Arenes Flashcards
What is the bond angle in benzene?
120
Describe the bonding in benzene
Each C has 4 bonding electrons, it uses 3 to form σ bonds to 2 other Cs and 1 H. The 4th electron from each C if found in a 2p orbital which extends above and below the plane of the molecule. These p orbitals overlap sideway with those of neighbouring C atoms, creating a ring of electron density above and below the plane of the molecule. This overlap produces a system of π bonds which spread over all 6 C atoms. The p electrons are delocalised
Describe the shapes of benzene and Kekulé’s model of benzene
Kekulé’s: circular, 3 long single bonds, 3 shorter double bonds
Actual: planar, regular hexagonal ring, equal bond lengths
Why evidence is there against Kekulé’s structure of benzene?
Evidence from x-rat crystallography and tunnelling electron microscopy shows benzene has equal bond lengths
Benzene does no undergo electrophilic addition reactions with bromine as its electron density is insufficient to polarise Br2 molecules
Benzene has a less exothermic enthalpy change of hydrogenation than expected
Reaction of phenol with aqueous NaOH
C6H5OH + NaOH –> C6H5O-Na+ + H2O
Reaction of phenol with sodium
2C6H5OH + 2Na –> 2C6H5O-Na+ + H2
Effervescence will be seen
Reaction of phenol with bromine, observations and conditions
C6H5OH + 3Br2 –> C6H2Br3OH (2,4,6 - tribromophenol) + 3HBr
The bromine water will decoluorise and a white precipitate will form
This reaction takes place at room temperature
Describe the nitration of benzene
Concentrated nitric acid (to form NO2+) and concentrated H2SO4 (acts as a catalyst) are added to benzene
Formation of NO2+ : HNO3 + H2SO4 –> NO2+ + HSO4- + H2O
The NO2+ acts as a electrophile and reacts with benzene to form C6H5NO2 + H+
H+ + HSO4- –> H2SO4
This is an electrophilic substitution reaction
Describe the halogenation of benzene
Br+ is needed to react with bromine
C6H6 + Br+ –> C6H5Br + H+
Formation of Br+ : Br2 + FeBr3 –> Br+ + FeBr4-
FeBr4- + H+ –> FeBr3 + HBr
What halogen carriers can be used for bromination of benzene?
For bromination, AlBr3, FeBr3 or Fe
For chlorination, AlCl3, FeCl3 or Fe
Why will bromine react readily with cyclohexene?
Cyclohexene has π bonds, which contain localised π electrons above and below the 2 carbon atoms in the double bond, which produces a region of high electron density. This polarises the Br2 molecules
Why will bromine not react readily with benzene?
Benzene has delocalised π electrons spread all over 6 carbon atoms in the ring.
Benzene has a lower electron density than alkenes, and so attracts the Br2 molecule less
Benzene polarises Br2 less
Why will bromine react more readily with phenol than with benzene?
A lone pair of electrons on the oxygen atom in the phenol group is drawn into the benzene ring
This creates a higher electron density in the ring structure
This increased electron density polarises bromine molecules, which are then attracted more strongly towards the ring structure than in benzene
What is phenol used for?
In the production of plastics, antiseptics, disinfectants and resin for paints
