CD.5 Flashcards
Making and modifying dye molecules
What is the reaction known as when benzene reacts with concentrated nitric acid in the presence of an acid catalyst?
Nitration
What acid catalyst is used in nitration?
Concentrated sulfuric acid.
What is the combination of conc nitric acid and conc sulfuric acid known in nitration?
Nitrating mixture.
What condition is required for nitration and why?
The temperature of the reaction should be kept below 55 °C, this is because at higher temperatures, further substitution of the nitro group would take place, leading to di and tri nitro compounds, which may be explosive.
What acts as the electrophile in nitration?
NO2 +,
How is the electrophile NO2+, generated in nitration?
By reactioins between nitric acid and sulfuric acid
HNO3 + 2H2SO4 → NO2+ + 2HSO4 - + H3O+
The sulfuric acid catalyst is regenerated at the end from the hydrogen thats been substitued off.
What are the products of nitration?
Nitrobenzene and water.
Why is it important to be able to insert nitro groups into a benzene ring that is within the chromophore?
It will modify the properties of the chromophore and allow it to absorb a different wavelength and thus have a different colour.
What is the reaction known as when benzene reacts with concentrated sulfuric acid?
Sulfonation
What condition is required for sulfonation?
Heat under reflux.
What acts as the electrophile in sulfonation?
SO3, which is present in concentrated sulfuric acid as a result of dissociation.
What is the structure of SO3 and why is it a good electrophile?
Three oxygens each double bonded to the sulfur, which gives the sulfur in the centre a large partial positive charge, making it a good electrophile to be attacked and binded to the benzene ring.
What are the products of sulfonation?
Benzene sulfonic acid and a hydrogen ion.
Is benzene sulfonic acid a strong acid or a weak acid?
Strong acid, meaning that it would dissociate completely in solution.
What would form when benzene sulfonic acid is placed in an alkaline solution?
Benzene sulfonate salt.
Why is in important to add sulfonic acid groups to dye molecules?
It makes the dyes more soluble because the sulfonic acid group is able to form ionic salts. Therefore sulfonation provides a way of making more soluble derivatives of aromatic compounds.
What is the azo functional group?
R1-N=N-R2
What two types of reactants are reacted together to form azo compounds?
A diazonium salt and a coupling agent.
What is the reaction that forms azo compounds known as?
Coupling reaction.
When is the azo compound most stable?
When R1 and R2 are arene groups, this is because the -N=N- group is stablised by becoming part of an extended delocalised system involving arene groups.
What does the stability of aromatic azo compounds allow it to be used as?
Dyes, if they are coloured.
What is the diazomiun group?
R - N+ ≡ N
How are diazonium compounds very unstable?
Because the diazonium group is very easily lost, forming nitrogen gas and a carbocation.
How can the diazonium group be temporaryily stablised?
By attaching it to a benzen ring, this allows electrons in the N ≡ N to become part of benzene’s delocalisation.
