Aromatic Chemistry
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What are the 4 problems with Kekules suggested structure for benzene? (cyclic with a double bond every other bond)
1 - would expect electrophilic addition reactions, but doesn’t react 2 - Would expect 4 isomers of dibromobenzene, however 1,2 and 1,6 models were indistinguishable, so in reality only 3 (irregular = bonds) 3 - enthalpy changes of benzene didn’t match predictions based off double and single bond data 4 - x-ray crystallography should show = to be shorter than - –> actually showed perfect hexagon
What did the equilibrium model for benzene suggest?
There were 2 forms that rapidly inter-converted
What is the name of the accepted benzene model?
Delocalised Model
What does the delocalised model for benzene show its structure to be?
A cyclic C hexagon, with p orbitals above and below. These orbitals are close enough to overlap, and so form a continuous pi bond above and below the planar of the ring
What are the conditions for nitration?
conc. HNO3 conc H2SO4 50*C
How is the nitronium ion generated for nitration?
1) HNO3 + H2SO4 –> H2NO3+ + HSO4- 2) H2NO3+ –> NO2+ + H2O
What are the uses of nitrobenzenes?
As explosives (TNT) To make aromatic compounds (amines, used to make dyes)
What are the conditions for Friedel Crafts acylation?
AlCl3 catalyst, dry
What are the reactants and products of Friedel-Crafts acylation?
Reactants - a benzene-based compound and either acyl chloride or an acid anhydride Products - An aromatic ketone and either HCl or a carboxylic acid
What type of reaction mechanism occurs in Friedel-Crafts acylation?
Electrophilic substitution
How does the AlCl3 catalyst form the electrophilic ion necessary for Friedel-Craft acylation, and what is the name of this ion?
AlCl3 + RCOCl –> AlCl4- + RCO+ RCO+ –> Acyllium ion
Describe and explain the difference in stability between the hypothetical cyclohexa-1,3,5-triene and benzene
Benzene is more stable Has 3 double bonds rather than 1, and its double bonds are close enough to overlap, thus allowing the electrons to become delocalised in a pi orbital above and below the ring, which is more stable