Aromatic Compounds

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Structure Of Benzene

  • Problems with Kekulé structure of Benzene:
    -Low reactivity: if C=C bonds were present then benzene would react with Br2, similar to alkenes, decolourising bromine water - which doesn't occur.
    -Also Kekulé structure contains both C=C and C-C alternating bond, whereas methods have shown that all bond lengths are equal and with length between both the previous.
    -Hydrogenation: the three C=C proposed bonds would have a hydrogenation enthalpy of -360kJmol-1 whereas the actual enthalpy is -208kJmol-1 which is less than expected.
  • Therefore a delocalised model of benzene was produced, which has the following features:
    -Benzene is a cyclic hydrocarbon with 6C's and 6H's.
    -The 6C's form a planar hexagonal ring.
    -The shape around each C is trigonal planar with bond angle of 120 degrees.
    -Each C has 4 outer shell electrons, 3 of which are bonded and 1 is not. The 3 form sigma bonds and the other 1 resides in a 2p orbital above and below the plane.
    -The electrons in the p-orbitals overlap and form a ring of electron density above and below.
    -This overlap produces a system of pi-bonds, which spread over all 6C's; forming a delocalised ring. 
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  • Just to illustrate how the orbitals overlap above and below the plane. 
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Benzene's Reactions

  • Under normal conditions Benzene does not:
    -Decolourise bromine water
    -React with strong acids such as HCl
    -React with halogens
  • Instead of taking part in addition reactions, Benzene takes part in substitution reactions; which retains the delocalisation and the maintains stability.
  • Typically benzene reacts with electrophiles, taking part in electrophilic substitution.
    -To recap an electrophile is an atom or group that is attracted to an electron-rich centre; and accepts a pair of electrons to form a new covalent bond.
  • Benzene can be nitrated as such:
    -C6H6 + HNO3 -> C6H5NO2      by using a H2SO4 catalyst at 50 degrees C under reflux.
  • Methylbenzene can also be nitrated to form TNT. Fun fact woo.
  • Although benzene does not react with halogens alone, it can with the presence of a halogen carrier:
    -C6H6 + Cl2 -------- FeCl3/AlCl3--------> C6H5Cl + HCl
    -This is the same for Br2 etc, and thus the carrier changes accordingly (ie. FeBr3) 
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Reaction Mechanism

  • Electrophilic substitution reactions occur in benzene chemistry because of the delocalised ring above and below the plane:
    -The electron-dense ring attracts the electrophile, which accepts a pair of e- and forms a covalent bond.
    -The intermediate forms that contains both the electrophile and the H atom that is being substituted; the intermediate is unstable though.
    -The unstable intermediate rapidly loses H as H+ and stability is restored.
  • Reaction mechanim:
  • The curly arrows show the movement of an e- pair to form a new covalent bond. 
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Nitration Mechanism

  • A mixture of concentrated HNO3 and H2SO4 is used.
  • The sulfuric acid is needed to generate an electrophile from the nitric acid, which is the nitryl cation and has the formula NO2+.
  • HNO3 + H2SO4 -> NO2+ + HSO4- + H2O
    and reforms: H+ + HSO4- -> H2SO4
    (and thus is a catalyst) 
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Reactivity Of Cyclohexene

  • Cyclohexene and bromine water:
    -Acting as a typical alkene and the Br forms across the double bond.
  • Cyclohexane:
    -The Br2 is non-polar but the pi-bond (C=C) contains localised electrons above and below the 2 C's, producing an area of high electron density.
    -This induces a dipole in the Br-Br bond; making it polar. 
    -The pi-electron pair from the C=C bond is attracted to the positive end of Br2
    -Causing the C=C bond to break and a new bond to be formed, forming a carbocation
    -Finally the Br- ion is attracted towards the intermediate carbocation; forming 1,2-dibromocyclohexane.
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Benzene And Bromine

  • Benzene does not react with just Br, requiring Fe filings to catalyse.
  • Because:
    -Benzene has delocalised pi-electrons spread over all 6C's, whereas Cyclohexene has localised pi-electrons above and below the 2C's.
    -Thus benzene has a lower electron density.
    -When a non-polar molecule such as Br approaches the benzene ring there is insufficient electron density to cause necessary polarisation in Br2.
    -A halogen carrier is required to make a more powerful electrophile, Br+


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  • Class of hydroxyl, -OH, attached to a benzene ring directly.
  • Phenol is a pink solid crystal at room temperature; however is is less soluble than most alcohols due to the presence of a benzene ring.
  • Reactions with salts:
    -Dissolves in H2O to form a weak acidic solution by losing H+ from the -OH group:
    C6H5OH + aq <-> C6H5O- + H+
    -Phenol is neutralised by aq. NaOH to form a salt:
    C6H5OH + NaOH -> C6H5O-Na+ + H2O
  • Reactions with Na:
    -Metal effervesces forming H2(g), the organic product is C6H5O-Na+ which is a salt:
    2C6H5OH + 2Na -> 2C6H5O-Na+ + H2
  • Phenol reacts with (3)Br2 without the need for a halogen carrier at room temperature, this is due to the O, from which a lone pair dissociate into the delocalised ring. This activates the ring and so the increased electron density polarises Br2 upon proximity.
  • Phenol is used for: detergents, antiseptics, disinfectants, preparation of aspirin, epoxy resins 
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