aromatic chemistry

benzene molecular structure and bonding

• C6H6

• cyclic planar molecule

• 120 degree bond angle

• each carbon has 3 covalent bonds

• all C-C bonds are the same length

• 0.140nm bond length

• spare electrons in p orbital overlap to form layers

kekulè structure and bond length

intermediate bond length

why is benzene more stable?

because of ring of delocalised electrons

hydrogenation of benzene and cyclohexatriene

-120×3= -360

-360-(-208)= -152

• expected ΔH hydrogenation of cyclohexatriene: -360 kJmol⁻¹

• ΔH hydrogenation of benzene is less exothermic by 152kJmol⁻¹

• so benzene is more stable than cyclohexatriene

  • due to ring of delocalised e-

enthalpy of hydrogenation of cyclohexa-1,3-diene and cyclohexa-1,4-diene

benzene vs phenyl naming

Where is the functional group?	Naming approach
On the alkyl chain (not directly on benzene)	Phenyl- as a substituent with exceptions of carboxylic acid and aldehyde
Directly on benzene ring	Named as substituted benzene (e.g. benzoic acid, nitrobenzene)

Reactions of benzene

• Benzene does not generally undergo addition reactions because these would involve breaking up the delocalised system

• Benzene has a high electron density and so attracts electrophiles

  • It reactions are usually electrophilic substitutions

nitration of benzene

• change in functional group: benzene —> nitrobenzene

• reagents: conc. HNO3 and conc. H2SO4

• mechanism: electrophilic substitution

• electrophile: NO₂⁺

• NOTE: THE + MUST BE ON THE N, NOT THE O2 OR U WILL LOSE MARKS

equation for formation of electrophile: HNO₃ + 2H₂SO₄ —> NO₂⁺ + 2HSO₄⁻ + H₃O⁺

The mechanism for the formation of the electrophile

mechanism for nitration of benzene

• This reaction with benzene is done at 60 degrees

• On using higher temperatures a second nitro group can be substituted.

Reducing a nitroarene to aromatic amines

• Mechanism: reduction

• Reagent: Sn/Fe and HCl

• Conditions: Heating

As the reaction is carried out in HCl, the ionic salt C₆H₅NH₃⁺Cl^- will be formed, which is soluble in water

friedel-crafts acylation

• Change in functional group: benzene —> phenyl ketone

• Reagents: acyl chloride in the presence of anhydrous aluminium chloride catalyst

• Conditions: heat under reflux

• Mechanism: Electrophilic substitution

Equation for Formation of the electrophile:

AlCl₃ + CH₃COCl —> CH₃CO⁺AlCl₄ ⁻

mechanism for friedel-crafts acylation