Electrophilic Substitution Reactions …Reactions of the benzene ring

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.

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

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.

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-

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

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’

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.

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

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.

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

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.

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

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?

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.

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

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

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.