SAJC_H2_CHEM_P3_ANS Prelim

1 Solutions for 2015 SAJC H2 Chemistry Prelim Paper 3 1 (a) Organic compounds have various applications. For instance, Ketoprofen is a kind of nonsteroidal anti-inflammatory drug, which can be used as a substitute for aspirin. (i) Friedel Craft Acylation was first discovered by Charles Friedel and James Crafts in 1877. This reaction allowed for the acylation of an aromatic ring with acyl chloride. One such example is given below. Benzophenone, which can be used to synthesise Keto profen, has the structure below. Suggest the reagent needed to convert 2 moles of benzene into 1 mole of benzophenone. [1] COCl2

2 1 (a) (ii) Ketoprofen may be synthesised from benzophenone in the following route. State the reagents and conditions for Steps 1 to 3. Draw the structural formula of A. [4] Step 1: AlCl3 or FeCl3 OR AlBr3 or FeBr3 OR AlBr3 or FeBr3 Step 2: NaOH(aq), heat Step 3: H2SO4 (aq), K2Cr2O7, heat Structure of A: (iii) Ketoprofen is commonly sold as its sodium salt form for better absorption by the body, which is made up of 70% water. Explain, with the aid of a diagram, how this better absorption comes about. [2] The ions in the sodium salt form can form ion-dipole interactions with water , thus enhancing its solubility and absorption by the body.

3 (iv) Nitrogen-containing derivatives of Ketoprofen may be synthesised and some examples are as shown. Describe a simple chemical test to distinguish between compound D and compound E. [2] Add NaOH(aq) to each of the unknown, followed by heating. Add Br2 (aq) at room temperature to each of the resultant products. Orange Br2 (aq) will decolourise, forming a white ppt in the presence of compound E while the orange colour remains in compound D. OR Add NaOH(aq) to each of the unknown, followed by heating. Compound D will produce a colourless, pungent gas which turns moist red litmus paper blue / forms white fumes with a glass rod dipped in concentrated HC l. Compound E does not produce such gas. (v) Ketoprofen can react with cold alkaline HCN to form a product which exists as a mixture of 4 stereoisomers. Give the structure of the product formed. Hence, state the type of isomerism exhibited by this product and draw the stereoisomers. [4] Optical isomerism

4 1 (b) Another use of organic compounds is in the area of food preservation. To prevent food spoilage, the pH of food should not fluctuate too much. Hence, a mixture of lactic acid, CH3CH(OH)COOH, and sodium lactate is comm