Benzene: Benzene is C6H6. Classed as an aromatic hydrocarbon or an arene. Two ways to draw
benzene. Questions may use either, even though right one is misleading.
Joined double benzene ring has 10 carbons and not 12.
Physical Properties: Benzene is colourless, sweet smelling and flammable liquid. Found in crude
oil. Carcinogen- causes cancer.
Kekulé Model: Kekulé suggested that structure of benzene was a six carbon ring with alternate
double and single bonds. This structure can be disproved because…
Lack of Reactivity of Benzene: Benzene only reacts with bromine (undergo electrophilic
addition/ decolourise bromine) in presence of halogen carrier. No catalyst needed for alkenes.
Lengths of C- C Bonds: Bond length of benzene in between bond length of C-C and C=C and all
the C-C bonds are same length. Using X- ray diffraction.
Hydrogenation Enthalpies: Enthalpy change of hydrogenation less exothermic than expected
when compared to enthalpy change of hydrogenation of cyclohexene/ Kekulé model.
Hydrogenation- when C=C reacted with H2. Delocalised structure is more stable.
Delocalised Model of Benzene, developed after evidence disproved
Kekulé structure. According to model, benzene is a planar with six
carbon and six hydrogen. Bond angle is 120. C- C bonds are same length.
Electrons: Each carbon atom uses three out of four electrons for
bonding. Each carbon atom has one delocalised electron in a p- orbital
at right angles to the plane. Forms pi bonds.
Six pi- electrons all together.
Delocalised Electrons: Delocalised electrons are electrons that are
spread over more than two atoms.
π- Bond: A pi- bond formed by overlap of p-orbitals.
Benzene made of 12 sigma bonds and 3pi bonds/ 1 pi region. 6 sigma
bonds between C-H and 6 between C-C.
Compare Kekulé and delocalised model for benzene:
– Kekule- Sideways overlap of p orbitals between two carbon atoms
forms 3 x π Bonds. Localised electron density in π bond. Both C-C
and C=C bonds present. σ bonds between C-H.
– Delocalised model- Sideways overlap of p-orbitals between adjacent carbon atoms above
and below the plane of the ring forms 1 x π bonding region. Delocalised pi electrons have
lower electron density. Equal length C-C bonds present. σ bonds between C-H.
Naming Aromatic Compounds:
Benzene Parent Chain: Benzene considered parent chain if attached are alkyl groups, halogen
and nitro NO2. Disubstituted compounds have two substituent groups. Substituent groups are
prefixes, listed alphabetical. Ring is numbered starting from one of the substituent groups.
Phenyl: When a benzene ring attached to an alkyl chain with a functional group OR an alkyl chain
with 7+ carbons, benzene considered as substituent. The prefix phenyl is used for
benzene. E.g. phenylethanone.
Exceptions: Just need to learn these exceptions.