2 Electrophilic substitution

Nitration of Benzene Importance of this reaction Nitration of benzene and other arenes is an important step in synthesising useful compounds e.g. explosive manufacture (like TNT, trinitrotoluene/ 2,4,6- trinitromethylbenzene) and formation of amines from which dyestuffs are manufactured. (The reaction for this is covered in the amines section.) Change in functional group: benzene  nitrobenzene Reagents: conc nitric acid in the presence of concentrated sulphuric acid (catalyst) Mechanism: Electrophilic Substitution Electrophile: NO2+ Equation for Formation of electrophile: (Learn!) HNO3 + 2H2SO4  NO2+ + 2HSO4 – + H3O+ + NO2 + NO2  + H + The horseshoe shape of the intermediate must not extend beyond C’s 2 to 6 Mechanism Overall Equation for reaction The H+ ion rejoins with the HSO4 – to reform H2SO4 catalyst. + H + This reaction is done at 60oC. On using higher temperatures a second nitro group can be substituted. N Goalby chemrevise.org NO2+ + H NO2 NO2 Friedel Crafts Acylation Change in functional group: benzene  phenyl ketone Reagents: acyl chloride in the presence of anhydrous aluminium chloride catalyst Conditions: heat under reflux (50OC) Mechanism: Electrophilic Substitution Any acyl chloride can be used RCOCl where R is any alkyl group e.g. –CH3 , -C2H5 . The electrophile is the RCO+ . Equation for Formation of the electrophile. AlCl3 + CH3COCl  CH3CO+ AlCl4 – CH3CO+ AlCl4 – + AlCl + 3 + HCl C O CH3 phenylethanone Overall Equation for reaction These are important reactions in organic synthesis because they introduce a reactive functional group on to the benzene ring The H+ ion reacts with the AlCl4 – to reform AlCl3 catalyst and HCl. H+ + AlCl4 –  AlCl3 + HCl Mechanism C O CH3 C CH3 + O + H C CH3 O 4 Reducing a Nitroarene to aromatic amines The nitro group on an arene can be reduced an amine group as follows Reagent: Sn and HCl or Fe and HCl Conditions: Heating Mechanism:reduction NO2 NH2 + 6[H] + 2H2O As the reaction is carried out in HCl the salt C6H5NH3+Cl- will be formed. Reacting this salt with NaOH will give phenylamine. phenylamine nitrobenzene This reduction reaction can also be done with catalytic hydrogenation (H2 using a Ni catalyst) Effect of delocalisation on side groups with lone pairs If a –OH group, a Cl atom or an NH2 group is directly attached to a benzene ring the delocalisation in the benzene ring will extend to include the lone pairs on the N,O and Cl. This changes the properties and reactions of the side group. Cl chlorobenzene The C-Cl bond is made stronger. Typical halogenoalkane substitution and elimination reactions do not occur. Also the electron rich benzene ring will repel nucleophiles. OH phenol Delocalisation makes the C-O bond stronger and the O-H bond weaker. Phenol does not act like an alcohol- it is more acidic and does not oxidise. NH2 phenylamine Less basic than aliphatic amines as lone pair is delocalised and less available for accepting a proton. N Goalby chemrevise.org 5 NO2 conc nitric acid + conc sulphuric acid Electrophilic substitution C O CH3 acyl chloride in the presence of anhydrous aluminium chloride catalyst Electrophilic substitution NH2 Sn and HCl reduction NH C O CH3 CH3COCl Nucleophilic add-el CH O CH3 H NaBH4 Red Nu Add NH CH3 CH3Cl Nu sub C O CH3 H NaCN + H2SO4 CN Nu Add C OH CH3 CH2 NH2 LiAlH4 reduction Aromatic Synthetic Routes H3C CH O C O CH3 CH3CO2H + H2SO4 heat esterification Remember that many questions about aromatic compounds will actually be about normal aliphatic reactions of the side chains and not about the benzene ring. CH CH2 Conc H2SO4 dehydration