Preparation of Haloalkanes - UNSOLVED PRACTICE SET

Q7. Write the reaction for the preparation of chlorobenzene from benzene diazonium chloride. Name the reaction.

Q8. Why is the preparation of iodoalkanes from alcohols using HI preferred over other methods? Write the reaction.

Q9. What is the Finkelstein reaction? Write the reaction showing the conversion of ethyl chloride to ethyl iodide.

Q10. Explain why the reaction of tertiary alcohol with HCl occurs readily at room temperature, while primary alcohol requires anhydrous ZnCl₂.

Q11. Write the mechanism for the addition of HBr to propene in the presence of peroxide (anti-Markovnikov addition).

Q12. Your school lab has a bottle of 1-propanol. How would you convert it to 1-bromopropane? Write the reaction and name the reagent.

Q13. (a) Describe the preparation of haloalkanes from alcohols using:

(i) Hydrogen halides (HX)

(ii) Phosphorus halides (PCl₅, PBr₃)

(iii) Thionyl chloride (SOCl₂)

Write the reactions and mention the advantages of each method.

Q14. (a) Describe the preparation of haloalkanes from alkenes by:

(i) Addition of hydrogen halides (Markovnikov and anti-Markovnikov addition)

(ii) Addition of halogens

(b) Explain the mechanism of addition of HBr to propene:

(i) In the absence of peroxide

(ii) In the presence of peroxide

(c) Why does HCl not follow the peroxide effect?

Q15. (a) Describe the preparation of haloalkanes from alkanes by free radical halogenation.

(b) Why is this method not suitable for preparing pure monohalogenated products?

(c) Write the mechanism of chlorination of methane.

(d) Explain why fluorination is too vigorous and iodination is too slow for practical preparation.

Q16. A student wants to prepare 1-bromobutane from 1-butanol.

(a) Write three different methods for this conversion, naming the reagents used in each.

(b) Which method would give the highest yield? Explain why.

(c) If the student starts with 14.8 g of 1-butanol (molar mass = 74 g/mol) and obtains 20.5 g of 1-bromobutane (molar mass = 137 g/mol), calculate the percentage yield.

(d) What are the likely by-products in each method? How can they be removed?

(e) Why is SOCl₂ preferred over PBr₅ for converting 1-butanol to 1-bromobutane in the laboratory?

Q17. The following reactions are used to prepare haloalkanes:

Reaction 1: CH₃CH₂OH + HBr → CH₃CH₂Br + H₂O (with H₂SO₄)

Reaction 2: CH₃CH₂OH + SOCl₂ → CH₃CH₂Cl + SO₂ + HCl

Reaction 3: CH₂=CH₂ + HBr → CH₃CH₂Br (peroxide present)

Reaction 4: CH₄ + Cl₂ → CH₃Cl + HCl (hv)

(a) Classify each reaction by type (substitution, addition, free radical).

(b) For Reaction 1, explain why concentrated H₂SO₄ is used. What is the role of H₂SO₄?

(c) For Reaction 2, calculate the atom economy. Why is this reaction considered "green"?

(d) For Reaction 3, if 2.8 g of ethene is used, calculate the theoretical yield of bromoethane. (Molar masses: C₂H₄ = 28 g/mol, C₂H₅Br = 109 g/mol)

(e) For Reaction 4, why is the reaction difficult to stop at the monochlorination stage? Calculate the percentage of chlorine in the product mixture if the mole ratio of CH₃Cl : CH₂Cl₂ : CHCl₃ : CCl₄ is 1:0.3:0.1:0.05.

Q18. In a school laboratory, students prepare chlorobenzene from benzene.

(a) Write the reaction for the preparation of chlorobenzene from benzene. What catalyst is used? Why is anhydrous FeCl₃ preferred over FeCl₃·6H₂O?

(b) The students are warned not to add too much chlorine. Why? What by-products form if excess chlorine is used?

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

(d) A student suggests preparing chlorobenzene from aniline via diazotization. Write the reactions involved. Why might this method be preferred in some cases?

(e) The teacher asks: "Why can't we prepare bromobenzene by the direct bromination of benzene using HBr?" What is your answer?