4.2 Aromaticity Flashcards
Why is the kekule model not possible to be the structure of benzene?
- Kekule states is has single and double bonds, but benzene does not react as an alkene and undergo addition reactions easily (e.g aqueous bromine)
- If there was double and single bonds, there would be two different bond length in benzene, but x-rays disproved this.
- The enthalpy change is 152 kj mol less than expected suggesting benzene is more stable than the kekule structure.
Describe the structure of benzene:
In benzene each carbon atom is bonded to two other carbon atoms and one hydrogen atom by sigma bonds. The fourth outer shell electron of a carbon atom is in a 2p orbital, above and below the plane of the carbon atoms. These p- orbitals overlap to give a delocalised electron structure, above and below the plane of carbon atoms. This delocalised electron pie structure is often shown as a circle within the hexagon.
What would happen to the stability of benzene if it underwent an addition reaction?
An addition reaction would disrupt the stable delocalised electron system and the resulting product would become less stable.
What happens when benzene is hydrogenated?
gives cyclohexane under the conditions of a nickel or platinum catalyst and high temperatures.
Why does benzene generally react by substitution reaction?
As the delocalised system of electrons are retained.
What is an electrophile?
An electron deficient species that can accept a lone pair of electrons.
Why is benzene susceptible to attack by an electrophile?
As benzene has a delocalised ring of electrons above and below the plane of carbon atoms causing an area of high electron density that is susceptible to electrophiles (+) looking for a lone pair of electrons (-).
What happens when benzene is attacked by an electrophile?
Electrophilic substitution- a hydrogen atom (as H+) is replaced by the incoming electrophile. During this process to replace a hydrogen atom, the stability of the ring needs to be disturbed, giving an unstable WHELAND INTERMEDIATE, which then looses a hydrogen ion , H+, in a rapid step.
what happens during the nitration of benzene?
A hydrogen atom is replaced by a nitro group (NO2). This happens as the electrophile that attacks is in the form of NO2+ and this is produced by the reaction of conc nitric acid and conc sulfuric(VI) acid.
The NO2+ reacts with benzene giving nitrobenzene, C6 H5 NO2, as the organic product. (Yellow liquid observed)
Now practice this mechanics on a whiteboard.
How does the halogenation of benzene work with Br2?
- Benzene and bromine only react when a catalyst such as iron(III) bromide or aluminium bromide is present . This is due to the pie electron system of benzene is not positive enough to polarise the bromine molecule (Br2) to give Br delta plus - Br delta minus. Therefore iron(III) bromide is used as in this presence the bromine molecules polarisation is more pronounced.
Use whiteboard for this mechanism
How does the halogenation of benzene work which Cl2?
Again Cl2 and benzene only react when the catalyst aluminium chloride or iron(III) chloride act as a catalyst for the same reason as in the use with bromine.
Use whiteboard for mechanism.
What does Friedel-Crafts alkylation method do?
Provides a way of producing a new carbon-to-carbon bond, giving alkyl derivatives of benzene such as methylbenzene and ethylbenzene.
What is used for the friedel-crafts alkylation to work?
Uses a halogenoalkane reacting with a benzene and aluminium chloride as a catalyst (AlCl3)
Why is benzenes C-Cl bond stronger than a regular halogenoalkanes C-Cl bond?
The stronger bond between carbon and chlorine in chlorine zone results from a non-bonding p electron pair on chlorine overlapping with the ring pie-system of electrons. The resulting bond requires much more energy to be broken thus forcing conditions needed for this such as 300°C and increased pressure.
