Preparation and Reactions of Haloarenes - UNSOLVED PRACTICE SET

Q7. Why is chlorobenzene less reactive than alkyl halides towards nucleophilic substitution? Explain using resonance structures.

Q8. Write the reaction for the conversion of chlorobenzene to phenol. What conditions are required?

Q9. What is the Wurtz-Fittig reaction? Write the reaction of chlorobenzene with ethyl bromide in the presence of sodium.

Q10. Explain why p-nitrochlorobenzene is more reactive than chlorobenzene towards nucleophilic substitution.

Q11. Write the reaction of chlorobenzene with Mg in dry ether. What is the product called and what is its use?

Q12. Why does chlorobenzene undergo electrophilic substitution at the ortho and para positions rather than meta?

Q13. (a) Describe the preparation of chlorobenzene from benzene.

(b) Explain why chlorobenzene is less reactive than alkyl halides towards nucleophilic substitution. Discuss:

(i) Resonance in chlorobenzene

(ii) The nature of the C–Cl bond

(iii) The stability of the phenyl cation

(c) Under what conditions can chlorobenzene undergo nucleophilic substitution?

Q14. (a) Describe the following reactions of chlorobenzene:

(i) Conversion to phenol (Dow process)

(ii) Conversion to aniline

(iii) Wurtz-Fittig reaction

(iv) Fittig reaction

(b) Write the mechanism of nucleophilic substitution in p-nitrochlorobenzene.

(c) Why does the presence of -NO₂ group at ortho or para position activate chlorobenzene towards nucleophilic substitution?

Q15. (a) Compare the reactivity of chlorobenzene, benzyl chloride, and alkyl chloride towards nucleophilic substitution. Explain the differences.

(b) Arrange the following in order of increasing reactivity towards nucleophilic substitution:

C₆H₅Cl, C₆H₅CH₂Cl, p-NO₂C₆H₄Cl, C₆H₅CHClCH₃

Give reasons for your arrangement.

Q16. A student prepares chlorobenzene from benzene and analyses the product.

(a) If 39 g of benzene (C₆H₆, molar mass = 78 g/mol) is used and the theoretical yield of chlorobenzene is 100%, calculate the mass of Cl₂ required. (Molar mass of Cl₂ = 71 g/mol)

(b) In practice, the student obtains 55 g of chlorobenzene (C₆H₅Cl, molar mass = 112.5 g/mol). Calculate the percentage yield.

(c) The crude product contains unreacted benzene, dichlorobenzene, and HCl. Describe how you would separate and purify chlorobenzene from this mixture.

(d) If the student wants to convert all the chlorobenzene to phenol, how many grams of NaOH are theoretically required? (Molar mass of NaOH = 40 g/mol)

(e) The Dow process uses 10% NaOH at 300°C and 150 atm. Why are such harsh conditions required? What does this tell you about the C–Cl bond in chlorobenzene?

Q17. The following reactions involve chlorobenzene and its derivatives:

Reaction 1: C₆H₅Cl + NaOH (aq, 300°C, 150 atm) → C₆H₅OH

Reaction 2: C₆H₅Cl + 2Na + C₂H₅Br → C₆H₅C₂H₅ + NaCl + NaBr

Reaction 3: p-NO₂C₆H₄Cl + NaOH (aq, 100°C) → p-NO₂C₆H₄OH

Reaction 4: C₆H₅Cl + Mg → C₆H₅MgCl

(a) Name each reaction.

(b) Compare the conditions of Reaction 1 and Reaction 3. Why is Reaction 3 much milder?

(c) In Reaction 2, if 11.25 g of chlorobenzene is used with excess ethyl bromide and sodium, calculate the theoretical yield of ethylbenzene. (Molar masses: C₆H₅Cl = 112.5, C₆H₅C₂H₅ = 106 g/mol)

(d) In Reaction 4, the Grignard reagent formed is treated with CO₂ followed by acid hydrolysis. What is the final product? Write the reactions.

(e) A student suggests that chlorobenzene can be converted to toluene by reacting with CH₃Cl and AlCl₃. Will this work? Explain why or why not.

Q18. In a school project on "Aromatic Compounds in Daily Life," students investigate haloarenes.

(a) A student finds that her grandmother's old mosquito repellent contains DDT (dichlorodiphenyltrichloroethane). Draw the structure of DDT. Why has DDT been banned in many countries?

(b) Another student learns that the antibacterial drug chloramphenicol contains a nitro group and a dichloroacetyl group attached to a benzene ring. Why is the C–Cl bond in chloramphenicol more reactive than in chlorobenzene?

(c) A third student discovers that PCBs (polychlorinated biphenyls) were once used in electrical transformers. Why were they banned? What property made them useful initially?

(d) The students test the reactivity of chlorobenzene, benzyl chloride, and p-nitrochlorobenzene with aqueous NaOH. Only one reacts readily at room temperature. Identify it and explain why the others do not react.

(e) The teacher asks: "If you were designing a new insecticide that is biodegradable and non-toxic to humans, what structural features would you include? What would you avoid?" Give your answer with reasoning.18. In a school project on "Aromatic Compounds in Daily Life," students investigate haloarenes.

(a) A student finds that her grandmother's old mosquito repellent contains DDT (dichlorodiphenyltrichloroethane). Draw the structure of DDT. Why has DDT been banned in many countries?

(b) Another student learns that the antibacterial drug chloramphenicol contains a nitro group and a dichloroacetyl group attached to a benzene ring. Why is the C–Cl bond in chloramphenicol more reactive than in chlorobenzene?

(c) A third student discovers that PCBs (polychlorinated biphenyls) were once used in electrical transformers. Why were they banned? What property made them useful initially?

(d) The students test the reactivity of chlorobenzene, benzyl chloride, and p-nitrochlorobenzene with aqueous NaOH. Only one reacts readily at room temperature. Identify it and explain why the others do not react.

(e) The teacher asks: "If you were designing a new insecticide that is biodegradable and non-toxic to humans, what structural features would you include? What would you avoid?" Give your answer with reasoning.