6.1 - Aromatics - benzene and phenol Flashcards
Benefit of benzene’s delocalised structure
Very stable compared to other molecules of similar size
Define arene
Aromatic compound that contains a benzene ring
Evidence for benzene structure
- enthalpy change of hydrogenation for benzene less exothermic than cyclohexene
- all bond lengths between carbon atoms are equal
- resistance to electrophilic addition reactions
Why does benzene undergo electrophilic substitution instead of electrophilic addition?
EA would involve breaking the stable delocalised ring of electrons
Halogenation of benzene e.g bromination
- ES reaction
- Br2
- FeBr3 catalyst (acts as halogen carrier)
- room temperature
Nitration of benzene
- ES reaction
- heat
- concentrated HNO3
- concentrated H2SO4 catalyst
- 55°C for monosub, greater for multiple subs (must be mono for production of aromatic amines)
Friedel-Crafts Acylation
- AlCl3 catalyst
- produces phenylketone
Reaction of benzene with bromine water
No reaction due to delocalised electron density of pi system - relatively stable so resistant to bromination
Properties of phenol
- aromatic alcohol
- weak acid (neutralised with NaOH but will not react with carbonates)
Reaction of phenol with bromine water and why
- produces 2,4,6-tribromophenol
- white precipitate
- decolourises bromine water
- why: lone pair on O atom increases electron density of ring
Reaction of phenol with dilute HNO3
Produces a mixture of 2 and 4-nitrophenol
Directing groups
2-4 (and 6) directing - electron donating e.g NH2, OH
3 directing - e.g NO2
Why is phenol more reactive than benzene?
- lone pair from O atom overlaps with delocalised pi electron system in benzene ring
- donates electron density into the ring, increasing its electron density = more reactive towards electrophiles
Bromination of phenol
Br2 (aq), room temp
Nitration of phenol
dilute HNO3, room temp
