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A-level Chemistry > CbD - Dyes > Flashcards

CbD - Dyes Flashcards

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1
Q

What are aromatic amines used to make?

A

Azo dyes.

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2
Q

What are used to make azo dyes?

A

Aromatic amines.

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3
Q

What are azo dyes?

A

Man-made dyes that contain the azo group:

-N=N-

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4
Q

What is the azo group?

A

-N=N-

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5
Q

What does the azo group do in most azo dyes?

A

Links two aromatic groups.

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6
Q

What links two aromatic groups in most azo dyes?

A

The azo group.

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7
Q

What is an aromatic group?

A

Any group with a benzene ring.

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8
Q

What does having two aromatic groups create?

A

A very stable molecule.

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9
Q

What does the azo group become part of?

A

The delocalised electron system.

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10
Q

What are the colours of dyes a result of?

A

Light absorption by the delocalised electron system.

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11
Q

How are different colours of dyes made?

A

By combining different aromatic groups and amines.

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12
Q

Example of a typical azo compound

A

Methyl orange.

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13
Q

What reaction can make azo dyes?

A

Coupling reaction.

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14
Q

How can azo dyes be made in a coupling reaction?

A

First make a diazonium salt (diazonium compounds contain the group -+N≡N-).

The azo dye is then made by coupling the diazonium salt with an aromatic compound that is susceptible to electrophilic attack.

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15
Q

Explain the method for creating a yellow-orange azo dye

A

React phenylamine with nitrous acid to make a diazonium salt:
1) Nitrous acid (HNO2) is unstable, so it has to be made in situ from sodium nitrate and hydrochloric acid.
NaNO2 + HCl –> HNO2 + NaCl
2) Nitrous acid reacts with phenylamine and HCl to form benzenediazonium chloride. The temperature must be below 5 degrees to prevent a phenol forming instead.

Make the azo dye by coupling the diazonium salt with a phenol:

1) Dissolve the phenol on sodium hydroxide solution to make sodium phenoxide solution.
2) The stand it in ice, and add chilled benzenediazonium chloride.
3) The azo dye precipitates out of the solution immediately.
4) Phenol is a coupling agent. The lone pairs on its oxygen increase the electron density of the benzene ring, especially around carbons 2,4 and 6. This gives the diazonium ion (a weak electrophile) something to attack.

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16
Q

Is the diazo group stable or unstable?

A

Unstable.

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17
Q

What does the diazo group decompose to form?

A

Nitrogen gas.

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18
Q

Explain the process of diazotization

A

Aromatic compound dissolved in dilute HCl.

Cold solution of sodium nitrite (sodium nitrite(III)), NaNO2, added.

Temp kept below 5 degrees by performing the reaction in an ice-bath.

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19
Q

Explain how the coupling reaction to make azo dyes happens

A

A solution of the coupling agent (usually containing hydroxy- or amino- groups attached to the benzene ring) is made up.

An ice cold solution of the diazonium salt is added.

A coloured precipitate of the azo compound is immediately formed.

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20
Q

What is done to the coupling agent if it is a phenol?

A

Usually dissolved in alkali first.

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21
Q

What state is the phenylamine when it is a coupling agent?

A

Liquid.

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22
Q

What properties does a good dye have to have?

A

Has got to be colourfast - it can’t wash out too easily or fade in the light.

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23
Q

What does colourfastness depend on?

A

The strength of the bonding between the dye and fibre molecules.

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24
Q

What can dye molecule have added to them to help them bind to a material?

A

Functional groups.

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25
Q

What will a dye that binds well to one type of material often not do?

A

Won’t bind as strongly to other types of material.

26
Q

What does adding an amine functional group (-NH2) to a dye molecule enable?

A

Enables dye molecules to form hydrogen bonds with fibre molecules.

27
Q

What does adding an amine functional group (-NH2) to a dye molecule to enable dye molecules to form hydrogen bonds with fibre molecules work well with and why?

A

Cellulose fibres like cotton, rayon and linen because they contain loads of -OH groups.

28
Q

How colourfast are dye molecules with amine groups attached and why?

A

These dyes aren’t particularly colourfast because the hydrogen bonds just aren’t strong enough.

29
Q

What does adding acidic groups like carboxylic acid (-COOH) or sulfonic acid (-SO3H) to a dye molecule enable?

A

Helps dyes to bind to alkaline -NH- links in fibres.

30
Q

What does adding acidic groups like carboxylic acid (-COOH) or sulfonic acid (-SO3H) to a dye molecule help dyes to bind to alkaline -NH- links in fibres and why?

A

These links are found in wool, silk and nylon.

H+ ions move from the dye to fibre molecule, and ionic interactions then hold them together.

