Aldehydes, Ketones and Carboxylic Acids

Aldehydes, ketones and carboxylic acids contain the carbonyl group, C=O, in which the carbon is sp2 hybridised and the bond is polarised, making the carbon electrophilic. In this chapter you will learn their nomenclature and structure, and methods of preparation: oxidation and dehydrogenation of alcohols, ozonolysis of alkenes, hydration of alkynes, Rosenmund reduction, the Stephen reaction, reduction of nitriles and esters with DIBAL-H, Etard reaction, Gatterman-Koch reaction and Friedel-Crafts acylation. You will explain their physical properties and their characteristic nucleophilic addition reactions with HCN, NaHSO3, Grignard reagents, alcohols and ammonia derivatives such as hydroxylamine and 2,4-DNP. The chapter also covers reduction by Clemmensen and Wolff-Kishner methods, oxidation tests with Tollens' and Fehling's reagents, the haloform reaction, aldol condensation, the Cannizzaro reaction and electrophilic substitution. For carboxylic acids you will study preparation from alcohols, alkylbenzenes, nitriles and Grignard reagents, the effect of substituents on acidity, and reactions such as esterification, decarboxylation, Kolbe electrolysis, the Hell-Volhard-Zelinsky reaction and ring substitution in benzoic acid.

Detailed Worksheet: Aldehydes, Ketones and Carboxylic Acids Section A - Definitions (10 marks) 1. Write the IUPAC names of: CH3CHO, CH3COCH3, CH3CH2CH(CH3)CHO and C6H5COCH3. (2 marks) 2. What is the Rosenmund reduction? Write the equation for the preparation of benzaldehyde by this method. (2 marks) 3. What is the Cannizzaro reaction? Which aldehydes undergo it? (2 marks) 4. Distinguish between Clemmensen reduction and Wolff-Kishner reduction. (2 marks) 5. What is the Hell-Volhard-Zelinsky reaction? Write the equation for ethanoic acid. (2 marks) Section B - Calculations and Applications (15 marks) 6. 2.2 g of ethanal is warmed with excess Tollens' reagent. Using RCHO + 2[Ag(NH3)2]+ + 3OH- -> RCOO- + 2Ag + 2H2O + 4NH3, calculate the mass of silver deposited (C = 12, H = 1, O = 16, Ag = 108). (3 marks) 7. Calculate the pH of a 0.1 M solution of ethanoic acid. Ka = 1.8 x 10^-5 (take sqrt(1.8) = 1.34 and log 1.34 = 0.127). Also calculate its pKa (log 1.8 = 0.255). (3 marks) 8. Arrange in increasing order of acid strength using the pKa values: CH3COOH 4.76, ClCH2COOH 2.86, Cl2CHCOOH 1.26, HCOOH 3.75, C6H5COOH 4.19. Explain the trend in terms of inductive effect. (3 marks) 9. An organic compound C3H6O does not reduce Tollens' reagent but forms an addition product with sodium hydrogensulphite and gives a positive iodoform test. Identify the compound and write the equations for the iodoform reaction. Calculate the percentage of carbon in it. (3 marks) 10. 15 g of ethanoic acid is heated with excess ethanol in the presence of concentrated H2SO4. Calculate the theoretical mass of ethyl ethanoate formed, and the actual mass if the yield is 66% (CH3COOH = 60, CH3COOC2H5 = 88). (3 marks) Section C - Diagrams (10 marks) 11. Draw the orbital diagram of the carbonyl group showing the sigma and pi bonds and the planar arrangement, and draw the mechanism of nucleophilic addition of HCN to ethanal. (4 marks) 12. Draw the mechanism of aldol condensation of ethanal in the presence of dilute NaOH, showing the formation of the enolate ion, the aldol and the final alpha,beta-unsaturated aldehyde. (3 marks) 13. Draw the structure of the hydrogen-bonded dimer of ethanoic acid and explain why carboxylic acids have higher boiling points than alcohols of comparable mass. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain why aldehydes are generally more reactive than ketones in nucleophilic addition reactions, considering both electronic and steric factors. Arrange ethanal, methanal, propanone and butanone in increasing order of reactivity towards HCN. (5 marks) 15. An organic compound A (C8H8O) forms an orange-red precipitate with 2,4-DNP, gives a yellow precipitate on heating with iodine and NaOH, does not reduce Tollens' or Fehling's reagent, and on vigorous oxidation gives benzoic acid. Identify A and write all the reactions involved. (5 marks) 16. Explain why carboxylic acids do not give the characteristic reactions of the carbonyl group, why benzoic acid does not undergo Friedel-Crafts reaction, and why the -COOH group is meta directing. Write the equation for the nitration of benzoic acid. (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.