Q1. List the major methods used for the preparation of haloalkanes.
Haloalkanes can be prepared from hydrocarbons, from alkenes by addition of HX, from alcohols by replacing OH with halogen, and by halogen exchange reactions such as Finkelstein and Swarts reactions.
Q2. Explain the preparation of haloalkanes from alkanes by photochemical halogenation.
Alkanes react with chlorine or bromine in presence of sunlight or UV light by free radical substitution. Example: CH₄ + Cl₂ → CH₃Cl + HCl. Further chlorination can form CH₂Cl₂, CHCl₃, and CCl₄.
Q3. Write short notes on initiation, propagation, and termination steps in free radical halogenation.
Initiation: homolytic cleavage of halogen molecule, e.g. Cl₂ → 2Cl·. Propagation: radical attacks alkane and chain continues, e.g. CH₄ + Cl· → CH₃· + HCl; CH₃· + Cl₂ → CH₃Cl + Cl·. Termination: radicals combine to stop the chain, e.g. CH₃· + CH₃· → C₂H₆.
Q4. How are haloalkanes prepared from alkenes? Illustrate with one example.
Haloalkanes are prepared from alkenes by addition of hydrogen halide across the double bond. Example: CH₂=CH₂ + HBr → CH₃CH₂Br.
Q5. What is meant by symmetric and unsymmetric alkenes in haloalkane preparation?
In symmetric alkenes, the two double bonded carbon atoms are equivalent, so addition gives one constitutional product. Example: CH₂=CH₂. In unsymmetric alkenes, the two carbons are not equivalent, so orientation of addition matters. Example: CH₃CH=CH₂.
Q6. State Markovnikov rule with an example from preparation of haloalkanes.
Markovnikov rule states that in the addition of HX to an unsymmetrical alkene, hydrogen goes to the carbon having more hydrogen atoms, and halogen goes to the more substituted carbon. Example: CH₃CH=CH₂ + HBr → CH₃CHBrCH₃ (2-bromopropane).
Q7. What is anti-Markovnikov addition or peroxide effect? Give one example.
In presence of peroxide, HBr adds to an unsymmetrical alkene in the opposite orientation to Markovnikov rule. This is called anti-Markovnikov addition or peroxide effect. Example: CH₃CH=CH₂ + HBr / peroxide → CH₃CH₂CH₂Br (1-bromopropane).
Q8. Explain the preparation of haloalkanes from alcohols. Mention important reagents.
Alcohols are converted into haloalkanes by replacing the OH group with a halogen atom. Reagents used include HX, PCl₅, PCl₃, PBr₃, and SOCl₂. Example: R-OH + HCl → R-Cl + H₂O or R-OH + SOCl₂ → R-Cl + SO₂ + HCl.
Q9. Write notes on Finkelstein reaction and Swarts reaction.
Finkelstein reaction converts alkyl chloride or bromide into alkyl iodide using NaI in acetone. Swarts reaction converts alkyl chloride or bromide into alkyl fluoride using metallic fluorides such as AgF, Hg₂F₂, CoF₂, or SbF₃.
Q10. Why is thionyl chloride often preferred in the preparation of alkyl chlorides from alcohols?
Thionyl chloride is preferred because it converts alcohol into alkyl chloride and the by-products SO₂ and HCl are gases, which escape easily. This leaves the alkyl chloride relatively pure and simplifies purification.