Amines

Amines are derivatives of ammonia in which one, two or all three hydrogen atoms are replaced by alkyl or aryl groups, giving primary, secondary and tertiary amines. In this chapter you will learn their nomenclature and pyramidal structure, with an sp3 nitrogen carrying a lone pair. You will study methods of preparation: reduction of nitro compounds with iron scrap and HCl, ammonolysis of alkyl halides, reduction of nitriles and amides, Gabriel phthalimide synthesis for pure primary amines, and Hoffmann bromamide degradation, which gives an amine with one carbon atom less than the amide. The chapter explains the basic character of amines using Kb and pKb values, the order of basicity in the gas phase and in water due to inductive, solvation and steric effects, and why aniline is much weaker than ammonia. You will study acylation, the carbylamine reaction, reaction with nitrous acid, Hinsberg's test, and electrophilic substitution in aniline. Finally, you will learn the preparation of benzenediazonium chloride and its reactions: Sandmeyer and Gatterman reactions, other replacement reactions, and coupling reactions forming azo dyes.

Detailed Worksheet: Amines Section A - Definitions (10 marks) 1. Classify the following as primary, secondary or tertiary amines and give their IUPAC names: CH3NH2, (CH3)2NH, (CH3)3N, C6H5NH2. (2 marks) 2. What is Gabriel phthalimide synthesis? Why can it not be used to prepare aromatic primary amines? (2 marks) 3. What is the carbylamine reaction? Which class of amines gives this test? (2 marks) 4. What is Hinsberg's reagent? How does it distinguish primary, secondary and tertiary amines? (2 marks) 5. What is diazotisation? Why is it carried out at 273-278 K? (2 marks) Section B - Calculations and Applications (15 marks) 6. The pKb of methanamine is 3.38. Calculate its Kb. Calculate the hydroxide ion concentration and pH of a 0.1 M aqueous solution, using [OH-] = sqrt(Kb x C) (take Kb = 4.2 x 10^-4, sqrt(42) = 6.48, log 6.48 = 0.81). (3 marks) 7. In Hoffmann bromamide degradation, CH3CONH2 + Br2 + 4NaOH -> CH3NH2 + Na2CO3 + 2NaBr + 2H2O. Calculate the mass of methanamine obtained from 11.8 g of ethanamide, assuming 100% yield (C = 12, H = 1, N = 14, O = 16). (3 marks) 8. Calculate the mass of aniline obtained by reducing 24.6 g of nitrobenzene with Fe and HCl, if the yield is 80% (C6H5NO2 = 123, C6H5NH2 = 93). (3 marks) 9. Arrange the following in decreasing order of basic strength in aqueous solution using pKb values: (C2H5)2NH 3.00, (C2H5)3N 3.25, C2H5NH2 3.29, NH3 4.75, C6H5NH2 9.38. Explain why aniline is the weakest base. (3 marks) 10. Calculate the percentage of nitrogen in (i) methanamine (ii) aniline (iii) N,N-dimethylmethanamine. Which has the highest percentage? (3 marks) Section C - Diagrams (10 marks) 11. Draw the pyramidal structure of trimethylamine showing the lone pair on nitrogen and the C-N-C bond angle of 108 degrees. Draw the resonance structures of aniline showing delocalisation of the lone pair into the ring. (4 marks) 12. Draw a flowchart showing the conversion of benzenediazonium chloride into chlorobenzene, bromobenzene, benzonitrile, iodobenzene, fluorobenzene, benzene and phenol, with reagents. (3 marks) 13. Draw the structures of the products of coupling of benzenediazonium chloride with phenol and with aniline, and state their colours. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain why the order of basicity of methyl-substituted amines in water is (CH3)2NH > CH3NH2 > (CH3)3N > NH3, whereas in the gas phase it is (CH3)3N > (CH3)2NH > CH3NH2 > NH3. Discuss inductive, solvation and steric effects. (5 marks) 15. Direct nitration of aniline gives a substantial amount of meta-nitroaniline along with tarry oxidation products. Explain why, and describe how acetylation is used to obtain p-nitroaniline as the major product, writing all the steps. (5 marks) 16. An aromatic compound A (C7H9N) on treatment with NaNO2 and HCl at 273 K gives B, which on warming with water gives p-cresol. A gives a carbylamine test. Identify A and B, write the reactions, and explain why aromatic diazonium salts are more stable than aliphatic ones. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Write balanced chemical equations and show all structures clearly. Use the given data for calculations.