Chapter 25

• Structure of benzene


  6 C atoms in a hexagonal ring, with alternating single and double C-C bonds

  
• Structure of benzene


  • Suggests it reacts in same way as unsaturated alkene, but not the case
  • Bond angle for C-C and C=C is different but in benzene the lengths are equal

  
• Formation of benzene structure

  1. Each carbon atom in the ring forms 3 pi bonds using the sp2 orbitals

  2. The P orbitals overlap with other p orbitals forming a pi system

  3. The pi system made by overlap of p orbitals results in electrons being delocalised and able to freely spread making pi system - 2 ring one above plane, one below

  
• Benzene and other aromatic compounds are regular and planar compounds with 120 degree bond angles

  
• Delocalisation of electrons means that all C-C bonds in these compounds are identical and have both single and double bond characteristics

  
• Bonds in benzene are the same length - is evidence for delocalised ring structure

  
• Hydrogenation of cyclohexane


  Each molecule has one C=C double bond, enthalpy change is -120 kJ mol-1

  
• Hydrogenation of benzene (Kekule structure)

  Benzene has three double C=C bonds, so expected enthalpy change would be 3 x -120 kJ mol-1 = -360 kJ mol-1

  
• But when benzene reacts with hydrogen, the enthalpy change is less exothermic at -208 kJ mol-1, indicating benzene is more stable and needs less energy

  
• Aromatic


  Refers to pleasant fragrant, in chemistry used to describe the molecule benzene
• Benzene has many uses - pharmaceuticals, pesticides, polymers and dyes
• Alkenes undergo bromination easily


  Cyclohexane reacts with Br2 to form 1,2-dibromocyclohexane by electrophilic addition

  
• In benzene there is no localised areas of high electron density, preventing it from being polarised by the bromine molecule

  
• For benzene to undergo electrophilic substitution with bromine, a halogen carrier like AlBr3 must be present

  
• Electrophilic substitution reactions of benzene

  Involve the replacement of the 6 hydrogen atoms from the benzene ring, which is different to the reaction of alkenes

  
• Electrophile


  An atom or group of atoms that attacks areas of high electron density and accepts a pair of electrons to form a new covalent bond
• The delocalised pi system in benzene is very stable with high electron density

  
• Electrophilic substitution reaction


  Involve a positive electrophile or positive end of a polar molecule
• Many electrophiles can react with benzene but certain conditions and reagents are required for the reaction to occur
• Mechanism of benzene nitration

  1. Generation of the electrophile NO2+

  2. Electrophilic attack

  3. Regenerating aromaticity

  
• Nitration of benzene is an example of electrophilic substitution where the hydrogen is replaced by the nitro group -NO2

  
• The delocalisation of electrons (aromatic stabilisation) is the main reason for substitution over addition reactions in arenes

  
• In substitution reactions, the aromaticity is restored, while in addition reactions the aromaticity is not restored and can be completely lost
• Halogenation of benzene


  1. Metal halide carrier (e.g. FeBr3, AlCl3) acts as a catalyst to create the electrophile X+

  2. Overall equation: C6H6 + X2 → C6H5X + HX

  
• Friedel-Crafts acylation


  Substitution of an acyl group (alkyl group containing a carbonyl) into the benzene ring, using a metal halide (e.g. AlCl3) catalyst to generate the necessary alkyl electrophile

  
• Aromatic compounds


  React with halogens in presence of metal halide carrier (e.g. iron (III) bromide, aluminium chloride)
• Reaction of metal halide carrier

  1. Acts as a catalyst and creates Electrophile X+ (X is halogen atom)

  2. Regenerated at the end

  
• Metal halide carriers


  • AlCl3
  • FeBr3

  
• Overall equation for halogen addition to benzene

  C6H6 + X2 → C6H5X + HX

  
• One hydrogen on benzene ring is substituted with 1 halogen atom, therefore HX is the product

  
• Halogen addition to benzene follows the same general mechanism
• Halogen addition to benzene occurs at room temperature (25°C)

  
• Friedel-Crafts Acylation


  Substitution of an acyl group into the benzene ring

  
• Acyl group


  An alkyl group containing a carbonyl
• Friedel-Crafts Acylation


  1. Benzene ring reacts with acyl chloride in the presence of an AlCl3 catalyst

  2. Generates the necessary alkyl Electrophile

  
• Friedel-Crafts Acylation


  • Reaction of methylbenzene with propanoyl chloride to form an acyl benzene

  
• The acyl group is on the 4 position due to the -CH3 group on the benzene

  
• Phenol


  OH is directly bonded to the benzene, not an alcohol, and the reactivity is different to alcohol

  
• Phenol


  Very weak acid
• Phenol dissociation


  C6H5OH + H2O ⇌ C6H5O- + H3O+