18.1.1 ARENES
Saturday, 19 March 2022
15:30
Arenes are hydrocarbons which contain rings of C atoms in which there are
delocalised electrons.
Delocalised electrons are not fixed between atoms in a bond, but shared
between 3 or more atoms.
18.1.2 STRUCTURE OF BENZENE
Saturday, 19 March 2022
15:38
Benzene has the formula C6H6, with a cyclic structure of 6 C atoms joined together.
Kekule proposed that the benzene ring was a planar ring of C atoms with alternating
single and double bonds.
Each C atom bonded to a H atom.
He later adapted the model stating that the benzene molecule was constantly
flipping between isomers by switching over the double bonds.
X-ray diffraction studies showed that all C-C bond lengths in benzene were the same,
with each bond between a C-C and C=C.
Shows that his model isn't completely correct.
18.1.3 DELOCALISATION IN BENZENE
Saturday, 19 March 2022
16:10
In the delocalised model, each C atom forms 3 σ bonds, with a H and the 2
neighboring C atoms.
Form due to the overlapping of orbitals.
The formation of the sigma bonds leave each C atom with one electron in a p orbital.
The p orbitals stick above and below the plane, and overlap sideways to form
a ring of pi bonds which are delocalised around the ring.
The delocalised pi bonds are made up of 2 ring-shaped clouds of electrons, above
and below the plane.
The delocalised electrons are shared between the 6 C atoms.
,18.1.4 NAMING ARENES
Saturday, 19 March 2022
16:42
18.1.5 PROPERTIES & REACTIONS OF
BENZENE & ARENES
Saturday, 19 March 2022
16:44
Arenes are non-polar compounds and so have weak London forces.
Alkenes have a single localised high electron density which strongly attracts
electrophiles.
Undergo electrophilic addition.
Arenes such as benzene have a delocalised electron density pi bond ring.
Spread out the negative charge, making the ring very stable and electrophiles
are less attracted.
As the ring is very stable, much more energy is required to add an electrophile as this
would destroy the stable ring.
Benzene prefers to react by electrophilic substitution as this retains the stable
arrangement.
Benzene combusts with a smoky flame in air.
Not enough oxygen in the air to burn benzene completely, so there are
particles of soot in hot gas - smoke.
, 18.1.6 ELECTROPHILIC SUBSTITUTION
REACTIONS OF BENZENE
Saturday, 19 March 2022
19:35
In electrophilic substitution, a H atom is substituted with the electrophile.
The mechanism involves the addition of the electrophile to form a positively charged
intermediate, followed by the loss of H+ from the C attached to electrophile.
Reforms the delocalised ring.
As the charge density is spread across the ring due to the delocalised pi bonds, the
electrophile must have a strong positive charge to attack the benzene ring.
Most compounds aren’t polarised enough, i.e. induced dipoles in Br2.
The electrophile can be made stronger by the use of a catalyst, halogen
carrier.
Halogen carriers accept a lone pair of electrons from a halogen atom on an
electrophile.
As the lone pair is pulled away, the polarisation increases and sometimes a
carbocation can form.
Makes the electrophile stronger.
Benzene reacts with halogens at room temperature in the presence of a halogen
carrier catalyst to form a halogenobenzene.
Benzene can be warmed in the presence of Br2 and Fe fillings to form iron(III)
bromide, FeBr₃.
This catalyst polarises the halogen, allowing one of the halogens to act as an
electrophile.
Nitration of benzene is when a H atom is replaced by a nitro group, NO₂.
Saturday, 19 March 2022
15:30
Arenes are hydrocarbons which contain rings of C atoms in which there are
delocalised electrons.
Delocalised electrons are not fixed between atoms in a bond, but shared
between 3 or more atoms.
18.1.2 STRUCTURE OF BENZENE
Saturday, 19 March 2022
15:38
Benzene has the formula C6H6, with a cyclic structure of 6 C atoms joined together.
Kekule proposed that the benzene ring was a planar ring of C atoms with alternating
single and double bonds.
Each C atom bonded to a H atom.
He later adapted the model stating that the benzene molecule was constantly
flipping between isomers by switching over the double bonds.
X-ray diffraction studies showed that all C-C bond lengths in benzene were the same,
with each bond between a C-C and C=C.
Shows that his model isn't completely correct.
18.1.3 DELOCALISATION IN BENZENE
Saturday, 19 March 2022
16:10
In the delocalised model, each C atom forms 3 σ bonds, with a H and the 2
neighboring C atoms.
Form due to the overlapping of orbitals.
The formation of the sigma bonds leave each C atom with one electron in a p orbital.
The p orbitals stick above and below the plane, and overlap sideways to form
a ring of pi bonds which are delocalised around the ring.
The delocalised pi bonds are made up of 2 ring-shaped clouds of electrons, above
and below the plane.
The delocalised electrons are shared between the 6 C atoms.
,18.1.4 NAMING ARENES
Saturday, 19 March 2022
16:42
18.1.5 PROPERTIES & REACTIONS OF
BENZENE & ARENES
Saturday, 19 March 2022
16:44
Arenes are non-polar compounds and so have weak London forces.
Alkenes have a single localised high electron density which strongly attracts
electrophiles.
Undergo electrophilic addition.
Arenes such as benzene have a delocalised electron density pi bond ring.
Spread out the negative charge, making the ring very stable and electrophiles
are less attracted.
As the ring is very stable, much more energy is required to add an electrophile as this
would destroy the stable ring.
Benzene prefers to react by electrophilic substitution as this retains the stable
arrangement.
Benzene combusts with a smoky flame in air.
Not enough oxygen in the air to burn benzene completely, so there are
particles of soot in hot gas - smoke.
, 18.1.6 ELECTROPHILIC SUBSTITUTION
REACTIONS OF BENZENE
Saturday, 19 March 2022
19:35
In electrophilic substitution, a H atom is substituted with the electrophile.
The mechanism involves the addition of the electrophile to form a positively charged
intermediate, followed by the loss of H+ from the C attached to electrophile.
Reforms the delocalised ring.
As the charge density is spread across the ring due to the delocalised pi bonds, the
electrophile must have a strong positive charge to attack the benzene ring.
Most compounds aren’t polarised enough, i.e. induced dipoles in Br2.
The electrophile can be made stronger by the use of a catalyst, halogen
carrier.
Halogen carriers accept a lone pair of electrons from a halogen atom on an
electrophile.
As the lone pair is pulled away, the polarisation increases and sometimes a
carbocation can form.
Makes the electrophile stronger.
Benzene reacts with halogens at room temperature in the presence of a halogen
carrier catalyst to form a halogenobenzene.
Benzene can be warmed in the presence of Br2 and Fe fillings to form iron(III)
bromide, FeBr₃.
This catalyst polarises the halogen, allowing one of the halogens to act as an
electrophile.
Nitration of benzene is when a H atom is replaced by a nitro group, NO₂.
