Aromatic compounds

• What is benzene?


  A cyclic, planar molecule with the general formula C6H6.
• What is Benzene's structure?


  Each carbon atom is bonded to 2 other carbon atoms and 1 hydrogen atom.

  The final lone pair is in a p-orbital which sticks out above and below the planar ring.
• What do the lone pairs in benzene do?


  The lone pairs in p-orbitals combine to form a delocalised ring of electrons.
• What does Benzenes delocalised electron structure do?


  Cause all C-C bonds to be the same and same length (139pm).

  The C-C bond length lies between 154pm (c-c) and 134pm (c=c).
• How do we measure stability?


  By comparing enthalpy of hydrogenation (adding on H2 atom).
• What is benzene's stability compared to cyclohexa-1,3,5-triene?


  More stable as more energy is required to break the benzene bonds due to its delocalised electrons.

  Its enthalpy change is -208kjmol-1 compared to -360kjmol-1.
• What are aromatic compounds?


  Molecules containing a benzene ring.
• What is an Arene?


  Molecule that contains a benzene ring
• What is Phenol?


  An arene with an OH- group attached.

  
• What is phenylamine?


  An arene containing an NH2- group.

  
• What type of mechanism are arenes?


  Electrophilic substitution reaction.

  Benzene's high electron density is attractive to electrophiles.

  The hydrogen or functional group is substituted for an electrophile.
• Friedel-Crafts Acylation?


  An acyl group is added onto benzene to make it weaker so we can react benzene.

  Heat reactants under reflux in dry ether.
• Why do we use a halogen carrier?


  AlCl3 acts as a catalyst accepting a lone pair leaving a carbocation, producing a stronger electrophile to react with benzene.

  
• Nitration of benzene?


  Heat benzene with conc HNO3 and H2SO4 to form nitrobenzene.

  
• What are Amines?


  Derived from ammonia molecules.

  All contain a Nitrogen atom where H atoms are replaced by an organic group.

  There are different types of amine: Primary, secondary, tertiary, quaternary and aromatic.
• what are non aromatic amines known as?


  Aliphatic amines
• What are cationic surfactants?


  Quaternary ammonium salts are used to make cleaning products eg shampoo, detergents.

  They are made from a positive end and attracted to negative ions.

  They have a long hydrocarbon tail.
• How does water and oil mix using cationic surfactants?


  Non-polar tail is attracted to grease.

  Polar head is attracted to water.

  This allows water and oil to mix, removing grease.
• Why is cationic surfactants used in conditioner and fabric softeners?


  They have a positive ammonium ion attracted to negatively charged fibres and hair. This removes static.
• Are amines a base?


  Yes they have a lone pair of electrons allowing them to accept a proton.
• How does a proton bind to an amine?


  Via a dative bond.

  Both electrons originate from the Nitrogen.
• What does the strength of base depend on?


  The availability of the lone pair. The higher the electron density, the more readily available electrons are.
• What is the strongest base?


  Aliphatic amines as the alkyl groups push electrons towards the Nitrogen, increasing the electron density, so lone pair availability increases.
• What is the weakest base?


  Aromatics as benzene is an electron withdrawing group pulling electrons away from Nitrogen. Electron density decreases at Nitrogen so lone pair availability is reduced.
• Reacting halogenoalkanes with EXCESS ammonia= primary amine?


  Negative: reaction produces secondary, tertiary, quaternary salts too so we have an impure product.

  This occurs as the primary amine nitrogen still has a lone pair to act as a nucleophile to react with any remaining halogenoalkanes.

  2CH3NH2 + CH3CH2Cl > CH3NHCH2CH3 + CH3NH3+CL-

  
• Reducing nitriles with H gas and nickel catalyst to make amine?


  Cheapest way to make primary amines.

  Catalytic hydrogenation reaction (produces only primary amines).

  Nickel catalyst, high temp/pressure.

  R-CH2-C=N + 2H2 > R-CH2-CH2-NH2
• Reducing nitriles using LiAlH4 and dilute acid to make amine?


  More expensive.

  Reduction reaction and is dissolved in dry ether.

  R-CH2-C=N + 4[H] > R-CH2-CH2-NH2
• Reducing nitro compounds to make aromatic amine?


  Aromatic amines are used to make dye stuffs and pharmaceuticals.

  Heat nitrobenzene with concentrated HCl and tin to =salt.

  The salt is reacted with alkali to produce aromatic amine.

  
• What are Amides?


  Derivatives of carboxylic acids.

  Functional group= -CONH2
• How to make NO2+ electrophile?

  
• What can nitro compounds be used for?


  They can be reduced forming aromatic amines used in dyes and pharmaceuticals.

  Nitro compounds decompose when heated, so used in explosives eg 2,4,6-trinitromethylbenzene (TNT).
• Recrystallisation practical step analysis?


  Minimum quantity of hot water= ensures saturated solution.

  Flask cools before filtering crystals= yield will increase.

  Crystals compressed in funnel= Air passes through sample not just around it.

  Cold water poured through crystals= Wash away soluble impurities.
• In recrystallisation what impurity would cause melting point to be lower than data book?


  Water.

  Ensure to press crystal sample between filter paper to dry.
• Calculate % yield?


  Actual yield / theoretical yield

  Answer X100.
• Why is benzene more stable than cyclohexa-1,3,5-triene?


  Because of its delocalised ring of electrons.
• Aromatic bond lengths?


  C-C bond= 154pm

  C=C bond= 134pm

  Benzene C-C bonds= 139pm
• Mechanisms with benzene?


  Electrophilic substitution as benzenes high e- density is attractive to electrophiles.
• Reducing benzene to cyclohexane?


  Reducing agent= H2.

  Bond angles in benzene= 120.

  Bond angles in cyclohexane= 109.5
• Ketone to alcohol.


  Reducing agent= NaBH4

  Nucleophilic addition.

  
• Aluminium chloride as a catalyst?


  eg Ch3COCl + AlCl3 > Ch3CO+ + AlCl4-

  AlCl4- + H+ > AlCl3 + HCl