Reactions of Benzene Benzene does not generally undergo addition reactions because these would involve breaking up the delocalised system. Most of Benzene’s reactions involve substituting one H for another atom or group of atoms. It reactions are usually electrophilic substitutions. Comparison of Benzene with alkenes: reaction with Bromine Alkenes react with Bromine easily at room temperature. Benzene does not react with Bromine without additional halogen carrier chemicals. In benzene, electrons in π-bond(s) are delocalised. In alkenes, π- electrons are localised between two carbons. Benzene therefore has a lower electron density than C=C. Benzene therefore polarises bromine less and induces a weaker dipole in bromine than an alkene would. Toxicity of Benzene Benzene is a carcinogen (cancers causing molecule) and is banned for use in schools. Methylbenzene is less toxic and also reacts more readily than benzene as the methyl side group releases electrons into the delocalised system making it more attractive to electrophiles. 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 onto different positions on the ring N Goalby chemrevise.org H+ + HSO4 – H2SO4 If the benzene ring already has a side group e.g. methyl then the Nitro group can also join on different positions. A-level does not require knowledge of what positions the groups go on. Change in functional group: benzene Bromobenzene Reagents: Bromine Conditions: iron(III) bromide catalyst FeBr3 Mechanism: Electrophillic Substitution This reaction can be done with chlorine. The catalyst can be AlCl3 or FeCl3 Halogenation of Benzene + Br2 Br + HBr Overall Equation for reaction Equation for Formation of electrophiles: (Learn!) AlCl3 + Cl2 AlCl4 – + Cl+ FeBr3 + Br2 FeBr4 – + Br+ Cl+ Cl Mechanism The H+ ion reacts with the AlCl4 – to reform AlCl3 catalyst and HCl. H+ + AlCl4 – AlCl3 + HCl Cl H + N Goalby chemrevise.org 4 Friedel Crafts Alkylation Change in functional group: benzene alkylbenzene Reagents: chloroalkane in the presence of anhydrous aluminium chloride catalyst Conditions: heat under reflux Mechanism: Electrophilic Substitution Any chloroalkane can be used RCl where R is any alkyl group Eg –CH3 , -C2H5 . The electrophile is the R+ . Formation of the electrophile. AlCl3 + CH3CH2Cl CH3CH2 + AlCl4 – CH3CH2 + AlCl4 – + AlCl3 + HCl ethylbenzene + Overall Equation for reaction CH2CH3 +CH2CH3 The H+ ion reacts with the AlCl4 – to reform AlCl3 catalyst and HCl. H+ + AlCl4 – AlCl3 + HCl CH2CH3 + H CH2CH3 Mechanism 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
/
~
~
~
/
6.1.1 Aromatic compounds
Electrophilic substitution (d) the electrophilic substitution of aromatic compounds with: (i) concentrated nitric acid in the presence of concentrated sulfuric acid (ii) a halogen in the presence of a halogen carrier (iii) a haloalkane or acyl chloride in the presence of a halogen carrier (Friedel–Crafts reaction) and its importance to synthesis by formation of a C–C bond to an aromatic ring (see also 6.2.4 d) Halogen carriers include iron, iron halides and aluminium halides. (e) the mechanism of electrophilic substitution in arenes for nitration and halogenation (see also 4.1.1 h–i) For nitration mechanism, learners should include equations for formation of NO2 +. Halogen carriers include iron, iron halides and aluminium halides. For the halogenation mechanism, the electrophile can be assumed to be X+. HSW1,2,8 Use of reaction mechanisms to explain organic reactions.(f) the explanation of the relative resistance to bromination of benzene, compared with alkenes, in terms of the delocalised electron density of the π-system in benzene compared with the localised electron density of the π-bond in alkenes (see also 4.1.3 a, 6.1.1 a) HSW2,5 Use of delocalised benzene model to explain reactivity. (g) the interpretation of unfamiliar electrophilic substitution reactions of aromatic compounds, including prediction of mechanisms Extra information may be provided on exam papers
6.1.1 aromatic compounds Page 3 - 4
Oxford Textbook Pages : 437 - 441
CGP Revision Guide Pages : 176 - 177
