2. 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.

3. 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.

4. 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.

5. 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.

6. Nitration of Benzene Neither nitric acid nor sulphuric acid will react with benzene. However NITRATING MIXTURE does, readily and exothermically. At 50 o C use equal volumes conc HNO 3 and conc H 2 SO 4 C 6 H 6 +HNO 3 → C 6 H 5 NO 3 + H 2 O Nitrobenzene Yellow oil bp 211 o C

7. TY 2002 production of the electrophile the benzene ring is a centre of high electron density which makes it susceptible to electrophilic attack the nitryl cation, NO 2 +, 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

8. TY 2002 the initial reaction HNO 3 + H 2 SO 4 HSO 4 - + H 2 NO 3 + the sulphuric acid acts as a catalyst in this reaction it is regenerated later in the reaction mechanism H 2 NO 3 + → NO 2 + + H 2 O i.e. HNO 3 + H 2 SO 4 → NO 2 + + HSO 4 - +H 2 O

9. TY 2002 mechanism the benzene molecule has 6 delocalised electrons in a molecular orbital the NO 2 + 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 NO 2 + forming a new covalent bond

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

11. TY 2002 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.

12. 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

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

14. TY 2002 the products the H + reacts with HSO 4 - to regenerate sulphuric acid

15. TY 2002 Electrophilic Substitution 1 – Nitration of benzene H 2 SO 4 + HNO 3 NO 2 + + HSO 4 - + H 2 O home + NO 2 H+NO 2 NO 2 H+H+ Summary:

16. 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

17. TY 2002 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.

18. TY 2002 How do halogen carriers work? Both FeCl 3 and AlCl 3 are covalent and soluble in benzene. Draw a covalent diagram for AlCl 3. This is electron deficient. It can react with Cl 2 to form AlCl 4 - -dative bonding. Heterolytic fission of Cl-Cl bond to give a Cl +. Fully positive ion can now attack the benzene ring.

19. TY 2002 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).

20. TY 2002 Electrophilic Substitution 2 – Bromination of benzene Br 2 + AlCl 3 Br + + AlBrCl 3 - home + BrH+Br Br HBr AlCl 3