CD - Dyes

Question 1

What functional groups do azo dyes contain?

Answer 1

An azo group

(often) Aromatic groups

Question 2

What happens to the electrons in an azo group linking 2 aromatic compounds?

Answer 2

They become part of the delocalised electron system

Question 3

How is the structure of an azo dye related to its colour?

Answer 3

The colour is a result of light absoption of certain frequencies by the delocalised electron system.
Different colours are made by combining different phenols and amines

Question 4

What type of reaction produces an azo dye?

Answer 4

Coupling reactions

Question 5

Give the structure for a diazonium group

Answer 5

• N⁺ ≡ N -

Question 6

Give the two steps involved in creating an azo dye

Answer 6

1) creating an diazonium salt

2) Coupling the diazonium salt with an aromatic compound that is susceptible to electrophilic attack (e.g. phenol)

Question 7

Give the reactants, products and conditions required to produce a diazonium salt

Answer 7

Nitrous acid, phenylamine and hydrochloric acid below 10°C (to prevent the formation of a phenol) forms benzenediazonium chloride and water

N.B. Nitrous acid must be formed in situ from sodium nitrite and hydrochloric acid because it is so unstable:
NaNO₂ + HCl → HNO₂ + NaCl

Question 8

Give the reactants, products and conditions required for the coupling of phenol with benzenediazonium chloride

Answer 8

• Phenol dissolved in sodium hydroxide solution
• It is then stood in ice and chilled benzenediazonium chloride is added
• This produces an azo dye linking benzene and phenol, NaCl and water
• The azo dye precipiatates out of the solution immediately

Question 9

Define colourfast

Answer 9

A dyes ability to not be washed out easily or fade in the light

Question 10

What does colourfastness depend on?

Answer 10

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

Question 11

What type of fibres do amine groups (-NH₂) bind well with, what bonds are formed and why?

Answer 11

Cellulose fibres (e.g. cotton, rayon and linen)
This is because they contain many -OH groups so form hydrogen bonds with the amine groups

Question 12

What types of fibres do acidic groups bind well with, what bonds are formed and why?

Answer 12

Wool, silk and nylon
H ions move from the dye to fibre molecule, and ionic interactions hold them in place.
Because they contain alkaline links (-NH-)

Question 13

What is the most colourfast type of dye and why?

Answer 13

Fibre reactive dyes are the most permanent (colourfast) type of dye because they have functional groups that react with -OH or -NH- groups in fibres to form strong covalent bonds

Question 14

Define a chromophore

Answer 14

An extended delocalised system of electrons in a compound which give its colour

Question 15

Describe how a chromophore causes a compound to become coloured

Answer 15

• When light hits the chromophore, certain wavelengths are absorbed by the electrons in the delocalised system
• The wavelengths not absorbed in the visible spectrum will be seen as a particular collour

Question 16

Explain the effect on colour when functional groups containing O and N atoms are p

Answer 16

The lone pair of electrons for O and N atoms become part of the delocalised system responsible for absorbing light
Therefore, modifying the chromophore would change the frequency of light absorbed and thus the colour would change

Question 17

Explain how dyes are made more soluble in water and give examples

Answer 17

Dyes are made more soluble in water by incorporating solubilising functional groups into the dye molecule.
These are often ionic groups (e.g. sulfate ion - usually in the for of its sodium salt)
Water dissolves ionic substances because of its polar nature

Question 18

Explain why dyes need to be soluble

Answer 18

Because the dying process involves soaking materials in a solution of the dye compound

Question 19

Give the term for a coloured compound if it is:

i) soluble
ii) insoluble

Answer 19

i) dye

ii) pigment