3.10 Aromatic Flashcards
What is the molecular formula of benzene?
C6H6
Describe Kekule’s structure of benzene
An organic cyclohexene, with alternating double and single bonds.
Ie Cyclohex-1,3,5-diene
Describe Lonsdale’s structure of benzene
A cyclohexane with a delocalised electron ring
What evidence is there for Lonsdale’s structure over Kekule’s structure of benzene?
The hydrogenation of benzene has a lower enthalpy change than the hydrogenation of cyclohex-1,3,5-triene, showing us that the structure of benzene is more stable than the kekule structure.
Describe some of the properties of benzene
- Carbon-carbon bonds are all the same length, and are between single and double bonds in length.
- Benzene does not undergo addition reactions readily
- enthalpy change of hydrogenation of benzene is about 152Kjmol-1 less than expected
How do you source NO2+ ions?
A mixture of nitric acid and sulfuric acid
HNO3 + H2SO4 -> H2NO3+ +HSO4-
H2NO3+ + H2SO4 -> HSO4- + NO2+ + H2O
Describe the mechanism Electrophilic substitution (Nitration) of benzene
1) An arrow is drawn from the delocalised electron ring to the NO2+ ion, forming a positive intermediate, with a +horseshoe ring (this must be opposite the new bond formed). You must draw the C-H and C-NO2 bonds.
2) An arrow is drawn from the C-H bond to the horseshoe electron structure
3) Nitrobenzene + a proton is formed
Describe the mechanism Electrophilic Substitution (Friedel Crafts acylation) of benzene
1) An arrow is drawn from the delocalised electron ring to the C+OCH3 (Acylium) ion, forming a positive intermediate, with a +horseshoe ring (this must be opposite the new bond formed). You must draw the C-H and C-C+OCH3 bonds.
2) An arrow is drawn from the C-H bond to the horseshoe electron structure
3) An aromatic ketone + a proton is formed
How are Acylium ions sourced for friedel crafts acylation?
By reacting an acyl chloride with AlCl3 to for, an Acylium ion + AlCl4-
CH3COCl + AlCl3 -> CH3CO + AlCl4-
