6.1.1 Aromatic Compounds Flashcards
what is the Kekulé model of benzene?
- 3 double bonds between carbon atoms
what is the delocalised model of benzene?
- each carbon bonded to 2 other carbon atoms and one hydrogen atom ( 3 sigma bonds)
- this forms a trigonal planar shape around each carbon (hexagonal planar)
- 4th electron on each carbon above and below the plane of the ring
- these p-orbitals can OVERLAY sideways to form a delocalised pi bonding system
- spell out the π-ELECTRONS are delocalised
- the 6 pi electrons now spread out above and below the plane of the ring and are delocalised (so less repulsion = more stable benzene)
what are the experimental evidence for a delocalised rather than Kekulé model?
Bromine water
- doesn’t react like alkenes and doesn’t decolourise bromine water
Bond lengths
- double bonds are stronger and shorter than single bonds but in benzene all C-C bond lengths are equal
Enthalpy of hydrogenation
- is less exothermic than expected suggesting benzene is more stable than Kekulé’s structure suggests
what is the mechanism of benzene reactions?
- electrophilic substitution
explain electrophilic substitution of benzene in words
- electrophile is attracted to the electron rich pi system
- electrophile accepts a pair of pi electrons from the ring
- unstable intermediate formed
- H+ leaves, putting 2 electrons back into the ring
draw the general mechanism for electrophilic substitution of benzene
see notes
what are the conditions for the nitration of benzene?
- conc nitric acid in the presence of conc sulphuric acid catalyst
- reflux at around 50C
what happens at higher temperatures?
- further substitution
draw the mechanism for the nitration of benzene
see notes
what is the equation for the generation of the electrophile for nitration of benzene?
HNO3 + H2SO4 -> HSO4- + NO2+ + H2O
what is the equation for the regeneration of the catalyst for the nitration of benzene?
HSO4- + H+ -> H2SO4
What are the conditions for the halogenation of benzene?
- halogen in the presence of a halogen carrier catalyst
What are examples of halogen carrier catalysts?
- aluminium chloride
- iron chloride
- iron metal
Draw the mechanism for the halogenation of benzene
See notes
What is the equation for the regeneration of the catalyst?
- AlCl4- + H+ -> AlCl3 + HCl
What is the equation for the catalyst generating the electrophile?
- Cl2 + AlCl3 -> AlCl4- + Cl+
What are the conditions for the alkylation of benzene? (Friedel-Crafts)
- halogen is replaced by an alkyl group
- a haloalkane in the presence of a halogen carrier catalyst
What is the mechanism for the alkylation of benzene?
See notes
What are the conditions for acylation of benzene?
- hydrogen is replaced by acyl group
- acyl chloride in the presence of a halogen carrier catalyst
- heating at reflux
- anhydrous conditions
What is the mechanism for the acylation of benzene
- see notes
What is an acyl group?
R - C = O
What are halogen carrier catalysts used for?
- takes a hydrogen atom from the reagent and forms a covalent bond to the halogen
- the H+ ion is then used to regenerate the catalyst
- the catalyst generates an electrophile
Why are alkenes more reactive than benzene?
- the electrons are localised in the double bond (benzene has the delocalised π bonding system)
- higher electron density (lower electron density)
- polarises the electron more
- electrophile is more strongly attracted
- alkenes are more reactive
- by reacting with electrophilic substitution benzene retains the delocalised π bonding system
Will benzene decolourise bromine?
- no reaction
- needs halogen carrier catalyst
- less reactive than alkenes
