Electrophilic Substitution Reactions of Benzene Aim: To describe the electrophilic substitution of arenes with concentrated nitric acid in the presence of concentrated sulphuric acid. Outline the mechanism of the above reaction.

The Reactions of Benzene The reactions of benzene usually involve the replacement of 1or more hydrogen atoms on the ring being replaced by a functional group in a SUBSTITUTION reaction. Substitution allows the ring to retain its delocalised Π system of electrons and its stability. Stability means it is far less reactive than its electron cloud would suggest. In fact it needs a catalyst for reactions to take place.

Why are aromatic compounds so unreactive? Although there is a delocalised Π cloud above and below the plane of the ring electrophiles are far less attracted to the cloud than they are in alkenes. In ethene, for example there are 2 Π electrons per single covalent bond which form a region of localised high electron density which attracts electrophiles readily. In benzene there are 6 electrons delocalised over the whole 6 bonds in the ring, leading to a much lower (1/2?) electron density.

Mechanism of electrophilic substitution The electrophile must carry a full positive charge. Catalysts are usually required. Benzene is very important in the chemical industry. Used to make: Explosives, dyes, pharmaceuticals, insecticides, detergents, polymers etc. Explosives and dyes are derived from NITROBENZENE.

Neither nitric acid nor sulphuric acid will react with benzene. Nitration of Benzene Neither nitric acid nor sulphuric acid will react with benzene. However NITRATING MIXTURE does, readily and exothermically. At 50oC use equal volumes conc HNO3 and conc H2SO4 C6H6 +HNO3 → C6H5NO3 + H2O Nitrobenzene Yellow oil bp 211oC

production of the electrophile the benzene ring is a centre of high electron density which makes it susceptible to electrophilic attack the nitryl cation, NO2+, is the electrophile in this reaction it is produced by the reaction of concentrated nitric acid with concentrated sulphuric acid TY 2002

H2 NO3+ + H2SO4 → H3O+ + NO2+ + H3O+ the initial reaction HNO3 + H2SO4 HSO4- + H2 NO3+ H2 NO3+ + H2SO4 → H3O+ + NO2+ + H3O+ i.e. HNO3 + 2H2SO4 → NO2+ + 2HSO4- +H3O+ the sulphuric acid acts as a catalyst in this reaction it is regenerated later in the reaction mechanism TY 2002

mechanism the benzene molecule has 6 delocalised electrons in a molecular orbital the NO2+ electrophile is attracted to this centre of high electron density situated above and below the ring an electron pair move from the delocalised ring system to the NO2+ forming a new covalent bond TY 2002

electrons move from the benzene ring towards the electrophile electrophilic attack TY 2002

the intermediate formed is very unstable Note that the positive charge is delocalised over 5 carbon atoms TY 2002

mechanism the stable delocalised ring system is reformed as the unstable intermediate breaks down the covalent bond between the hydrogen and the carbon atom in the benzene ring breaks heterolytically the delocalised benzene ring system is reformed TY 2002

the hydrogen - carbon bond breaks the delocalised molecular orbital reforms TY 2002

the H+ reacts with HSO4- to regenerate sulphuric acid the products the H+ reacts with HSO4- to regenerate sulphuric acid TY 2002

mechanism the product of this reaction is nitrobenzene this is an electrophilic substitution reaction the halogens e.g.chlorine will undergo a similar reaction with benzene in the presence of a catalyst such as aluminium chloride producing chlorobenzene CARE!!benzene and chlorine undergo an addition reaction in the presence of UV light the product is 1,2,3,4,5,6-hexachlorocyclohexane TY 2002

+ home Electrophilic Substitution 1 – Nitration of benzene H2SO4 + HNO3 NO2+ + HSO4- + H2O NO2 +NO2 + NO2 H H+ home TY 2002

+ home Electrophilic Substitution 2 – Bromination of benzene Br2 + AlCl3 Br+ + AlBrCl3- Br +Br + Br H HBr home AlCl3 TY 2002

+ home Electrophilic Substitution 3 – Alkylation of benzene CH3Cl + AlCl3 CH3+ + AlCl4- CH3 +CH3 + CH3 H HCl home AlCl3 TY 2002

+ home Electrophilic Substitution 4 – Acylation of benzene CH3COCl + AlCl3 CH3CO+ + AlCl4- CH3 +C O + CH3 C O H CH3 C O HCl home AlCl3 TY 2002

+ home Bromination of methylbenzene – ring substitution Br2 + AlBr3 Br+ + AlBr4- CH3 CH3 + H Br CH3 Br Br+ HBr home AlBr3 TY 2002