Electrophilic substitution reactions of benzene

    Cards (32)

    • Benzene and its derivatives undergo substitution reactions in which a hydrogen atom on the benzene ring is replaced by another atom or group of atoms
    • Benzene typically reacts with electrophiles and most of the reactions of benzene proceed by electrophilic substitution
    • Benzene reacts slowly with nitric acid to form nitrobenzene. The reaction is catalysed by sulfuric acid and heated to 50 degrees to obtain a good rate of reaction. A water bath is used to maintain the steady temperature
    • If the temperature rises above 50 degrees, further substitution reactions may occur leading to the production of dinitrobenzene - this shows the importance of temperature control in the preparation of organic compounds.
    • Nitrobenzene is an important starting material in the preparation of dyes, pharmaceuticals and pesticides. IT can be used as a starting material in the preparation of paracetamol.
    • The reaction involves an electrophile - however nitric acid is not the electrophile involved in the mechanism
    • The electrophile involved is the nitronium ion, NO2+, produced by the reaction of cconcentrated nitric acid with concentrated sulfuric acid
    • In step 2, the electrophile, NO2+, accepts a pair of electrons from the benzene ring to form a dative covalent bond. The organic intermediate formed is unstable and breaks down to form the organic product, nitrobenzene and the H+ ion. A stable benzene ring is reformed
    • Finally the H+ ion formed in step 2 reacts with the HSO4- ion from step 1 to regenerate the catalyst, H2SO4
    • Step 1: HNO3 + H2SO4 = NO2+ +HSO4- + H20
    • Step 3: H+ + HSO4- = H2SO4
    • Halogenation of benzene: the halogens do not react with benzene unless a catalyst called a halogen carrier is present
    • Common halogen carriers = FeCl3, AlCl3, AlBr3, FeBr3 - which can be generated in situ (in the reaction vessel) from the metal and the halogen
    • Bromination of benzene: at RTP and in the presence of a halogen carrier, benzene reacts with bromine in an electrophilic substitution reaction
    • In bromination, one of the hydrogen atoms on the benzene ring is replaced by a bromine atom
    • Benzene is too stable to react with a non-polar bromine molecule. The electrophile is the bromonium ion Br+, which is generated when the halogen carrier catalyst reacts with bromine in the first stage of the reaction
    • In step 2, the bromonium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond. The organic intermediate is unstable and breaks down to form the organic product, bromobenzene and an H+ ion
    • Finally, the H+ formed in step 2 reacts with the halogen carrier ion from step 1 to regenerate the halogen carrier catalyst
    • step 1: Br2 + FeBr3 = FeBr4- + Br+
    • step 3: H+ + FeBr4- = FeBr3 + HBr
    • Chlorination of benzene: chlorine will react with benzene in the same way as bromine and following the same mechanism. The halogen carrier used is FeCl3, AlCl3 or iron metal and chlorine, which react to make FeCl3
    • Alkylation reactions: the alkylation of benzene is the substitution of a hydrogen atom in the benzene ring by an alkyl group
    • The reaction is carried out by reacting benzene with a haloalkane in the presence of AlCl3, which acts as a halogen carrier catalyst, generating the electrophile.
    • Alkylation increases the number of carbon atoms in a compound by forming C-C bonds
    • The reaction is sometimes called a Friedel-Crafts alkylation after the two chemists who first carried out the reaction
    • Acylation reaction: when benzene reacts with an acyl chloride in the presence of an AlCl3 catalyst and aromatic ketone is formed. This is called an acylation reaction and is another example of electrophilic substitution, the reaction forms C-C bonds and is again useful in organic synthesis.
    • Ethanoyl chloride, CH3COCl is the first member of the acyl chloride homologous series. Phenylethanone is produced in the reaction between benzene and ethanoyl chloride - used in the perfume industry
    • Comparing the reactivity of alkenes with arenes:
      • alkenes decolourise bromine by an electrophilic addition reaction
      • e.g. cyclohexane and bromine
    • in this reaction - bromine adds across the double bond in cyclohexane:
      1. the Pi-bond in the alkene contains localised electrons above and below the plane of the 2 carbon atoms in the double bond. this produces an area of high electron density
      2. the localised electrons in the pi-bond induce a dipole in the non polar bromine molecule making one bromine atom sightly positive and the other slightly negative
      3. the slightly + bromine atom enables the brome molecule to act as an electrophile
      electrophilic addition
    • unlike alkenes, benzene does not react with bromine unless a halogen carrier is present:
      • bc benzene has delocalised pi-electrons spread above and below the plane of the carbon atoms in the benzene ring structure
      • the electron density around any 2 carbon atoms in the benzene ring is less than that in a C=C bond in an alkene
      • when a non-polar molecule such as bromine approaches the benzene ring, there is insufficient pi-electron density around any 2 carbon atoms to polarise the bromine molecule - prevents any reaction taking place
    • the mechanism for the reaction of bromine with benzene in the presence of a halogen carrier is electrophilic substitution