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21. Answer ALL parts of this question. The structure shown below represents one of the stereoisomers of naproxen, used to relieve pain, fever and swelling: H CH3 со,Н CH,0 (a) Using the four substituents attached to the chiral centre, outline the rules proposed by Cahn, Ingold and Prelog to assign priority to such substituents. (5 marks) (b) Explain how this order of priority may be used to define a particular enantiomer as either Ror S. You should use the structure of naproxen shown above to illustrate this, defining the structure shown as either Ror S. (3 marks) (c) Draw the structure of the enantiomer of this compound and comment on why it might produce a different reaction in vivo (in fact the enantiomer has no analgesic effect and may cause liver damage). (2 marks)