13616.08RRR 12 Benzaldehyde is an aromatic aldehyde with an almond odour. It can be obtained from natural sources and is widely used in the preparation of dyes, perfumes and flavourings. C O H benzaldehyde (a) Describe how you would practically carry out the reaction of a solution of benzaldehyde with Tollens’ reagent and with acidified potassium dichromate(VI) solution and state all observations which occur. Write an equation for the oxidation of benzaldehyde and half equations for the reduction of each reagent. In this question you will be assessed on using your written communication skills including the use of specialist scientific terms. [6] 13616.08RRR [Turn over (b) (i) Benzaldehyde undergoes an addition reaction with hydrogen cyanide to form a hydroxynitrile called mandelonitrile. Outline the mechanism for this reaction. Include an equation for the formation of the nucleophile from hydrogen cyanide. [4] (ii) The addition of cyanide ions to a carbonyl group is carried out at a pH of 4 – 5. Suggest why a very low pH is not used. [1] (iii) Explain why the mandelonitrile formed from the reaction in (b)(i) is optically inactive. [2] 13616.08RRR (c) The hydrolysis of mandelonitrile forms mandelic acid C6H5CH(OH)COOH. This is a weak acid with a Ka value of 3.88 × 10-4 mol dm-3. Excess mandelic acid reacts with sodium hydroxide to form a buffer solution. (i) Write an equation for the reaction of mandelic acid with sodium hydroxide. [1] (ii) Define the term buffer solution. [1] (iii) Calculate the pH of the buffer formed on mixing 35.0 cm3 of 0.125 mol dm-3 mandelic acid with 15.0 cm3 of 0.175 mol dm-3 sodium hydroxide solution. Give your answer to 2 decimal places. Answer                      [4] 13616.08RRR BLANK PAGE DO NOT WRITE ON THIS PAGE (Questions continue overleaf) [Turn over