What was the original Kekule structure?
Cyclohexane-1,3,5,triene.
3 single bonds and 3 double bonds.
What were the three problems with kekule’s model?
Double bonds - the double bonds meant that benzene should react easily with halogens eg turning bromine water colourless. however, it doesn’t do this. Benzene has a very low chemical reactivity.
Bond length- the alternating double and single bonds, kekule’s proposes, should mean alternating bond lengths. However, benzene has 6 symmetrical sides.
Enthalpy changes - kekule’s structure: enthalpy change of -360kJmol-1
However, benzene actually has an enthalpy change of -208kJmol-1. Benzene Is more stable than kekule thought!
Describe the delocalised module
- all bonds have the same length and strength
- carbon atoms in ring form 3 sigma bonds; 2 with carbon atoms and 1 with a H atom.
- 1 electron from each carbon atom is delocalised, forming a ring of electron density above and below the plane
- trigonal planar, 120 degrees Celsius
Why doesn’t benzene take part in addition reactions?
Addition reactions would disrupt the delocalisation of the ring structure
Describe the electrophilic substitution of arenes with conc nitric acid in the presence of conc sulphuric acid.
NITRATION.
overall equation: C6H6 + HNO3 -> C6H5NO2 + H2O
reaction mechanism:
1) formation of NO2+
HNO3 + H2SO4 -> NO2+ + HSO4- + H2O
2) NO2 electrophile then reacts with benzene
3) H+ reforms H2SO4 by combining with HSO4- from step 1. Therefor H2SO4 is the catalyst.
Describe the electrophilic substitution of arenes with a halogen in the presence of a halogen carrier.
HALOGENATION
Overall equation: C6H6 + Br2 -> C6H5Br + HBr
Reaction mechanism:
1) formation of Br+
Br2 + FeBr3 -> Br+ + FeBr4-
2) Br electrophile then reacts with benzene
3) H+ reacts with FeBr4- from the first step to reform FeBr3
Does benzene react readily with Br2? Explain why.
- Benzene is unable to react with Br2 without the help of a halogen carrier.
- this is because benzene has delocalised electrons spread over all 6 carbon atoms. This means there Is a much lower pi-electron density than alkenes. In alkenes, the pi-electron density is concentrated above and below the 2 carbon atoms.
- the halogen carrier generates the more powerful electrophile Br+
What is a salt?
When H+ in an acid has been replaced by a positive ion.
Describe the reactions of phenol with sodium hydroxide.
Phenol is neutralised by sodium hydroxide to form a salt, sodium phenoxide and water.
C6H5OH + NaOH -> C6H5O-Na+ + H2O
Describe the reactions of phenol with sodium
Sodium is added to phenol, metal effervesces -> hydrogen gas is produced, sodium phenoxide is the salt.
Describe the reactions of phenol with bromine.
- Room temperature without need for a halogen carrier catalyst.
- When Br2 is added to phenol, the colour changes from Orange to colourless.
- a white precipitate of 2,4,6-tribromophenol is formed.
Explain the relative ease of bromination of phenol compared with benzene.
- lone pair of electrons on oxygen p orbital of the phenol interacts with pi electrons of ring.
- this increases electron density, activating ring.
- this increased electron density polarises Br2 molecules so they are attracted more strongly towards the phenol ring.
State the uses of phenols
- disinfectants
- antiseptics
- production of plastics
- resin for paint
