3.10 Aromatic Chemistry

• Describe the bonding in benzene.

  Contains 6 carbons. Each carbon forms 3 sigma bonds. Fourth electron is delocalised in the p-orbital which forms a ring above and below by overlapping sideways
• What does the bond lengths in benzene provide evidence for?

  Double bonds are smaller in length than a single bond as the extra overlap of the pie orbitals brings the atoms closer together. In benzene all the bonds have a length of 0.142nm therefore providing evidence against Kekule's structure
• What are some properties of benzene?

  -six p-electrons have a stabilising effect. Increase in stability means that it is difficult for an addition reaction to take place

  -benzene is planar in shape

  -all the bonds are the same length

  -bond angles are all 120 degrees

  -when added to bromine water benzene does not decolourise - can't undergo addition reaction so this can be used as a test to show not Kekule
• Describe the bonding and stability of benzene. (6 marks)

  Bonding:

  -each C has 3 covalent bonds

  -spare electrons in p orbital overlap

  -delocalisation leads to ring

  Shape:

  -planar

  -hexagon/ 6 carbons/ 120 degree bond angle

  Stability:

  -expected of Kekule = -360kJmol-1

  -hydrogenation of benzene is less exothermic by -152kJmol-1

  -benzene is lower in energy therefore more stable
• What makes benzene susceptible to attack by electrophiles?

  Delocalised electrons constitute an electron-rich area
• What catalyst is used to form the nitronium ion (NO2+) in nitration of benzene? Give an equation


  Concentrated sulphuric acid

  HNO3 + H2SO4 -> NO2+ + HSO4- + H2O
• Give an equation to show the reformation of the H2SO4 catalyst.

  HSO4- + H+ -> H2SO4
• What is the overall equation for nitration of benzene?

  C6H6 + HNO3 -> C6H5NO2 + H2O
• what are the reagents and conditions for nitration of benzene?

  Conc nitric acid and conc sulphuric acid

  Warm to 50 degrees
• Why mustn't the reaction rise over 55 degrees?

  Further substitution will occur (additional nitration) which leads to a very explosive compound that is toxic
• What might nitration compounds be used for?

  Manufacture of explosives as they decompose violently when heated

  Manufacture of amines and dyes:

  -nitrobenzene can be reduced to phenylamine by treating it with tiny and conc HCl then heating under reflux

  -phenylamine can subsequently be used to manufacture dyes
• Give an equation to show the reduction of nitrobenzene.


  C6H5NO2 + 6[H] -> C6H5NH2 + 2H2O
• What catalyst is used in friended-crafts acylation? Give an equation

  Anhydrous AlCl3

  
  

  R-COCl + AlCl3 -> AlCl4- + RCO+
• Give an equation to show the catalyst being regenerated.

  AlCl4- + H+ -> AlCl3 + HCl
• Give an overall equation for friedel-crafts acylation.

  C6H6 + RCOCl -> (AlCl3 over the arrow) C6H5C(R)=O + HCl