Aromatic Chemistry

• Prior information:

  • Benzene was first isolated by Michael Faraday in 1825.
  • In 1865, Kekulé suggested a cyclic structure for Benzene with three alternating carbon-carbon double bonds. Suggests benzene's low reactivity was due to rapid equilibrium between two isomers, rapid changing prevents reactions.
• Issues with Kekulé's Model:

  • One produces one isomer but should have many.
  • C-C (0.153nm) and C=C (0.134nm) have different bond lengths, therefore benzene would be an irregular hexagon.
  • Cyclohexene = ΔH of -120 kJ mol-1, therefore we would predict Kekulé’s Model (Cyclohex-1,3,5-triene) to have ΔH of -360 kJ mol-1. In reality benzene has a ΔH of -208 kJ mol-1
• Delocalised Model: MOST RECENT CORRECT MODEL

  • Benzene is a planar, hexagonal molecule of six carbon atoms.
  • All carbon-carbon bond lengths are intermediate in length between that of a C-C and C=C.
  • Each carbon uses three of its outer electrons to form three sigma bonds to two other carbon atoms and one hydrogen atom, leaving each carbon atom with one electron in a p-orbital at a right angle above and below the carbon atom, creating a ring of delocalised π-electrons, making it stable and allowing equal bond length.
• Electrophilic substitution:

  The delocalised ring in benzene is an area of high electron density making it susceptible to attack from electrophiles. Benzene undergoes substitution reactions, where one or more of the hydrogen atoms will be replaced by another atom or group.
• Electrophilic Substitution:

  1. Pair of electrons from the delocalised ring form a bond with the electrophile, breaking the electron ring. Producing a highly unstable intermediate.
  2. C-H bond breaks in intermediate, and the two electrons in the bond move back into the pi electron system, reforming the stable delocalised electron ring. Hydrogen ion are lost.

  A) Electrophile

  B) Unstable intermediate

  
• Nitration:

  Nitro group (NO2) replaces one of the hydrogen atoms.

  Conditions= A mixture of concentrated nitric and sulfuric acid, at 50°C.

  1. Conc. H₂SO₄ donates a proton to Conc. HNO₃, forming an intermediate of H2NO3+, that decomposes to produce NO2+, which act as the electrophile.
  2. Pair of electrons from the delocalised ring form a bond with the nitronium ion, breaking it and producing an unstable intermediate.
  3. C-H bond breaks in intermediate, and the two electrons in the bond move back into the pi electron system, reforming the delocalised ring.

  A) Nitronium ion

  B) Unstable intermediate

  
• Nitration: relevant equations

  • H2SO4 + HNO3 ⇄ HSO4+ H2NO3+
  • H3NO3+→NO2+ + H2O

  = 2H2SO4 + HNO3 → 2HSO4– + H3O+ + NO2+

  • H2SO4  + H2O → HSO4- + H3O+
• Products and uses of nitration:

  • Nitration is often one of the steps of organic synthesis.
  • Nitrobenzene (yellow, oily liquid) is used for explosives, such as TNT
  • Formation of amines, Nitrobenzene can be reduced to phenylamine, used in the production of dyes
• Acylation:

  Acylation or Friedel-Crafts reactions are electrophilic substitution reactions in which an acyl group is attached to the delocalised ring, replacing a hydrogen atom. Benzene can be acylated using acyl chloride, in the presence of aluminium chloride, which acts as a catalyst. Conditions must be anhydrous to prevent its hydrolysis.
• Acylation:

  1. Acyl chloride is polarised by the aluminum chloride to generate the electrophile required.
  2. Pair of electrons from the delocalised ring form a bond with C+ in acyl chloride, producing an unstable intermediate.
  3. C-H bond breaks in the ring, delocalised ring reforms.
  4. Hydrogen ion reacts with AlCl4– , regenerating aluminum chloride, the catalyst.
• Acylation: relevant equations

  Step 1 - Formation of electrophile

  • R-Cl + AlCl3 → Rδ+–Clδ- ---AlCl3
  • R-Cl + AlCl3 → R+ + AlCl4– 

  Step 4 - Regenerating catalyst

  • H+ + AlCl4– → AlCl3 + HCl
• Acylation or Friedel-Crafts reactions are important steps in synthesis
• Amines are derivatives of ammonia, where hydrogen atoms have been replaced by alkyl groups. 

  Types of amines:

  • Primary amines=Contains one alkyl group
  • Secondary amines=Contains two alkyl groups
  • Tertiary amines=Contains three alkyl groups
  • Four alkyl groups attached to a nitrogen atom are quaternary ammonium salts, not amines.
• Preparation of Primary aliphatic amines: Reaction of ammonia with halogenoalkanes

  • Condition: Heat in a sealed flask with excess ammonia in ethanol.
  • Overall equation - R-X + 2NH2 → R-NH2 + NH4X

  1. Lone pair on N attacks δ+ C in halogenoalkane, bond between halogen and C breaks and H-N bond breaks
  2. H is substituted for NH3.
  3. Another ammonia molecule attacks the N+, breaking H-N bond, producing NH4.

  • An issue with this reaction is the product could undergo further substitution, producing a mixture of products.
• Preparation of Primary aliphatic amines: Reduction of nitrile compounds 

  • Condition: Hydrogen in the presence of a nickel catalyst.
  • R-C☰N + 4[H] → R-CH2NH2

  OR

  • Condition: Reducing agent of LiAlH4
  • R-C☰N + 4[H] → R-CH2NH2

  = Preferred method because only one amine is formed.
• Preparation of aromatic amines: Reduction of nitro compounds

  Aromatic amines are prepared by reducing nitro compounds. Phenylamine is prepared by reducing nitrobenzene, using tin and concentrated hydrochloric acid as a reducing agent. 

  • Condition: Heat under reflux with tin and excess concentrated hydrochloric acid, followed by adding concentrated sodium hydroxide.
  • RNO2 + 6[H] → RNH2 + 2H2O
• Aromatic amines are used in the manufacture of dyes.
• Base strength of amines:

  • Amines are weak bases.
  • Base strength depends on how well the N lone pair can accept H+. The higher the electron density of the N lone pair, the stronger the base. Therefore, a tertiary amine is a stronger base than a primary amine because they have more electron density.
  • The more alkyl groups, the greater the inductive effect (acyl groups crowd N, preventing it from accepting a proton) pushing electron density onto the N lone pair.

  More acyl groups, more stable, more electron dentistry, therefore stronger base.

  = 3y > 2y > 1y > NH3 > aromatic