6.1.1: Aromatic compounds Flashcards
Arenes
Aromatic compounds; have a structure based upon C6H6. Simplest arene is benzene.
Why are the electrons in benzene delocalised?
- π bonds overlap in ring –> occupy a greater volume than in alkenes
- e- are more spread out, ∴ delocalised
Evidence for the delocalisation of benzene
- Hydrogenation energies are less exothermic than expected (should be 2 x -120 but is only -208kJmol-1)
- Benzene’s lack of reactivity: does not decolourise bromine or undergo electrophilic addition reactions
- Carbon-carbon bond lengths are equal in the delocalised system (hence perfect hexagon)
Bond angles in benzene
120°
Describe how the delocalised system of benzene arises
Side on overlap of atomic p orbitals leads to the formation of a “halo” of electron density above and below the sigma bonds in the plane.
Why is a halogen carrier catalyst required in the halogenation of benzene?
- Benzene has delocalised e-
- Alkenes have e- contained in a pi bond
- These pi bond electrons are localised, leading to high electron density –> can induce a dipole without requiring a catalyst.
Example of a halogen carrier (Friedel Crafts) catalyst
AlCl₃
Ring activator
Donates e- density into the ring
Ring activator example
OH in phenol
Ring deactivator
Withdraws e- density from the ring
Ring deactivator example
NO₂
Which groups have a 2 and 4 directing effect?
Ring activators e.g. OH and NH₂
Which groups have a 3 directing effect?
Ring deactivators e.g. NO₂
Phenol undergoes electrophilic substitution with bromine to give
2,4,6 - tribromophenol
Phenol undergoes electrophilic substitution with dilute nitric acid to form
2 - nitrophenol
Reagents needed for the nitration of benzene
conc H₂SO₄
conc HNO₃
50°C
What is the electrophile formed in the nitration of benzene?
NO₂⁺
Equation for electrophile production (nitration of benzene)
HNO₃ + H₂SO₄ ⟶ NO₂⁺ + HSO₄⁻ + H₂O
Reagents needed for nitration of phenol
just dilute nitric acid
What are the conditions for Friedel-Crafts alkylation?
• Reflux
What are the reagents for Friedel-Crafts alkylation?
- Benzene
- Haloalkane
- Halogen carrier catalyst
Why is Fridel-Crafts alkylation useful?
for the formation of C-C bonds; can increase the length of the carbon chain
What are the reagents needed for Friedel-Crafts acylation?
- Benzene
- Acyl chloride
- Halogen carrier catalyst
What are the conditions needed for Friedel-Crafts acylation?
• Reflux
Phenol is a weak acid (evidence)
It neutralises NaOH but does not react with carbonates
Evidence that phenol is more reactive than benzene
• Bromine and nitric acid react more readily with phenol than benzene
⟶ Phenol is nitrated with dilute nitric acid/Conc nitric and sulphuric acid are required for the nitration of benzene
⟶ No halogen carrier catalyst required for bromination of phenol
Reason why phenol is more reactive than benzene
• Increased reactivity of phenol due to lone pair of e- on p orbital of O of OH group
⟶ These e- are donated into the π system of phenol
⟶ Increases e- density of ring in phenol –> better attracts electrophiles than benzene
• E- density of phenol = sufficient to polarise bromine, hence no halogen carrier catalyst required.
