Eduqas Chemistry for A Level Year 2: Student Bk

123 OA1.2 Aromaticity Friedel–Crafts alkylation This method provides a way of producing a new carbon-to-carbon bond, giving alkyl derivatives of benzene such as methylbenzene, C 6 H 5 CH 3 . The reaction is similar to the halogenation of benzene but uses a halogenoalkane in place of the halogen. Anhydrous aluminium chloride is often used as the catalyst. + CH 3 CH 2 Cl AlCl 3 + HCl CH 2 CH 3 One problem with this reaction is that the introduction of an alkyl group onto the ring activates the ring towards further alkylation. As a result the product may also contain 1,2- and 1,4-diethylbenzenes. To reduce the chance of any polyalkylation, the halogenoalkane is added slowly to the benzene and the catalyst. Another problem that may occur is that a primary carbocation formed during the reaction may rearrange to a secondary carbocation. e.g. CH 3 CH 2 CH 2 CH 3 CHCH 3 + + The reaction of 1-chloropropane with benzene gives mainly (1-methylethyl)benzene rather than 1-propylbenzene as the organic product. + CH 3 CH 2 CH 2 Cl AlCl 3 CH 2 CH 2 CH 3 CH CH 3 H 3 C Acid chlorides, such as ethanoyl chloride, also react with benzene in a similar Friedel–Crafts reaction, giving a ketone. + HCl + CH 3 C C O O Cl CH 3 An important industrial process that uses a Friedel–Crafts style reaction is the manufacture of phenylethene (styrene), C 6 H 5 CH=CH 2 . + CH 2 CH 2 HCl ZnO AlCl 3 630ºC CH 2 CH 3 CH CH 2 & S C Stretch & Challenge Use the equation for the Friedel–Crafts alkylation of benzene and 1-chloropropane to suggest the formula of the major organic product when benzene reacts with 1-chloro-3-methylbutane. & S C Stretch & Challenge Give the systematic name of the choroalkane that will react with benzene to give C(CH 3 ) 3