Hydrocarbons

Hydrocarbons are compounds of carbon and hydrogen only, used as fuels such as LPG, CNG and petrol. This chapter classifies them as saturated alkanes (CnH2n+2), unsaturated alkenes (CnH2n) and alkynes (CnH2n-2), and aromatic hydrocarbons. For alkanes, you will study nomenclature, isomerism, preparation by hydrogenation, from alkyl halides by the Wurtz reaction, and from sodium salts of carboxylic acids. Their reactions include free radical halogenation, combustion, isomerisation, aromatisation and pyrolysis. The staggered and eclipsed conformations of ethane are also covered. For alkenes, you will learn about geometrical (cis-trans) isomerism and preparation by dehydrohalogenation and dehydration of alcohols. Their reactions include Markovnikov addition of HBr, the peroxide effect, ozonolysis, oxidation by Baeyer's reagent and polymerisation. Alkynes are made from calcium carbide, show acidic character, and undergo addition reactions and cyclic polymerisation to benzene. Finally, you will study benzene: its resonance structure, aromaticity by Huckel's (4n + 2) pi electron rule, electrophilic substitution (nitration, halogenation, sulphonation and Friedel-Crafts reactions), the directive influence of substituents, and the carcinogenic nature of some polynuclear hydrocarbons.

Detailed Worksheet: Hydrocarbons Section A - Definitions (10 marks) 1. State Markovnikov's rule with an example. (2 marks) 2. What is the Wurtz reaction? Give an equation. (2 marks) 3. State Huckel's rule of aromaticity. (2 marks) 4. What is the peroxide effect? Which halogen acid shows it? (2 marks) 5. Distinguish between staggered and eclipsed conformations of ethane. (2 marks) Section B - Calculations and Applications (15 marks) 6. A hydrocarbon contains 85.7% carbon and 14.3% hydrogen by mass, and its molar mass is 42 g/mol. Calculate its empirical and molecular formulae (C = 12, H = 1). Name it and write its structural formula. (3 marks) 7. Write the balanced equation for the complete combustion of propane. Calculate the volume of oxygen needed to burn 11 g of propane and the volume of carbon dioxide formed (take molar volume = 22.4 L at STP). (3 marks) 8. Calculate the mass of ethyne formed when 32 g of calcium carbide reacts with excess water: CaC2 + 2H2O -> C2H2 + Ca(OH)2 (Ca = 40, C = 12, O = 16, H = 1). What volume does it occupy at STP (22.4 L/mol)? (3 marks) 9. An alkene on ozonolysis gives ethanal and propanone. Deduce the structure and IUPAC name of the alkene. Write the reaction. (3 marks) 10. Write the products of the Wurtz reaction when a mixture of methyl bromide and ethyl bromide is treated with sodium in dry ether. Explain why this method is not suitable for preparing alkanes with an odd number of carbon atoms. (3 marks) Section C - Diagrams (10 marks) 11. Draw the Newman and sawhorse projections of the staggered and eclipsed conformations of ethane. (4 marks) 12. Draw the structures of cis and trans but-2-ene and explain why they are different. (3 marks) 13. Draw the mechanism of nitration of benzene showing the formation of the electrophile, the arenium ion and the product. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain the addition of HBr to propene with and without peroxide. Write the mechanisms and explain why the products differ. (5 marks) 15. Explain why ethyne is acidic while ethene and ethane are not. Write two reactions showing the acidic character of ethyne. (5 marks) 16. Explain the directive influence of functional groups on electrophilic substitution in monosubstituted benzene. Why is -OH ortho-para directing while -NO2 is meta directing? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Write structural formulae and equations clearly. Use '->' for reactions. Show all calculation steps.