Electrophilic Substitution Reactions

1. Electrophilic Substitution Reactions …Reactions of the benzene ring

2. Types of reaction • Alkenes will normally undergo addition reactions but due to its stability, benzene will usually only undergo electrophilic substitution reactions. • This allows the stability of the ring system to be kept intact.

3. Bromination • Requires an iron catalyst e.g. iron(III) bromide to make bromine more electrophilic.

4. Bromination • The Br2 molecule becomes polarised as it approaches the benzene ring Br δ- Br δ+ • Although the positive end is now electrophilic it will only react slowly with the benzene ring.

5. Bromination • So a catalyst is required… 2Fe + 3Br2 2FeBr3 • It is thought that the FeBr3 helps to polarise the bromine molecule by accepting a LP from one of the bromine atoms. • The bromine molecule becomes so polarised that it splits into Br+ and FeBr4-

6. Bromination • Br+ is then the electrophile that substitutes for a H+ • The H+ is then used to regenerate the catalyst • H+ + FeBr4-  HBr + FeBr3

7. Nitration • Concentrated nitric acid in presence of concentrated sulphuric acid produces electrophile NO2+ • HNO3 + 2H2SO4NO2++ 2HSO4-+ H3O+ • This is known as the ‘nitrating mixture’

8. Nitration • If temperature is kept below 55°C then the product is nitrobenzene • At higher temperatures, further substitutions of the ring give di- and tri- substituted compounds.

9. Sulphonation • Electrophile is sulphur trioxide, SO3. What is its structure? • Requires concentrating sulphuric acid and heating under reflux for several hours

10. Sulphonation • Benzenesulfonic acid is a strong acid which forms salts in alkaline solution. • Most detergents contain salts of this kind with a long alkyl group attached to the benzene ring. • The hydrocarbon part of the molecule mixes with fats, and the ionic part mixes with water.

11. AlCl3 Cl C Cl H Cl Chlorination • A chlorine atom may be substituted in a similar way to a bromine atom. An aluminium catalyst is often used.

12. Chlorination • The aluminium catalyst helps to polarise the chlorine molecule, this produces Cl+ electrophile, which reacts with the benzene ring. • Aluminium chloride reacts violently with water, so the reaction must be carried out under anhydrous conditions.

13. Alkylation by Friedel-Crafts reaction • Requires aluminium chloride catalyst and is carried out under reflux. • Chloroalkanes react with the catalyst to form a complex, R+AlCl4-, where R+ is the electrophile

14. Alkylation by Friedel-Crafts reaction • This type of reaction is known as an alkylation as an alkyl group is introduced to the benzene ring. • Acylation takes place via the same mechanism. • Can you draw the mechanism for the reaction between benzene and ethanoyl chloride? • What about with ethanoic anhydride?

15. The future… • More recently, Friedel-Crafts acylation reactions have been carried out using ‘ionic liquids’ as combined solvent/catalyst systems. • Ionic liquids are liquids at room temperature that contain only ions. • Usually made up of organic cations with either organic or inorganic anions and are highly conductive. • The formula of an ionic liquid can be written as [Q]Cl·2AlCl3 where Q is any of a range of organic cations.

16. The future… These liquids are promoted as a green alternative to conventional solvents because: • Their low volatility reduces emissions • Their low flammability and low toxicity increases safety • The temperatures at which F-C reactions are carried out are often lower than conventional methods • The ionic liquid can be easily recycled, saving resources and money

17. Addition Reactions • Requires SEVERE conditions for benzene to undergo any addition reactions. • Hydrogenation requires a nickel catalyst, 200°C and 30atm pressure.

18. Practise Time… • You need to add the summary diagram from page 286 to your notes and LEARN IT!! • Now have a go at CI 12.4 problems 1-2 and 5-6 to check your understanding.