arenes

• arenes are aromatic hydrocarbons like benzene
• an aryl group is a functional group derived from a simple aromatic ring where one hydrogen atom is removed from the ring (C6H5)
• a pi system is a system of connected p orbitals with delocalised electrons in a molecule, which lowers the overall energy of the molecule and increases its stability, the pi electrons do not belong to a single bond or atom but rather to a group of atoms
• an electrophile is a species that accepts a pair of electrons in a mechanism step
• electrophilic substitution is a chemical reaction in which an electrophile displaces a functional group in a compound, typically a hydrogen atom in a benzene ring
• halogen carriers are halogen compounds used during the halogenation , alkylation and acylation of benzene. they speed up electrophilic reactions by acting as a catalyst
• halogen carriers are needed to polarise molecules as benzene is not sufficiently rich in electrons to polarise molecules itself. it does this by pulling off a halogen atom from a molecule generating a positively charged electrophile
• an acyl group is a specific type of carbonyl group with a R group connected to the carbonyl

  
• a nitronium ion is a strong electrophile that is generated when concentrated nitric and sulphuric acids are mixed together and has the formula NO2+

  
• Kekule's model is a model of benzene C6H6, that was proposed in 1865, the carbons are arranged in a hexagon and he suggested alternating single and double bonds

  
• a directing effect is the fact that some groups attached to a benzene ring control whereabouts on the ring electrophilic substitution occurs
• an OH group has a directing effect on the 2 and 4 positions of a benzene ring
• a NO2 group has a directing effect in the 3 position on a benzene ring
• benzene is a colourless, planar and hexagonal shaped molecule
• arenes general formula is CnHn
• arenes empirical formula is always CH
• evidence to prove that Kekule's model is inaccurate:

  • chemical reaction with bromine
  • thermochemical data (enthalpy of hydrogenation)
  • bond length data
• benzene does not decolourise bromine water so it cannot be a triene (3 double bonds) like Kekule suggested as carbon double bonds cause bromine water to decolourise.
• hydrogenation of benzene is less exothermic than than the calculated value for Kekule's model
• the enthalpy change of hydrogenation of Kekule's model is -378 kJ/mol
• the enthalpy change of hydrogenation of benzene is -208 kJ/mol
• the differences between the enthalpy change of hydrogenation of Kekule's model and benzene is the enthalpy of stabilisation which is -170 kJ/mol

  
• x-ray diffraction crystallography can be used to give bond length data
• a C=C has a bond length of 0.134 nm
• a C-C bond has a bond length of 0.154 nm
• the actual bond length of benzene is 139 pm which is between C-C and C=C
• all the bonds in benzene are the same length so Kekule's model cannot be correct as there cannot be alternating single and double bonds as they are different lengths
• in benzene the carbon atoms in the ring are bonded to each other and their hydrogen atoms with sigma bonds, each carbon atom has one electron left in its p orbital
• in benzene the p orbitals overlap sideways to form a pi system which spreads over all 6 carbon atoms this makes the electrons delocalised. this delocalisation gives benzene its extra stability as electrons tend to repel one another so a system in which they are far apart from one another as possible will give benzene the greatest stability
• to achieve maximum overlap of p orbitals the molecule must be planar and the pi system means there is a high region of electron density above and below the ring which can attract electrophiles

  
• the addition reaction of bromine to cyclohexene readily occurs and it causes bromine water to be decolourised
• the addition reaction of bromine to benzene is very difficult as the pi system is very stable and requires too much energy to breaks so it undergoes electrophilic substitution
• steps of electrophilic substitution with benzene:

  1. generation of electrophile
  2. electrophile attaches to benzene which forms an intermediate ion
  3. H+ is lost from the intermediate and the delocalised pi ring system is reformed
  4. regeneration of the catalyst
• the generation of an electrophile for electrophilic substitution with benzene requires a catalyst called a halogen carrier
• to polarise benzene a full positive charge is needed rather than a partial charge as a result of an induced dipole, a partial charge is not strong enough to polarise the benzene ring
• 3 types of electrophilic substitution with benzene:

  • halogenation
  • alkylation
  • acylation
• example of halogenation step 1:

  Br2 + FeBr3 -> Br+ + FeBr4- (generation of electrophile Br+)
• example of halogenation steps 2 and 3:

  A) H+ ion produced

  B) C-H bond breaks and completes broken delocalised ring

  C) electron pair comes from pi system

  D) carbon becomes positively charged

  
• example of halogenation step 4:

  FeBr4- + H+ -> HBr + FeBr3 (iron bromide catalyst regenerated)
• a Friedel-Crafts reaction increases the carbon chain length by forming C-C bonds which is known as alkylation