6.1.1 - Aromatics - benzene and phenol Flashcards
Describe the similarities and differences between the Kekulé and delocalised model of benzene
Similarities:
- p-orbitals overlap
- pi bonds above and below bonding C atoms
Differences:
- Kekulé has 3 pi bonds whereas delocalised model has pi ring system
Describe the 3 pieces of 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
What is the benefit of benzene’s delocalised structure?
very stable compared to other molecules of similar size
Describe the test to distinguish between phenol and carboxylic acid
add Na2CO3
- carboxylic acid produces CO2
- phenol does not react
Define arene
aromatic compound that contains a benzene ring
Why does benzene undergo electrophilic substitution instead of electrophilic addition?
EA would involve breaking the stable delocalised ring of electrons
Describe the halogenation of benzene e.g bromination
- ES reaction
- Br2
- FeBr3 catalyst (acts as halogen carrier)
- room temperature
Describe the 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)
Describe Friedel-Crafts Acylation
- AlCl3 catalyst
- produces phenylketone
Describe the reaction of benzene with bromine water
- no reaction due to delocalised electron density of pi system
- relatively stable so resistant to bromination
Describe the properties of phenol
- aromatic alcohol
- weak acid (neutralised with NaOH but will not react with carbonates)
Describe the reaction of phenol with bromine water at room temp, and why does this happen?
- produces 2,4,6-tribromophenol
- white precipitate
- decolourises bromine water
- why: lone pair on O atom increases electron density of ring
Name the 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 (and benzoic acid)?
- lone pair on O delocalised into pi system
- donates electron density into the ring, increasing its electron density = more susceptible to electrophilic attack/attracts electrophiles more strongly
- COOH group is electron withdrawing
Describe the nitration of phenol
dilute HNO3, room temp
produces a mixture of 2 and 4-nitrophenol
Explain the relative resistance to bromination of benzene compared to phenol and cyclohexene
- benzene: pi bonds/electrons are delocalised
- phenol: LP on oxygen delocalised into ring
- cyclohexene: electrons are localised in pi bonds
- benzene has a lower electron density
- benzene cannot polarise Br2
Suggest ways of processing waste polymers
- landfill
- recycling
- use as an organic feedstock
- combustion for energy production
Define electrophile
electron pair acceptor
Explain what is meant by ‘delocalised pi bond electrons’
electrons are spread over more than 2 atoms formed by overlap of p-orbitals
Describe the bonding and structure of a benzene molecule with a diagram
- p-orbitals overlap above and below ring to form pi bonds
- electrons delocalised
- carbon-carbon bonds all equal lengths
- bond angles = 120°
- sigma bonds between C-C/C-H
Compare the different reactivities of benzene and alkenes towards e.g chlorine
- benzene: delocalised ring of electrons and pi bonds
- alkenes: electrons concentrated between 2 carbons
- electrophiles attracted to greater electron density in alkenes so alkenes more reactive
