Homolytic and Heterolytic Fission - UNSOLVED PRACTICE SET

Q7. Define homolytic and heterolytic fission. Show the difference using curved arrows (fishhook arrows for homolytic and double-headed arrows for heterolytic).

Q8. Why does the C–Cl bond in methyl chloride undergo heterolytic fission more readily than the C–C bond in ethane?

Q9. What are free radicals? Why are they highly reactive and short-lived?

Q10. Explain why homolytic fission is common in the halogenation of alkanes (like chlorination of methane), while heterolytic fission is common in the hydrolysis of alkyl halides.

Q11. Your teacher demonstrates the reaction of chlorine with methane in sunlight. Explain why sunlight is necessary and what type of bond fission initiates this reaction.

Q12. Draw the structures of a carbocation, a carbanion, and a free radical formed from the same alkyl group (e.g., from CH₃–X). How do their stabilities compare?

Q13. Discuss homolytic and heterolytic fission of covalent bonds in detail:

(a) Definition and mechanism of homolytic fission with examples

(b) Definition and mechanism of heterolytic fission with examples

(c) Factors favouring each type of fission (bond polarity, solvent, energy source)

(d) The reactive intermediates formed: free radicals, carbocations, and carbanions

(e) Show the fission of C–Cl bond in CH₃Cl by both mechanisms using curved arrows

Q14. Compare the reactive intermediates formed by homolytic and heterolytic fission:

(a) Free radicals — formation, structure, stability, and reactivity

(b) Carbocations — formation, structure, stability order, and reactivity

(c) Carbanions — formation, structure, stability order, and reactivity

(d) How the nature of the intermediate determines the reaction pathway (substitution, addition, elimination)

Q15. Understanding bond fission is crucial in industrial chemistry and environmental science. Discuss:

(a) How homolytic fission initiates the free radical chain reaction in the combustion of fuels

(b) Why heterolytic fission is important in the synthesis of pharmaceuticals in Indian drug industries

(c) The role of free radicals in atmospheric chemistry (ozone depletion, smog formation in Delhi)

(d) How antioxidants work by scavenging free radicals in the human body

Q16. The bond dissociation energies for various bonds are:

C–C: 347 kJ/mol

C–H: 413 kJ/mol

C–Cl: 339 kJ/mol

Cl–Cl: 243 kJ/mol

(a) Identify which bond is most likely to undergo homolytic fission when heated and explain why.

(b) Calculate the energy required to break 1 mole of Cl–Cl bonds homolytically.

(c) If UV light of wavelength 400 nm is used, calculate whether it has sufficient energy to break the Cl–Cl bond. (Use E = hc/λ, h = 6.626 × 10⁻³⁴ J·s, c = 3 × 10⁸ m/s)

Q17. In the chlorination of methane, the following bond dissociation energies are relevant:

CH₃–H: 439 kJ/mol

CH₃–Cl: 339 kJ/mol

H–Cl: 431 kJ/mol

Cl–Cl: 243 kJ/mol

(a) Calculate ΔH for the first step: Cl–Cl → 2Cl· (homolytic fission).

(b) Calculate ΔH for the propagation step: CH₄ + Cl· → CH₃· + HCl.

(c) Determine whether the overall reaction is exothermic or endothermic.

Q18. A pharmaceutical synthesis involves the heterolytic fission of a C–Br bond in a polar solvent.

(a) Draw the two possible products of heterolytic fission and identify which is more likely to form.

(b) If the bond dissociation energy is 285 kJ/mol and the ionic bond energy in the solvent-stabilised ions is 310 kJ/mol, calculate whether heterolytic fission is energetically favourable.

(c) Explain how a polar protic solvent (like water) would stabilise the ions formed compared to a non-polar solvent.