12201 16 Benzene is used to synthesise the analgesic, ibuprofen. This occurs using a multi-step reaction involving both Friedel–Crafts acylation and alkylation. COOH ibuprofen (a) The first step in this reaction is the acylation of benzene with 2-methylpropanoyl chloride. The structure of 2-methylpropanoyl chloride is shown below. H O H3C— C— C— CI CH3 2-methylpropanoyl chloride Show the mechanism for the catalysed reaction between 2-methylpropanoyl chloride and benzene, using aluminium chloride as a catalyst. [5] 12201 [Turn over (b) Draw a dot and cross diagram for the aluminium-containing intermediate species formed by the catalyst during this reaction. [1] (c) A later step in the synthesis is shown below. HO Suggest the structure of the organic reagent used for the step shown. [1] (d) The acylation and alkylation reactions of benzene are substitution reactions. Explain why benzene undergoes substitution reactions rather than addition reactions. [1]