3.10 Aromatic Chemistry
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What was the proposed structure of benzene of Kekulé’s?
C6H6 Alternating c single bonds and c double bonds
What are the problems with Kekulé’s proposal of benzene?
With bromine water there was no colour change - and we would expect one We would expect 4 disubstituted isomers but there are only 3 disubstituted isomers It would have different bond lengths (154 or 134) but they are all 139 pico meters Thermodynamic evidence
What is the thermodynamic evidence?
Cyclohexene + H2 -> cyclohexane = ΔH -120KJmol^-1 Cyclohexdiene + 2H2 -> cyclohexane = ΔH -240KJmol^-1 Benzene + 3H2 -> cyclohexane = ΔH -360KJmol^-1 But in reality Benzene + 3H2 -> cyclohexane = ΔH -208KJmol^-1
What are some facts about the actual structure of benzene?
Benzene is 149KJmol^-1 (resonance energy) which is more stable than expected It has 6 delocalised p electrons that confer stability on the structure These 6 delocalised p electrons form a region it electron density above and below the ring It is represented by the ring inside the hexagon
How do we name some benzene compounds?
NO2 group - nitrobenzene CH3 group - methylbenzene Cl/Br group - chloro/bromobenzene COOH group - benzene carboxylic acid COCH3 group - phenylethanone Remember to number!
What is involved in the nitration of benzene?
A nitronium ion is needed - as an electrophile + NO2
How do you form the nitronium ion?
+ cHNO3 + cH2SO4 -> H2NO3 + HSO4- \+ + H2NO3 -> NO2 + H2O Overall equation: cH2SO4 + cHNO3 -> HSO4- + + NO2 + H2O
What mechanism is used in nitration of benzene?
Electrophilic substitution
Describe the mechanism of nitration of benzene?
(Draw the benzene with a H) First arrow from the edge of the circle to the N on the NO2 (+) Second arrow from the middle of the H bond into a broken circle - from the points either side of the c, and a + sign in the middle Nitrobenzene and H+ formed H+ + HSO4- ->H2SO4 (regenerated)
What are the roles of the acids in the nitration of benzene?
cH2SO4 - catalyst cHNO3 - base
What is the overall equation for nitration of benzene?
Benzene + conc nitric acid -> nitrobenzene + water With conc sulfuric acid as a catalyst over the arrow
What are the uses of nitrobenzene?
Making amines: Nitrobenzene + 6[H] -> aminobenzene + 2H2O (Over the arrow - cHCl and Tin (Sn)) Making explosives
What does Friedel-Crafts acylation involve?
RCO AlCl3 catalyst
What are the conditions for Friedel-crafts acylation?
Dry - to prevent hydrolysis Dark - to prevent radical formation
How do you create the electrophile in Friedel-crafts acylation?
First arrow from the middle of the c-cl bond on the ethanoyl chloride onto the Al in the AlCl3 \+ Which forms CH3CO[AlCl4]
What mechanism is used in Friedel-crafts acylation?
Electrophilic substitution of benzene
Describe the mechanism for electrophilic substitution of benzene?
(Draw the benzene with a H) First arrow from the edge of the circle to the C+ on the CH3C(+)O[AlCl4] Second arrow from the middle of the H bond into a broken circle - from the points either side of the c, and a + sign in the middle Phenylethanone and H+ formed H+ + [AlCl4]- -> AlCl3 + HCl
What is the role of AlCl3 in Friedel-crafts acylation?
A catalyst
What is the overall equation for electrophilic substitution of benzene?
Benzene + CH3COCl -> phenylethanone + HCl With AlCl3 Dry and dark Over the arrow