Aromaticity

Question 1

Describe the structure of benzene

Answer 1

MF C6H6
Planar molecule, bond angle 120*
one p electron from each of the 6 carbon atoms overlap to form a delocalised ring of π electrons above and below the plane of the molecules

Question 2

Why is benzene unreactive?

Answer 2

The ring has a high e- density however since they are delocalised it is difficult for nucleophiles to attack

Question 3

How does the lack of reactivity disprove Kekule’s structure?

Answer 3

If benzene contained fixed C=C bonds it should turn bromine water brown but it doesn’t therefore benzene is not a standard alkene

Question 4

How does the bond length disprove Kekule’s structure?

Answer 4

The bonds in benzene are longer than C=C but shorter than C-C while benzene is still a perfect hexagon so the bonds cannot be fixed

Question 5

How does the enthalpy of hydrogenation disprove Kekule’s structure?

Answer 5

The ΔH for cyclohexane is -360kJmol-1 while ΔH for benzene is -208kJmol-1 therefore there is a difference of 152 known as the delocalisation energy; the energy that makes benzene so stable

Question 6

What is the reagent, conditions and nucleophile for the nitration of benzene?

Answer 6

Reagents - HNO3/H2SO4
conditions - reflux below 55
electrophile - NO2+

Question 7

What is the reagent, conditions and nucleophile for the chlorination of benzene?

Answer 7

Reagents - halogen carrier e.g AlCl3 FeCl3
- Halogen, Cl2, Br2
conditions - anhydrous absence of light room temp
electrophile - Cl+

Question 8

What is the reagent, conditions and nucleophile for the Friedel Crafts alkylation of benzene?

Answer 8

Reagents - halogen carrier e.g AlCl3 FeCl3
- CH3Cl CH3I
conditions - anhydrous absence of light room temp
electrophile - CH3+