Benzene - Chp 25

Benzene - Chp 25

1. Structure and Properties.

   • Colourless, sweet smelling, highly flammable liquid.
   • Found naturally in crude oil, component of petrol and found in cigarette smoke.
   • Classified as carcinogenic.
   • Kekule model.
     • Bezene is a 6C ring joined by alternate single and double bonds.
     • Evidence to disprove.
       • 1. Lack of reactivity.
         • If benzene contained C=C bonds, it should decolourise Br in electrophilic addition reactions.
           • But benzene doesn't do that.
       • 2. Bond lengths.
         • All bond lengths in benzene are the same length, however single and double bonds are different lengths.
           • Benzene length is between that of single and double.
       • 3. Hydrogenation enthalpies.
         • If it had 3 double bonds it should have the hydrogenation enthalpy 3x that of cyclohexene.
           • This is not true.
   • Delocalised model.
     • Each C uses 3 e- for bonding to other Cs and H.
       • The 4th e- in the p-orbital overlaps sideways with the adjacent C above and below the plane.
       • Overlapping creates a system of pi-bonds, which spread over the ring of carbons. The electrons are said to be delocalised.

2. Electrophilic Substitution Reactions.

   • Nitration of benzene.
     • Benzene reacts slowly with nitric acid to form nitrobenzene, catalysed by H2SO4 and heated to 50C.
       • Rise above 50C will cause multiple subs.
     • One H is replaced by NO2.
     • Electrophile is made by nitric acid. HNO3 + H2SO4 -> NO2+ +HSO4- + H2O. Catalyst must be regenerated at the end of the reaction.
   • Halogenation of benzene.
     • Must take place in the presence of a halogen carrier catalyst, e.g. AlBr3 or AlCl3.
     • Bromination - one H is replaced by Br.
     • Benzene is too stable to react with non-polar Br2. Needs Br+, which is formed using the halogen carrier.
       • AlBr3 + Br2 -> AlBr4- + Br+
   • Acylation.
     • One H is replaced by alkyl group.
     • Reaction of benzene with haloalkane and AlCl3.
     • Produces the product and HCl.
   • Alkylation.
     • Benzene reacts with acyl chloride in the presence of AlCl3 catalyst, forming an aromatic ketone.
   • Benzene is not as reactive as alkenes, due the the delocalised ring of electrons, so the electron density is not as high.

3. Phenol.

   • Weak acid, as it partially dissociates in water, forming a phenoxide ion and proton.
   • Less soluble in water than alcohols due to non-polar benzene ring.
   • More acidic than alcohols but less than CAs.
     • Ethanol doesn't react with NaOH (strong) or NaCO3 (weak) bases.
     • Phenols and CAs react with strong bases (NaOH).
       • Forms benzene attached to O-Na+ and water.
     • Only CAs are strong enough to react with weak bases (NaCO3)
   • Electrophilic substitution reactions.
     • More reactive than benzene as the lone pair of e- on O of OH extend into the delocalised ring, this increasing electron density.
   • Bromination.
     • Reacts with aq Br to form a white ppt, carried out at RT without catalyst.
   • Nitration.
     • Reacts with dilute nitric acid at RT.

4. Directing groups.

   • NH2 = 2,4,6.
   • NO2 = 3,5.
   • OH = 2,4,6.