Benzene Reactions

Benzene Reactions Addition causes a loss of delocalisation. Substitution can occur as the electron density is spread over a larger area. There are two steps to substitution: 1. Addition, 2. Elimination. Addition: The electrophile is attracted to the delocalised electron cloud. This forms a coordinate bond between one of the carbons and the electrophile. An unstable positively charged intermediate is formed.  Substitution: The C-H bond breaks. Its pair of electrons restores the unstable delocalised electron structure and a hydrogen ion is eliminated.  Nitration of Benzene Nitro group NO2 is substituted into benzene in place of hydrogens. Benzene becomes oil yellow liquid nitrobenzene.  Smells of bitter almonds. Overall equation: C6H6 + HNO3 ---> C6H5NO2 + H2O HNO3 isn't the electrophile To make the electrophile NO2, react concentrated sulphuric acid and concentrated nitric acid. This produces a nitronium ion, water and HSO4-. H2SO4 is produced at the end of the reaction by reacting H+ and HSO4-. This is why H2SO4 is the catalyst for the reaction.  The reaction has to occur at 50 degrees otherwise more nitro groups can be added meaning nitrobenzene will not be produced.  Halogenation Needs a halogen carrier as a catalyst AlCl3, FeCl3, AlBr3, FeBr3 Produced by mixing a halogen with the metal, even in the reaction vessel. Such as Br2 + FeBr3 ---> Br+ + FeBr4- The bromium ion is the electrophile The H+ reacts to reform the catalyst. H+ + FeBr4- ---> FeBr3 + HBr Alkylation Add an alkyl group Reacted with a haloalkane Halogen carrier catalyst. AlCl3 ---> Generates an electrophile Process increases number of carbon atoms. It is important for synthesis reactions.  Can be called Friedel-Crafts Alkylation.  AlCl3 + C2H5Cl ---> AlCl4- + +C2H5. +C2H5 is the electrophile AlCl4- + H+ ---> AlCl3 +HCl Acylation Requires a COCl group. Add acyl chloride which forms an aromatic keystone. Creates a C-C bond which is useful in synthesis.  

Additional Information Nitrobenzene is important for a number of reasons: starting material for dyes, pharmaceuticals, and pesticides. It is used as a starting material for paracetamol.  An electrophile is an electron acceptor.  Only room temperature is required for the bromination of benzene and the presence of a halogen carrier. The reason a halogen carrier is required is as benzene is too stable to react with the non-polar bromine molecule. Therefore a bromonium ion has to be generated by reacting Br2 and FeBr3. This is the same principle for chlorination. Comparing the reactivity of alkenes with arenes Alkenes decolourise bromine water, not alkanes. This is an example of an electrophilic addition reaction. A key example of this is the reaction between cyclohexene and bromine. Bromine adds across the double bond.  Alkenes contain a pi bond. This contains localised electrons above and below the plane between the two carbon atoms in the C=C bond. This creates an area of high electron density. These localised electrons induce a dipole in the non-polar molecule. This makes one of the Br atoms in Br2 slightly more positive and the other slightly negative.  The slightly positive Br enables Br2 to act as an electrophile. Benzene doesn't react in this way as it is stable, therefore a halogen carrier is required. 

Summary Questions from Kerboodle 1. a. An electrophile is an electron pair acceptor. b. Substitution is a reaction where an electrophile is attracted to an electron dense centre, which accepts a pair of electrons.  2. A halogen carrier is required. In this case, AlCl3 can be used. AlCl3 reacts with Cl2 to produced Cl+ and AlCl4-. Cl+ reacts with benzene in a substitution reaction as the Cl+ acts as an electrophile. A H+ ions are released from this. The catalyst can be regenerated at the end when AlCl4- reacts with H+, forming AlCl3 and HCl. 3. C6H6 + C3H7Cl ---> C6H5(C3H7Cl) + H+.