Electrophilic substitution

• Aromatic compounds are attacked by electrophiles because the aromatic ring is very electron rich due to the cloud of electrons above and below the ring
• Aromatics undergo substitution reactions where hydrogen atoms on the ring are replaced
• Aromatics do not readily undergo addition reactions as they would lose their delocalisation and therefore stability
• Electrophilic substitution by nitriles are used to make aromatic amines and explosives
• In nitration, a hydrogen is replaced by a nitro group (NO2)
• Nitration occurs at 50 degrees with the reagents concentrated HNO3 and concentrated H2SO4
• The electrophile in nitration is NO2+ (nitronium ion)
• Friedel-Crafts acylation makes aromatic ketones. It is very useful for adding carbon atoms to aromatic rings
• The reagents in Friedel-crafts acylation are Acyl chlorides or acid anhydrides (both with AlCl3)
• Friedel-Crafts acylation must be kept anhydrous to prevent the reaction of AlCl3
• When electrophilic substitution occurs with an acylium ion, it is a Friedel-Crafts acylation
• The electrophile in Friedel-Crafts acylation is RCO+ (acylium ion)
• Benzene has a lower electron density than alkenes, so benzene does not induce as large of a dipole within alkenes, so the alkenes acts as a worse electrophile
• Only under certain conditions can benzene react with bromine, and only via substitution, to form bromobenzene
• bromine cations are stronger electrophiles than bromine so can react with benzene
• Halogen carriers can react with bromine to form a bromine cation
• Aluminium bromide and iron bromide are examples of halogen carriers that can react with bromine
• A reaction with a halogen carrier forms a stronger electrophile
• During electrophilic substitution, a carbon forms four bonds, meaning it no longer has a free pi bond and the ring of delocalised electrons is broken. This is much less stable
• The halogen carrier breaks the carbon - hydrogen bond and is reformed when its spare halogen bonds with the hydrogen
• The halogen carrier in electrophilic substitution acts as a catalyst
• When benzene undergoes substitution, its delocalised electron ring is only broken temporarily. If it underwent an addition reaction, it would be broken permanently
• Friedel Crafts acylation is an example of electrophilic substitution
• The typical halogen carrier used in friedel-crafts acylation is aluminium chloride
• Aluminium chloride removes the chorine from an acyl chloride, to form a strong electrophile
• The general reaction for Friedel Crafts acylation is:

  C6H6 + RCOCl -> C6H5COR + HCl

  (r represents an alkyl group)
• The equation for the regeneration of the catalyst in friedel crafts acylation is:

  AlCl4- + H+ -> AlCl3 + HCl
• In the nitration of benzene, the catalyst is concentrated sulfuric acid
• In the nitration of benzene, both the nitric acid and sulfuric acid must be concentrated for the maximum amount of molecules
• Nitronium ion (NO2+) acts as the electrophile in the nitration of benzene
• The reaction for the nitration of benzene is:

  C6H6 + HNO3 -> C6H5NO2 + H2O
• We can generate a nitronium ion by reacting concentrated nitric acid with concentrated sulfuric acid
• sulfuric acid is a stronger acid than nitric acid so can donate a proton
• When acids are dilute, they are in solution. A lot of water means the equilibrium position for the nitration of benzene will move to the left, reducing the yield of the electrophile. Also, the electrophile can react with water instead of benzene