benzene

structure of benzene

• molecular formula: C₆H₆

• empirical formula: CH

• unsaturated hydrocarbon

cyclohexatriene

• also known as ‘Kekulé structure’

• alternating double bonds and single bonds

physical evidence for benzene

• bond lengths: x-ray diffraction shows that all carbon-carbon bonds in benzene are the same length (C

• number of isomers: kekulé structure predicts 1,2-disubstituted isomers because substituents could be attached across either a single or a double bond, however only one 1,2-disubstituted isomer exists - this supports delocalised structure of benzene

chemical evidence for benzene (thermochemical data)

• enthalpy of hydrogenation used to provide evidence

• cyclohexene (1 double bond) enthalpy of hydrogenation is −119 kJ mol⁻¹

• Kekulé (3 double bonds) enthalpy of hydrogenation is −119 kJ mol⁻¹ x 3 = −357 kJ mol⁻¹

• benzene enthalpy of hydrogenation is only −207 kJ mol⁻¹, more stable than Kekulé by 150 kJ mol⁻¹ due to delocalised electrons

chemical evidence for benzene (reaction type)

• if benzene had alternate double and single bonds, it should undergo addition reactions as an unsaturated compound (like alkenes)

• instead benzene more commonly undergoes substitution

true structure of benzene

• hexagonal planar structure

• each carbon atom forms 3 single bonds with 3 of its electrons

• 4th electron of each carbon atom overlaps with neighbouring carbon atoms to form a continuous delocalised pi-bond, 6 p-electrons give the molecule extra stability

• bond order (number of covalent bonds between 2 atoms) is 1.5

• benzene is the simplest aromatic hydrocarbon compound (contains a ring of delocalised electrons)

• unsaturated

• functional group is phenyl

• compounds that contain a ring of delocalised electrons are called arenes

electrophile

a positive or slightly positive species that can accept a pair of electrons in formation of a dative covalent bond

electrophilic substitution

nitration of benzene

halogenation of benzene

alkylation of benzene

acylation of benzene