Aromatic Chem

• Aromatic rings can be substituted by functional groups such as halogens or nitro (-NO2).
• The aromatic ring is planar, with alternating single and double bonds.
• Aromatic compounds are cyclic hydrocarbons that have delocalized electrons.
• The presence of an aromatic ring increases the stability of organic compounds.
• Alkanes are saturated hydrocarbons with single bonds between carbon atoms, while alkenes have double bonds and alkynes have triple bonds.
• Bonding in Benzene 


  Benzene is an aromatic compound consisting of a ring of six carbon atoms with six hydrogen atoms and a ring of delocalised electrons

  • Each bond in the benzene ring has an intermediate length in between that of a double and single bond
• Arenes


  Compound that contain benzene as part of their structure are called aromatic compounds. They have high melting points due to the high stability of the delocalised ring, but low boiling points as they are non-polar molecules and often cannot be dissolved in water
• Electrophilic Substitution

  The delocalised ring in benzene is an area of electron density making it susceptible to attack from nucleophiles

  When these species attack the electron ring, it is partially destroyed then restored in the process of electrophilic substitution, this means that aromatic amines and nitrobenzene can be produced from benzene
• Formation of Nitrobenzene - Electrophilic Addition

  Equation for formation of NO2+ ion (electrophile)


  The electrophile is the NO2+ ion, this is a reactive intermediate produced in the reaction of conc sulfuric acid (H2SO4) with conc nitric acid (HNO3)

  H2SO4 + HNO3 ---> H2NO3+ + HSO4-

  H2NO3+ ---> H2O + NO2+

  When heated with benzene these reagents lead to the substitution of the NO2+ electrophile onto the benzene ring, removing a hydrogen ion

  
• A benzene ring is _______.

  planar, hexagonal with 120 angles
• Thermochemical evidence of benzene


  Benzene was expected to have a value of -360 kj mol using cyclohexene as the standard but instead it came out to be -280kj mol. The benzene ring is more stable than cyclohexa-1,3,5-triene

  
• Benzene current structure is


  C-C are the same

  planar and hexagonal

  Each carbon uses 3 of its outer electrons to form sigma bond, the 1 left is delocalised.
• What makes benzene stable?


  The delocalised electron system ( Aromatic stability)
• Why can't benzene undergo addition reactions?


  That would mean breaking the delocalised electron system ( delocalised energy ).

  The benzene ring is too stable
• What are the 2 electrophilic substitution reactions benzene can undergo?

  Nitration and Friedel Craft
• How to prepare the nitrating mixture?


  HNO3 + 2H2SO4 --> NO2+ + 2HSO4- + H30+
• What's NO2+ called?


  Nitryl cation/ nitronium
• Outline the nitration of benzene. State the role of sulfuric acid

  A catalyst

  
• What are the uses of nitration with benzenes?


  Making explosives such as TNT

  Making dyes from nitrobenzenes --> phenylamines

  Organic synthesis
• Why is Friedel Craft Acylation important?


  Adds a reactive carbonyl functional group, much more reactive and the intermediate to make more compounds
• How to prepare the mixture to obtain CH3CO+


  CH3COCl + AlCl3 --> CH3CO+ + AlCl4 -
• What's the role of AlCl3 in Friedel Craft?Outline the reaction

  A catalyst.

  H+ + AlCl4 - --> AlCl3 + HCl
• Outline the mechanism of Friedel Craft

  
• Why is acylation of methyl benzene much faster than benzene ?


  Methyl group releases electrons

  Delocalised system is much more negative

  Electrophiles more attracted to it.
• What are the regeants to reduce nitroarines into aromatic amines? What kind of reaction is this ?

  Sn and HCl

  
  

  Fe and HCl

  
  

  Reduction reaction
• What is the effect of an -OH group attached to a benzene ring?


  Delocalisation makes C-O stronger and O-H weaker
• What is the effect of an -NH2 attached to a benzene ring?


  Basicity decreases as lone pair overlaps with delocalised electron system and become unavailable for accepting proton
• What is the effect of -Cl attached to a benzene ring?


  C-Cl bond is stronger

  More negative so will repel nucleophiles