to the hybrid.

(c) Explain how extended resonance in such dye molecules affects their colour (absorption of visible light).


Total: 30 Marks | Time: 40 mins

Explore more topics in Organic Chemistry Basic Principles

Classification of Organic Compounds Nomenclature IUPAC Rules Isomerism Structural and Stereoisomerism Hybridisation in Organic Molecules Inductive Effect Electromeric Effect Hyperconjugation Homolytic and Heterolytic Fission Types of Reagents Electrophile Nucleophile Types of Reactions Substitution Addition Elimination