27.3 Reactions of arenes

Question 1

How are arenes combusted

Answer 1

• They burn in air with smoky flames
• This is because they have a high carbon : hydrogen ratio compared with alkanes
• There is usually unburned carbon remaining so this produces soot

Question 2

Why is electrophilic substitution reaction the most common for aromatic compounds

Answer 2

It leaves the system unchanged, rather than addition which would require the input of the delocalisation energy to destroy the aromatic system

Question 3

Describe mechanism of electrophilic substitutions

Answer 3

. Delocalised system of the aromatic ring has a high electron density that attracts electrophiles.
At the same time, the electrons are attracted towards electrophile El+

. A bond forms between one of the carbon atoms and the electrophile, but to do this the carbon must use electrons from delocalised system

. This destroys aromatic system, so to get it stable again the carbon loses a H+ ion with the electron in the C-H bond returning to the delocalised system.

So overall there is a substitution of H+ by El+

Question 4

What is nitration

Answer 4

• It is the substitution of an NO2 group for one of the hydrogen atoms on an arene ring.
• The electrophile NO2+ is generated in the reaction mixture of concentrated nitric acid and sulphuric acid

Question 5

What is reaction of nitric and sulfuric acid
Aim is to generate NO2+ electrophile

Answer 5

HNO3 + H2SO4 –> H2NO3+ + HSO4-

Sulfuric acid is a stronger acid than nitric acid so it donates a proton H+ to the HNO3

H2NO3+ then loses a molecule of water to give NO2+ which is called nitronium ion or nitryl cation
H2NO3+ —> NO2+ + H2O

So overall equation is

H2SO4 + HNO3 —> NO2+ + HSO4- + H2O

Question 6

Describe mechanism for this reaction

What is the role of sulfuric acid

Answer 6

. NO2+ is electrophile, so arrow from the circle of electrons to the NO2+

. This means the carbon is connected to H and NO2
. And the circle opens slightly with a + in the middle to show it is missing an electron
. Then arrow from C-H bond to system shows electron returning
. A H+ ion is produced

Then, to regenerate H2SO4, this H+ is reacted with HSO4-.
So sulfuric acid is a catalyst in reaction

Question 7

Where are nitrated amines used

Answer 7

. TNT
. Nitration makes aromatic amines which are used to make industrial dyes