Organic Chemistry: Some Basic Principles and Techniques

Organic chemistry is the chemistry of carbon compounds. Carbon's tetravalence and its ability to form sp3, sp2 and sp hybridised bonds and long chains give rise to millions of compounds. Molecules are represented by complete, condensed and bond-line formulae, classified by functional groups and named by the IUPAC system. You will study structural isomerism (chain, position, functional group and metamerism) and stereoisomerism. You will also learn the fundamental concepts of reaction mechanisms: homolytic and heterolytic bond cleavage, carbocations, carbanions and free radicals, and nucleophiles and electrophiles. The electronic effects that control reactivity are explained: the inductive effect, resonance (with +R and -R effects), the electromeric effect and hyperconjugation, which explains why tertiary carbocations are the most stable. The chapter then describes methods of purification: sublimation, crystallisation, distillation (simple, fractional, under reduced pressure and steam), differential extraction and chromatography. Finally, you will study Lassaigne's test for nitrogen, sulphur and halogens, and quantitative estimation of carbon and hydrogen (Liebig), nitrogen (Dumas and Kjeldahl), halogens (Carius) and sulphur.

Detailed Worksheet: Organic Chemistry: Some Basic Principles and Techniques Section A - Definitions (10 marks) 1. What is a functional group? Give two examples. (2 marks) 2. Distinguish between homolytic and heterolytic cleavage. (2 marks) 3. What are nucleophiles and electrophiles? Give one example of each. (2 marks) 4. What is hyperconjugation? (2 marks) 5. What is the Rf value in chromatography? (2 marks) Section B - Calculations and Applications (15 marks) 6. In Liebig's method, 0.246 g of an organic compound gave 0.198 g of carbon dioxide and 0.1014 g of water on complete combustion. Calculate the percentage of carbon and hydrogen in the compound (C = 12, H = 1, O = 16). (3 marks) 7. In Kjeldahl's method, the ammonia evolved from 0.5 g of a compound neutralised 10 mL of 1 M H2SO4. Calculate the percentage of nitrogen in the compound. Name one class of compounds for which this method cannot be used. (3 marks) 8. In Dumas' method, 0.3 g of a compound gave 50 mL of nitrogen collected at 300 K and 715 mm pressure. Calculate the percentage of nitrogen if the aqueous tension at 300 K is 15 mm (molar volume of N2 at STP = 22,400 mL, mass 28 g). (3 marks) 9. In the Carius method, 0.378 g of an organic compound gave 0.574 g of silver chloride. Calculate the percentage of chlorine (Ag = 108, Cl = 35.5). (3 marks) 10. In paper chromatography, a spot moves 3.0 cm while the solvent front moves 7.5 cm. Calculate the Rf value. If another component has Rf = 0.8, how far would it move? Which component is more strongly adsorbed? (3 marks) Section C - Diagrams (10 marks) 11. Draw the bond-line, condensed and complete structural formulae of butan-2-ol. (4 marks) 12. Draw the resonance structures of the carboxylate ion and of phenol. (3 marks) 13. Draw a labelled diagram of the apparatus for fractional distillation or steam distillation. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain the inductive, resonance and electromeric effects with examples. How do they affect the acidity of carboxylic acids such as chloroacetic acid compared to acetic acid? (5 marks) 15. Explain the order of stability of carbocations using the inductive effect and hyperconjugation. Why is the tertiary butyl cation more stable than the methyl cation? (5 marks) 16. Explain the principle of Lassaigne's test. Write the reactions for the detection of nitrogen, sulphur and halogens. Why is the sodium extract boiled with nitric acid before testing for halogens? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Write structures and equations clearly. Show all calculation steps.