Benzene
Benzene, C6H6, is classed as an aromatic
hydrocarbon or arene.
The Kekulé model of the structure of benzene →
was based on a six membered ring of carbon atoms
joined by alternate single and double bonds.
Evidence to disprove the Kekulé model:
(1) The lack of reactivity of benzene…
• benzene does not undergo electrophilic addition
reactions
• benzene does not decolourise bromine under normal
conditions
… which suggests that benzene cannot have any C=C in
its structure.
(2) The length of the carbon-carbon bonds in benzene were
found to be equal and the measurement between the
lengths of a single bond and a double bond.
(3) The hydrogenation enthalpies;
if benzene did have the Kekulé structure, then it would be
expected to have an enthalpy change of hydrogenation that is
three times that of cyclohexene. The enthalpy change of
hydrogenation of benzene was actually less exothermic than
predicted, which means that less energy is produced than
expected. The actual structure of benzene is therefore more
stable, due to electron delocalisation.
The delocalised model of benzene:
Developed by scientists who decided that the experimental
evidence was sufficient to disprove Kekulé structure.
o benzene is a planar, cyclic, hexagonal hydrocarbon.
,o each carbon atom uses 3 of its 4 available electrons in
bonding to two other carbon atoms and to one hydrogen
atom.
o each carbon atom has one electron in a p-orbital at right
angles to the plane.
o adjacent p-orbital electrons overlap sideways, in both
directions, above and below the plane of the carbon atoms
to form a ring of electron density.
o this overlapping of the p-orbitals creates a system of π-
bonds which spread over all six of the carbon atoms in the
ring structure – said to be delocalised.
Delocalised pi
6 p-orbitals system
Naming aromatic compound:
In aromatic compounds, the benzene ring is often considered
to be the parent chain. Alkyl groups (CH3, C2H5) halogens (F,
Cl, Br, I) and nitro (NO2) groups are all considered the
prefixes to benzene e.g. 2-bromomethylbenzene,
1,4-dichlorobenzene and nitrobenzene.
When a benzene ring is attached to an alkyl chain with a
functional group or to an alkyl chain with 7+ carbon atoms,
benzene is considered to be the substituent. Instead of
benzene, the prefix phenyl is used in the name e.g.
phenylethanone and 2-phenyloctane.
Exceptions: benzoic acid, phenylamine and benzaldehyde.
, Comparing Reactivity of
Alkenes with Arenes
• Alkenes react with bromine by electrophilic addition.
• Benzenes react with bromine by electrophilic substitution.
Alkenes decolourise bromine by electrophilic addition
reactions. In the reaction bromine adds across the double
bond. The pi bond in the alkene contains localised electrons
above and below the plane of the two carbon atoms in the
double bond. This produces an area of high electron
density.
Unlike alkenes, benzene does not react with bromine unless a
halogen carrier catalyst is present. This is because benzene
has delocalised π-electrons spread above and below the
plane of the carbon atoms in the ring structure. The electron
density around any two carbon atoms in the benzene ring is
less than that in a C=C double bond in an alkene.
When a non-polar molecules such as bromine approaches the
benzene ring there is insufficient π-electron density
around any two carbon atoms to polarise the bromine
molecule. This prevents any reaction taking place.
Benzene, C6H6, is classed as an aromatic
hydrocarbon or arene.
The Kekulé model of the structure of benzene →
was based on a six membered ring of carbon atoms
joined by alternate single and double bonds.
Evidence to disprove the Kekulé model:
(1) The lack of reactivity of benzene…
• benzene does not undergo electrophilic addition
reactions
• benzene does not decolourise bromine under normal
conditions
… which suggests that benzene cannot have any C=C in
its structure.
(2) The length of the carbon-carbon bonds in benzene were
found to be equal and the measurement between the
lengths of a single bond and a double bond.
(3) The hydrogenation enthalpies;
if benzene did have the Kekulé structure, then it would be
expected to have an enthalpy change of hydrogenation that is
three times that of cyclohexene. The enthalpy change of
hydrogenation of benzene was actually less exothermic than
predicted, which means that less energy is produced than
expected. The actual structure of benzene is therefore more
stable, due to electron delocalisation.
The delocalised model of benzene:
Developed by scientists who decided that the experimental
evidence was sufficient to disprove Kekulé structure.
o benzene is a planar, cyclic, hexagonal hydrocarbon.
,o each carbon atom uses 3 of its 4 available electrons in
bonding to two other carbon atoms and to one hydrogen
atom.
o each carbon atom has one electron in a p-orbital at right
angles to the plane.
o adjacent p-orbital electrons overlap sideways, in both
directions, above and below the plane of the carbon atoms
to form a ring of electron density.
o this overlapping of the p-orbitals creates a system of π-
bonds which spread over all six of the carbon atoms in the
ring structure – said to be delocalised.
Delocalised pi
6 p-orbitals system
Naming aromatic compound:
In aromatic compounds, the benzene ring is often considered
to be the parent chain. Alkyl groups (CH3, C2H5) halogens (F,
Cl, Br, I) and nitro (NO2) groups are all considered the
prefixes to benzene e.g. 2-bromomethylbenzene,
1,4-dichlorobenzene and nitrobenzene.
When a benzene ring is attached to an alkyl chain with a
functional group or to an alkyl chain with 7+ carbon atoms,
benzene is considered to be the substituent. Instead of
benzene, the prefix phenyl is used in the name e.g.
phenylethanone and 2-phenyloctane.
Exceptions: benzoic acid, phenylamine and benzaldehyde.
, Comparing Reactivity of
Alkenes with Arenes
• Alkenes react with bromine by electrophilic addition.
• Benzenes react with bromine by electrophilic substitution.
Alkenes decolourise bromine by electrophilic addition
reactions. In the reaction bromine adds across the double
bond. The pi bond in the alkene contains localised electrons
above and below the plane of the two carbon atoms in the
double bond. This produces an area of high electron
density.
Unlike alkenes, benzene does not react with bromine unless a
halogen carrier catalyst is present. This is because benzene
has delocalised π-electrons spread above and below the
plane of the carbon atoms in the ring structure. The electron
density around any two carbon atoms in the benzene ring is
less than that in a C=C double bond in an alkene.
When a non-polar molecules such as bromine approaches the
benzene ring there is insufficient π-electron density
around any two carbon atoms to polarise the bromine
molecule. This prevents any reaction taking place.
