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CheMisTry - Colour by Design > CD.5 > Flashcards

CD.5 Flashcards

Making and modifying dye molecules

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1
Q

What is the reaction known as when benzene reacts with concentrated nitric acid in the presence of an acid catalyst?

A

Nitration

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2
Q

What acid catalyst is used in nitration?

A

Concentrated sulfuric acid.

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3
Q

What is the combination of conc nitric acid and conc sulfuric acid known in nitration?

A

Nitrating mixture.

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4
Q

What condition is required for nitration and why?

A

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.

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5
Q

What acts as the electrophile in nitration?

A

NO2 +,

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6
Q

How is the electrophile NO2+, generated in nitration?

A

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.

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7
Q

What are the products of nitration?

A

Nitrobenzene and water.

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8
Q

Why is it important to be able to insert nitro groups into a benzene ring that is within the chromophore?

A

It will modify the properties of the chromophore and allow it to absorb a different wavelength and thus have a different colour.

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9
Q

What is the reaction known as when benzene reacts with concentrated sulfuric acid?

A

Sulfonation

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10
Q

What condition is required for sulfonation?

A

Heat under reflux.

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11
Q

What acts as the electrophile in sulfonation?

A

SO3, which is present in concentrated sulfuric acid as a result of dissociation.

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12
Q

What is the structure of SO3 and why is it a good electrophile?

A

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.

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13
Q

What are the products of sulfonation?

A

Benzene sulfonic acid and a hydrogen ion.

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14
Q

Is benzene sulfonic acid a strong acid or a weak acid?

A

Strong acid, meaning that it would dissociate completely in solution.

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15
Q

What would form when benzene sulfonic acid is placed in an alkaline solution?

A

Benzene sulfonate salt.

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16
Q

Why is in important to add sulfonic acid groups to dye molecules?

A

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.

17
Q

What is the azo functional group?

A

R1-N=N-R2

18
Q

What two types of reactants are reacted together to form azo compounds?

A

A diazonium salt and a coupling agent.

19
Q

What is the reaction that forms azo compounds known as?

A

Coupling reaction.

20
Q

When is the azo compound most stable?

A

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.

21
Q

What does the stability of aromatic azo compounds allow it to be used as?

A

Dyes, if they are coloured.

22
Q

What is the diazomiun group?

A

R - N+ ≡ N

23
Q

How are diazonium compounds very unstable?

A

Because the diazonium group is very easily lost, forming nitrogen gas and a carbocation.

24
Q

How can the diazonium group be temporaryily stablised?

A

By attaching it to a benzen ring, this allows electrons in the N ≡ N to become part of benzene’s delocalisation.

25
Q

What are aromatic diazonium compounds normally prepared from?

A

Aromatic amines.

26
Q

What is the reaction known as when the amine group is converted into a diazonium group?

A

Diazotisation

27
Q

What is the process of diazotisation?

A
  1. The aromatic amine is dissolved in dilute hydrochloric acid.
  2. A cold solution of sodium nitrate(III) NaNO2, is added.
  3. The temperature is kept below 5°C by performing the reaction is an ice bath.
28
Q

What condition is require for diazotisation and why?

A

They should be prepared in ice-cold solutions, because solids are explosive.

29
Q

Can diazonium compounds be kept after it is made?

A

No, they should be used immediately.

30
Q

What are coupling agents?

A

Compounds that contain a relatively reactive benzene ring.

31
Q

What are the two common examples of types of coupling agents used to make azo compounds?

A

Phenols or aromatic amines.

32
Q

Why are coupling agents that contain phenol or amine groups attached to the benzene rings good for reacting with diazonium compounds?

A

The lone pairs of electrons from these groups increase the electron density on the benzene ring and therefore increases its reactivity towards electrophiles.

33
Q

What does the diazonium ion act as in the coupling reaction to form azo compounds?

A

The electrophile.

34
Q

What is the process of a coupling reaction?

A
  1. A solution of a coupling agent is made up.
  2. An ice-cold solution of the diazonium compound is added to it.
  3. A coloured precipitate of the azo compound immediately forms.
35
Q

What is a condition required when coupling with phenols?

A

The phenol should be dissolved in alkaline solution.

36
Q

Why should phenol be put in alkaline conditions before coupling?

A

It deprotonates the hydroxy group and pushes the delocalisation onto the benzene ring.

37
Q

In coupling reactions, is the hydroxy or amine groups reacting with the diazonium ion?

A

No, it is the actual benzene ring that it reacting with the diazonium ion, a hydrogen will be substituted off. The hydroxy or amine group will remain.

38
Q

By altering the structures of the diazonium compound and coupling agent used in the coupling reaction, a wide range of azo compounds can be made. This allows what properties of the azo compounds to differ?

A

Colour, water solubility and the ability to bind to fabrics.

CheMisTry - Colour by Design (11 decks)

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