6.1.1 - Aromatic Compounds Flashcards
Name the 3 problems with Kekule’s model for benzene
- Enthalpy change of hydrogenation
- Carbon-Carbon bond length
- Resistance to reaction with Bromine
Kekule vs delocalised model: Enthalpy change of hydrogenation
- The expected enthalpy change of hydrogenation for kekule’s model is -360KJ/mol-1
- It is actually less exothermic (-208KJ/mol-1)
- benzene is more stable than Kekules model suggests due to the pi-bond system
Kekule vs delocalised model: carbon-carbon bond length
- kekule’s model suggests the carbon-carbon bond lengths should be of alternating lengths
- however the lengths of the carbon bonds are all the same
Kekule vs delocalised model: resistance to reaction with bromine
- kekule’s model suggests benzene should undergo addition reactions with Bromine
- It doesn’t
Explain the bonding in the delocalised model of benzene
- each carbon forms 3 sigma bonds (2x C, 1x H) forming a cyclic molecule
- the remaining p-orbital overlap sideways with eachother forming a pi-bonding system above and below the plane of the molecule
- all the electrons in the pi-bond system are delocalised
- benzene has a low electron density
Describe the shape of a benzene molecule
- 120 degree bond angles
- planar
- equal c-c bond lengths
forming the electrophile: halogenation of benzene
X2 + AlX3 —> X+ + AlX4-
Regeneration of the catalyst: halogenation of benzene
AlX4- + H+ —-> AlX3 + HX
Conditions for the halogenation of benzene
Halogen carrier catalyst
examples of halogen carriers
- iron
- iron halides
- aluminium halides
What reactions can benzene undergo
electrophillic substitution
conditions for the nitration of benzene
conc. nitric acid
conc. sulfuric acid catalyst
Forming the electrophile: nitration of benzene
HNO3 + H2SO4 —> NO2+ + HSO4- + H2O
what is NO2+
nitronium ion
regenerating the catalyst: nitration of benzene
HSO4- + H+ —> H2SO4
Products of the nitration of benzene
nitrobenzene + H2O
Products of the halogenation of benzene
halobenzene + hydrogen halide
freidel-crafts acylation reagent
acyl chloride
Acyl chloride
R-C(=O)-Cl
Products of acylation
pentyl…one + HCl
Forming the electrophile: acylation
CH3COCl +AlCl3 –> CH3CO+ + AlCl4-
Regenerating the catalyst: acylation
AlCl4- + H+ –> HCl + AlCl3
conditions for acylation
acyl chloride
halogen carrier catalyst
conditions for alkylation
haloalkane
halogen carrier catalyst
forming the electrophile: alkylation
XR + AlX3 –> AlX4- + R+
Regenerating the catalyst: alkylation
AlX4- + H+ —> HX + AlX3
Why doesn’t benzene react with bromine without a halogen carrier catalyst
- arenes have delocalised electrons spread above and below the carbon plane in the pi-bonding system
- there is a lower electron density
- benzene is unable to polarise bromine to act as an electrophile
Naming when benzene is the substituant
Prefix: phenyl
When is benzene a substituant
When it is attached to an alkyl group that is either 7+ carbons long or has a functional group
Explain the electrons in the intermediate of benzen
- more unstable
- carbon 1 is bonded to a hydrogen and the electrophile and has no unbounded electrons disrupting the pi-bonding system
- there are 4 electrons spread over 5 carbons so they have restricted movement around the molecule
Reactivity of arenes vs. Alkenes
- benzene is more stable as the electrons in the pi-bond system are delocalised
- the electron density around a carbon-carbon bond is too low to induce a dipole in bromine
- the electrons in the carbon- carbon double bonds of alkenes are concentrated between the 2 atoms (not delocalised ) so a higher electron density to polarise bromine
- benzene is more stable so more energy required
