3.10 Aromatic Chemistry

Question 1

What was the proposed structure of benzene of Kekulé’s?

Answer 1

C6H6

Alternating c single bonds and c double bonds

Question 2

What are the problems with Kekulé’s proposal of benzene?

Answer 2

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

Question 3

What is the thermodynamic evidence?

Answer 3

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

Question 4

What are some facts about the actual structure of benzene?

Answer 4

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

Question 5

How do we name some benzene compounds?

Answer 5

NO2 group - nitrobenzene 
CH3 group - methylbenzene 
Cl/Br group - chloro/bromobenzene
COOH group - benzene carboxylic acid 
COCH3 group - phenylethanone 

Remember to number!

Question 6

What is involved in the nitration of benzene?

Answer 6

A nitronium ion is needed - as an electrophile

+
NO2

Question 7

How do you form the nitronium ion?

Answer 7

+
cHNO3 + cH2SO4 -> H2NO3 + HSO4-

 \+            + H2NO3 -> NO2 + H2O 

Overall equation: cH2SO4 + cHNO3 -> HSO4-
+
+ NO2 + H2O

Question 8

What mechanism is used in nitration of benzene?

Answer 8

Electrophilic substitution

Question 9

Describe the mechanism of nitration of benzene?

Answer 9

(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)

Question 10

What are the roles of the acids in the nitration of benzene?

Answer 10

cH2SO4 - catalyst

cHNO3 - base

Question 11

What is the overall equation for nitration of benzene?

Answer 11

Benzene + conc nitric acid -> nitrobenzene + water

With conc sulfuric acid as a catalyst over the arrow

Question 12

What are the uses of nitrobenzene?

Answer 12

Making amines:
Nitrobenzene + 6[H] -> aminobenzene + 2H2O
(Over the arrow - cHCl and Tin (Sn))

Making explosives

Question 13

What does Friedel-Crafts acylation involve?

Answer 13

RCO

AlCl3 catalyst

Question 14

What are the conditions for Friedel-crafts acylation?

Answer 14

Dry - to prevent hydrolysis

Dark - to prevent radical formation

Question 15

How do you create the electrophile in Friedel-crafts acylation?

Answer 15

First arrow from the middle of the c-cl bond on the ethanoyl chloride onto the Al in the AlCl3

                          \+ Which forms CH3CO[AlCl4]

Question 16

What mechanism is used in Friedel-crafts acylation?

Answer 16

Electrophilic substitution

of benzene

Question 17

Describe the mechanism for electrophilic substitution of benzene?

Answer 17

(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

Question 18

What is the role of AlCl3 in Friedel-crafts acylation?

Answer 18

A catalyst

Question 19

What is the overall equation for electrophilic substitution of benzene?

Answer 19

Benzene + CH3COCl -> phenylethanone + HCl

With AlCl3
Dry and dark
Over the arrow