31
Q

How colourfast are dye molecules with carboxylic acid (-COOH) or sulfonic acid (-SO3H) attached and why?

A

These give better colourfastness than dyes that form hydrogen bonds with the fibres.

(H+ ions move from the dye to fibre molecule, and ionic interactions then hold them together.)

32
Q

What does dyes having ionic salt groups such as -SO3^-Na^+ allow?

A

Allow the dye to bind with -NH- links.

33
Q

Why does dyes having ionic salt groups such as -SO3^-Na^+ allow the dye to bind with -NH- links?

A

The salt groups dissociate when you dissolve the dye in water to give -SO3^- groups.

34
Q

What do you need to make -+NH2- groups in dyes which have ionic salt groups?

A

You need to add acid to provide H+ ions to the -NH- links. The -SO3^- groups will then bind to the -+NH2- links.

35
Q

What are fibre reactive dyes?

A

The most permanent type of dye. They have a functional group that will react with the -OH or -NH- group in the fibre, forming strong covalent bonds.

They make use of a reactive group attached to the dye molecule to form a bridge between the dye and the fibre.

36
Q

What are the structures in molecules that give them their colour called?

A

Chromophores.

37
Q

What are chromophores?

A

The structures in molecules that give them their colour.

38
Q

What happens when light hits a chromophore?

A

Certain wavelengths are absorbed by electrons in the chromophore. Visible wavelengths not absorbed will be seen as a particular colour.

39
Q

What do chromophores tend to contain?

A

Double or triple bonds, lone pairs of electrons, or benzene rings.

40
Q

What do double or triple bonds, lone pairs of electrons, or benzene rings tend to form?

A

Part of a delocalised electron system across a large section of the molecule.

41
Q

What will modifying the chromophore do?

A

Change the frequency of light that is absorbs and so change the colour of the molecule.

42
Q

What can be added to a chromophore and why?

A

Functional groups containing O or N atoms with lone pairs of electrons can be added to adjust the colour of a dye molecule.

43
Q

What does adding functional groups containing O or N atoms with lone pairs of electrons to a chromophore do?

A

Adjusts the colour of a dye molecule.

44
Q

Why do functional groups containing O or N atoms with lone pairs of electrons adjust the colour of a dye molecule when added to a chromophore?

A

Because the lone pair of electrons becomes part of the delocalised system responsible for absorbing light.

45
Q

What does the process of dying usually involve?

A

Soaking material in a solution of the dye compound.

46
Q

What would a dye ideally dissolve easily in and why?

A

Water because it is cheap, readily available and non-toxic.

47
Q

Why do dyes need to be soluble?

A

Because the process of dying usually involves soaking material in a solution of the dye compound.

48
Q

What can be done to make a dye more soluble in water?

A

Solubilising functional groups can be incorporated into a dye molecule.

49
Q

What are solubilising functional groups often?

A

Often ionic groups, such as the sulfate ion (usually in the form of its sodium salt).

50
Q

Why does water dissolve ionic substances?

A

Because of its polar nature.

51
Q

Why do diazonium salts attack phenols?

A

Phenols are coupling agents.

52
Q

What are the 2 strategies for attaching dyes to fabrics using covalent bonds?

A

Mordanting.

Ading fibre.

53
Q

What is mordanting?

A

Makes use of a metal ions to join the dye to the fabric. Groups on the fabric and the dye form dative covalent bonds to the metal, forming chelate complex ions.

54
Q

What are examples of mordants?

A

Metal ions used as mordants include Al^3+ and Cr^3+ ions.

55
Q

Wool, silk and nylon (polyamides) are examples of fabrics dyed by what type of interaction?

A
  • Ionic bonds
  • SO3^- groups in dye molecule
  • NH3^+ groups in acidic solution in polymer molecule
56
Q

Cotton and cellulose are examples of fabrics dyed by what type of interaction?

A
  • Covalent bonds (fibre-reactive dyes)
  • Presence of reactive group, e.g. triazine derivatives, in dye molecule
  • OH or NH2 groups in polymer molecule
57
Q

Cotton is an example of a fabric dyed by what type of interaction?

A
  • Hydrogen bonds
  • Several NH2 groups in the LINEAR dye molecule
  • Frequent Oh groups in polymer molecule
58
Q

Polyesters are examples of fabrics dyed by what type of interaction?

A
  • Instantaneous dipole-induced dipole
  • Few polar groups in the SMALL dye molecule
  • No OH or NH groups in polymer molecule
59
Q

How does adding fibres allow covalent bonding to attach dyes to fibres?

A

Fibre-reactive dyes:

They make use of a reactive group attached to the dye molecule to form a bridge between the dye and the fibre.

60
Q

What groups cause things to be water soluble?

A

Sulfonium groups.

A-level Chemistry (51 decks)

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