6.1.1 Aromatic Compounds Flashcards
(25 cards)
What is the evidence that disproves Kekule’s model?
Lack of reactivity, lengths of carbon-carbon bonds, and enthalpy change of hydrogenation
How does the lack of reactivity of benzene disprove Kekule’s model
Does not undergo electrophilic addition reactions and does not decolourise bromine meaning it cannot contain carbon double bonds
How do the lengths of carbon-carbon bonds in benzene disprove Kekule’s structure
The X-ray diffraction results for bond lengths in benzene, all bonds were between the length of single and double bonds, disproving Kekule’s structure
How does the enthalpy change of hydrogenation disprove Kekule’s structure?
Does not have triple the enthalpy change of hydrogenation as cyclohexene (-120kJ/mol), the value is lower than expected, less exothermic, meaning benzene is more stable
Describe the delocalised model of benzene
Each carbon has one electron in a p orbital at right angles above and below the plane, adjacent p orbitals overlap sideways to form a ring of electron density
This creates π-bonds between delocalised electrons which spread equally across the 6 carbons
Common exceptions to naming aromatic compounds
Benzaldehyde, benzoic acid, phenylamine
When is the prefix ‘phenyl-‘ used
When benzene is attached to an alkyl chain with a functional group or an alkyl chain with 7+ carbons
Reagents and conditions for nitration of benzene
Concentration nitric acid
Concentrated sulfuric acid, 50ºC
Don’t let the temperature increase above 50ºC
Electrophile in nitration of benzene and equation that forms it
NO2+
HNO3 + H2SO4 -> NO2+ + HSO4- + H2O
Reagents for halogenation of benzene
Halogen carrier (AlCl3, FeCl3, AlBr3, FeBr3)
Electrophile in bromination of benzene
Br+
Alkylation of benzene reagents
Haloalkane and halogen carrier catalyst (AlCl3)
Acylation of benzene reagents
Acyl chloride, halogen carrier catalyst (AlCl3)
Product of acylation
Aromatic ketone
Why does benzene not react with bromine without a catalyst
Electron density around carbons in a benzene ring is less than C=C in alkenes, insufficient π-electron density to polarise a bromine molecule
Why is phenol classified as a weak acid?
Partially dissociates to form phenoxide ion and proton
What experiments demonstrate the weak acidity of phenol
Does not react with sodium carbonate, does react with aqueous sodium hydroxide to form sodium phenoxide salt and water
Bromination of phenol observations
Bromine water decolourised and white precipitate formed of 2,4,6-tribromophenol
Requirements for nitration of phenol
Room temperature, dilute nitric acid
Why is phenol more reactive than benzene
The lone pair of electrons on the oxygen p-orbital of the -OH group is donated into the π-system
This increases electron density in the benzene ring, this attracts electrophiles more strongly
The aromatic ring is more susceptible to attack
Directing effect of -NH2
2 or 4
Directing effect of -NO2
3
Directing effect of -OH
2 or 4
What does NH2 do to reactivity of phenol
Activates the aromatic ring, can react more readily with electrophiles
