Haloalkanes and Haloarenes

Haloalkanes and haloarenes are formed when hydrogen atoms in aliphatic or aromatic hydrocarbons are replaced by halogen atoms, and they are widely used as solvents, starting materials and medicines such as chloramphenicol. In this chapter you will learn their classification and nomenclature, the polar nature of the carbon-halogen bond, and methods of preparation: from alcohols using HX, PCl3, PCl5 or SOCl2, by free radical halogenation, by addition of HX to alkenes following Markovnikov's rule and the peroxide effect, by electrophilic substitution and the Sandmeyer reaction, and by halogen exchange through the Finkelstein and Swarts reactions. You will explain trends in their boiling points and densities. The chapter then explains nucleophilic substitution by SN2 and SN1 mechanisms, the stereochemistry of these reactions, including optical activity, chirality, inversion and racemisation, elimination following Saytzeff's rule, and reactions with metals to form Grignard reagents and in the Wurtz reaction. You will learn why haloarenes are less reactive towards nucleophiles, how nitro groups activate them, their electrophilic substitution, and the uses and environmental effects of polyhalogen compounds such as chloroform, freons and DDT.

Detailed Worksheet: Haloalkanes and Haloarenes Section A - Definitions (10 marks) 1. Write the IUPAC names of: CH3CH(Cl)CH3, (CH3)3CBr, CH2=CHCH2Cl and C6H5CH2Cl. Classify each as primary, secondary, tertiary, allylic or benzylic halide. (2 marks) 2. What is the Finkelstein reaction? Why is it carried out in dry acetone? (2 marks) 3. State Saytzeff's rule with an example. (2 marks) 4. What is a racemic mixture? Why is it optically inactive? (2 marks) 5. What is the peroxide effect? Which hydrogen halide shows it? (2 marks) Section B - Calculations and Applications (15 marks) 6. 0.15 g of an organic compound gave 0.12 g of silver bromide in the Carius method. Calculate the percentage of bromine in the compound (Ag = 108, Br = 80). (3 marks) 7. Calculate the mass of butane formed when 21.8 g of bromoethane reacts with excess sodium in dry ether (Wurtz reaction), assuming 70% yield (C2H5Br = 109, C4H10 = 58). Write the equation. (3 marks) 8. The SN2 reaction CH3Br + OH- -> CH3OH + Br- has rate = k[CH3Br][OH-]. If the initial rate is 3.0 x 10^-4 mol L^-1 s^-1 when both concentrations are 0.1 M, calculate k. What will the rate be if [CH3Br] is doubled and [OH-] is halved? How would the rate of an SN1 reaction of (CH3)3CBr change if [OH-] were doubled? (3 marks) 9. Write all the structural isomers of C4H9Br. Classify each as primary, secondary or tertiary, and identify the one that is optically active. (3 marks) 10. Arrange each set in increasing order of the property stated, giving reasons: (i) boiling point: CH3Cl, CH3Br, CH3I, CH3F (ii) reactivity towards SN2: (CH3)3CBr, CH3CH2Br, (CH3)2CHBr, CH3Br (iii) reactivity towards SN1: same set as (ii). (3 marks) Section C - Diagrams (10 marks) 11. Draw the mechanism of the SN2 reaction of hydroxide ion with chloromethane, showing the transition state and inversion of configuration. Draw the energy profile for this one-step reaction. (4 marks) 12. Draw the mechanism of the SN1 reaction of tert-butyl bromide with hydroxide ion, showing the carbocation intermediate, and explain why racemisation occurs for an optically active substrate. (3 marks) 13. Draw the resonance structures of chlorobenzene and explain why the C-Cl bond acquires partial double bond character. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain four reasons why haloarenes are much less reactive than haloalkanes towards nucleophilic substitution. Explain why the presence of a nitro group at the ortho or para position increases the reactivity of chlorobenzene, and compare the conditions needed for 4-nitrochlorobenzene and 2,4,6-trinitrochlorobenzene. (5 marks) 15. When 2-bromobutane is heated with alcoholic KOH, two alkenes are formed. Write their structures, identify the major product using Saytzeff's rule, and explain why aqueous KOH gives mainly butan-2-ol instead. Analyse how the nature of the base and solvent decides between substitution and elimination. (5 marks) 16. Explain why chloroform is stored in closed dark-coloured bottles filled to the brim, why DDT was banned in many countries despite its use against malaria, and how freons damage the ozone layer. Suggest one safer alternative practice for each. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Write balanced chemical equations and show mechanisms clearly. Use the given atomic masses for calculations.