Electrophilic Substitution Reactions of Benzene

1. 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.

2. The Reactions of Benzene • The reactions of benzene usually involve the replacement of 1or more hydrogen atoms on the ring 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 often needs a catalyst for reactions to take place.

3. 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.

4. 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.

5. 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

6. 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 • The nitric acid is a BASE and is protonated by the stronger acid, sulphuric acid

7. the initial reaction HNO3 + H2SO4 HSO4- + H2 NO3+ H2 NO3+→ NO2+ + H2O i.e. HNO3 + H2SO4→ NO2+ + HSO4- +H2O • the sulphuric acid acts as a catalyst in this reaction • it is regenerated later in the reaction mechanism

8. 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

9. electrons move from the benzene ring towards the electrophile electrophilic attack

10. the intermediate formed is very unstable Note that the positive charge is delocalised over 5 carbon atoms. The bond takes 2 of the 6 delocalised electrons leaving 4 in the ring.

11. 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 ringbreaks heterolytically • the delocalised benzene ring system is reformed

12. the hydrogen - carbon bond breaks the delocalised molecular orbital reforms

13. the products the H+ reacts with HSO4- to regenerate sulphuric acid

14. NO2 H NO2 + Summary: Electrophilic Substitution 1 – Nitration of benzene H2SO4 + HNO3 NO2+ + HSO4- + H2O +NO2 H+ home

15. 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

16. Formation of Halogenoarenes • Benzene does not react with halogens in the dark ( which it would if it was an alkene). • However if a ‘halogen carrier’ is used the reaction is straightforward. • Halogen carriers include iron, iron chloride, aluminium chloride. • Bubble chlorine through refluxing benzene in the presence of a catalyst. Electrphilic substitution takes place.

17. How do halogen carriers work? • Both FeCl3 and AlCl3 are covalent and soluble in benzene. • Draw a covalent diagram for AlCl3. • This is electron deficient. • It can react with Cl2 to form AlCl4- -dative bonding. • Heterolytic fission of Cl-Cl bond to give a Cl+. • Fully positive ion can now attack the benzene ring.

18. Reactivity of halogens • Reaction rate depends on bond strength of the halogen so Cl>Br>I. • Requires boiling benzene for reaction with bromine. • Iodine hardly at all. (not required by syllabus).

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