NYJC H2 Chem 2012 Prelim P2 Soln

2 H2 Chemistry 9647/02 NYJC J2/12 PX [ Turn over For E xaminer's Use Answer all questions in the spaces provided. 1 Planning (P) Paracetamol (acetaminophen) is commonly used for the relief of headaches and is a major ingredient in numerous cold and flu medicine. It is a white solid (m.p. = 169 oC) which can be prepared by a reaction between 4 -aminophenol and ethanoic anhydride. HOC6H4NH2 (CH3CO)2O CH 3CONHC6H4OH 4-aminophenol (Mr = 109) ethanoic anhydride (Mr = 102) paracetamol (Mr = 151) A typical yield, based on 4-aminophenol, is 70%. The crude product can be purified by recrystallisation from water. The purity of the recrystallised product can be confirmed by determining its melting point. Ethanoic anhydride can cause irritation of tissue, especially in nasal passages. 4-aminophenol is a skin irritant and is toxic. (a) Using the information above: (i) Write a balanced equation for the formation of paracetamol; HOC6H4NH2 + (CH3CO)2O → CH3CONHC6H4OH + CH3CO2H (i i) Calculate the minimum masses of reactants needed to prepare 5 g of pure paracetamol. Mass of 4-aminophenol required = 109 ×70 100×151 5 = 5.16 g M ass of ethanoic anhydride required = 5 100× ×102151 70 = 4.82 g [ 3]

3 H2 Chemistry 9647/02 NYJC J2/12 PX [ Turn over For E xaminer's Use (b) Write a full description of the purification of the crude paracetamol. List the essential apparatus you would use in the process. You do not have to describe the preparation of paracetamol. Apparatus: - Conical flask for dissolving crude solid (accept beaker) - Hot plate for heating (accept oil bath but not water bath or heating mantle) - Filter funnel/ conical flask for hot filtration - Buchner apparatus for collecting pure crystals. Method: - Dissolve crude solid in minimum amount of hot water - Filter solution while hot - Cool hot solution (in ice bath) - Collects crystals using vacuum filtration/ reduced pressure (using Buchner apparatus) - Wash with small amounts of cold water - Dries crystals between filter paper/ under IR lamp. ................................................. ............................................................................. [6] (c) Write a brief description showing how you would use the melting point of the purified product to confirm its purity. You do not have to describe how you would obtain the melting point. - Melts sharply/ over small temperature range - Melting point agrees with data value (i.e. 169 oC) ................................................. ............................................................................. [1] (d) Identify two potential safety hazards in this experiment and the relevant safety precautions you would take. Hazard Pr ecaution 4-aminophenol is a skin irritant and is toxic. Ethanoic acid is corrosive Avoid skin co