Summarise the nitration of benzene (5)
- electrophilic substitution with H2SO4 catalyst and dilute nitric acid.
1. Formation of electrophile: nitric acid (HNO3) + sulphuric acid = nitronium ion (electrophile) and HSO4 and water
- Nitronium ion breaks pi system and bonds with benzene
- H+ ion is released - reformation of catalyst:
H+ + HSO4- = H2SO4
Overall equation: under 50 degrees
Benzene + nitric acid = nitrobenzene and water
Summarise the bromination of benzene
- electrophilic substitution with iron bromide catalyst and bromine.
1. Formation of electrophile:
Br2 + FeBr3 = Br+ + FeBr4
- Benzene attacks Br+ and releases H+
- Reformation of catalyst:
H+ FeBr4 = FeBr3 + HBr
Overall:
Benzene + Br2 = Bromobenze and H+
Why is the Kekule’s model of benzene incorrect? (3)
- Bond Length: Kekule’s model shows 3 double bonds and 3 single bonds that vary in length (134nm or 153nm). However through X Ray diffraction it was revealed that all bonds in benzene are the same length: 0.139nm
- Hydrogenation enthalpy: Kekule’s model infers that benzene has triple the enthalpy of cyclohexene however it is 208 KJmol-1 instead of 360KJmol-1
- Reactivity: Benzene does not react like alkenes as it does not react with bromine without a catalyst.
Describe the bonding in Benzene (4)
- 6 carbons and 6 Hydrogens in a delocalised pi system
- The P orbitals with an electron overlap sideways and above and below the carbons so the electron density is shared across all 6 carbons
- The double bonds contain sigma bonds through the overlapping of S orbitals and pi bonds
- The C-H bond is a localised sigma bond
What are the conditions needed for the chlorination of benzene?(3)
- Electrophilic substitution
- Cl2
- Halogen carrier: AlCl3 (catalyst)
- Overall equation:
Benzene + Cl2 = chlorobenzene + HCl
Describe the Alkylation of benzene? (3)
- Electrophilic substitution
- Catalyst: AlCl3
- Reactants: Haloalkane and benzene
- overall equation:
Benzene + Haloalkane = RBenzene + HCl
Describe the acylation of benzene (5)
- Electrophilic substitution
- Reactants:
Benzene and Acyl halogen - Catalyst: AlCl3
- electrophile formed: COCH3+
- overall equation:
Benzene + Acyl chloride = Phenyl + HCl
Describe the reactivity of a phenol (4)
- More reactive than benzene
- Reacts with metals and metal hydroxides but not carbonates
- Is 2,4 directing
- Reacts with bromine without catalyst therefore decolourises
Why is phenol more reactive than benzene? (2)
- The O atom donates a lone pair of electrons to the pi system which increases the electron density of the aromatic ring
- This polarises the aromatic ring allowing it to induce dipoles in molecules like Br2 to undergo Electrophilic substitution without a catalyst
Describe the bromination of phenol (5)
- No catalyst needed
- Phenol is 2,4 directing so if 3Br2 is used then it will form 2,4,6- tribromophenol
- Observations:
Bromine is decolourised
White precipitate of 2,4,6 tribromophenol is formed at room temperature - Overall reaction:
Phenol + 3Br2 = 2,4,6 tribromophenol + 3HBr
Describe the nitration of phenol.
- No catalyst
- Reactants:
Phenol + dilute HNO3 at room temperature - Forms 2- nitrophenol and 4- nitrophenol as Phenol is 2,4 directing
How would you separate 2-nitrophenol and 4-nitrophenol
- Fractional distillation
- One will have a higher boiling point than the other
What directing group is phenylamine?
- 2,4 directing
- An activating group so no catalyst is needed in reactions
What directing group is Nitrophenol?
- 3, directing
- A deactivating group
How do you test for phenols? (3)
- Neutral iron chloride test.
- Pale yellow solution of FeCl3 is added which turns violet in the presence of phenol.
- FeCl2 replaces H in the OH group of phenol and HCl is made
What directing group are halides?
2,4
What directing groups are COOH, Aldehydes and ketones
3,5
Uses of phenol. (5)
Antiseptics
Aspirin
Detergents
Producing plastic
Component in paints
How to distinguish alkenes from phenols? (4)
React both solutions with bromine water.
Both solutions will decolourise bromine water.
Phenol reaction will form a white precipitate- 2,4,6 tribromophenol.
Alkene does not form a white ppt.
