CD - COLOUR BY DESIGN

Dyes and Colour

Dyes

Many early dyes were made from plants. Alizarin is an example extracted from the madder root. Alizarin only sticks fast to cloth which has been impregnated with a metal compound such as aluminium sulphate. The process is called mordanting. The colour obtained depends on the mordant used. An aluminium mordant gives a red colour, tin(II) give a pink colour and iron (II) gives a brown colour.

Mordanting takes place under alkaline conditions so that the metal hydroxide is precipitated in the fibres. The metal ions firmly attach themselves to the cloth and then bind the dye molecules by forming chelate rings.

A range of dyes have been developed based on phenylamine (aniline) as the starting material known as azo dyes.

Phenylamine reacts with a solution of sodium nitrate (III) and hydrochloric acid at a temperature below 5^oC (remember keeping that test-tube in ice?) to produce benzene diazonium chloride. This type of reaction is known as diazotisation.

The hydrochloric acid first reacts with sodium nitrate (III) to form unstable nitrous acid.

NaNO₂(aq) + HCl(aq)

HNO₂(aq) + NaCl(aq)

The nitrous acid then reacts with the amine.

+ HNO₂ + HCl

benzenediazonium chloride

+ 2H₂O

If the temperature is allowed to rise above 5^oC, the benzenediazonium chloride decomposes to form phenol and nitrogen gas is given off.

If the benzenediazonium chloride is reacted with another compound containing a benzene ring, called a coupling agent, an azo compound is produced. The diazonium salt acts as an electrophile. Phenols and amines are often used as coupling compounds. Many of the products are important dyes.

With phenol the following reaction occurs:

+ HCl

A similar reaction occurs with phenylamine:

+ HCl

The first dye is a yellow-orange shade, the second bright yellow. Many azo dyes are coloured in the range from red through to yellow. Some azo dyes are green or even blue or purple.

Many azo dyes are produced using a derivitive of naphthalene, one example of which uses naphthalen-2-ol to produce a red azo dye. Notice how this compond is formed from two benzene rings joined (naphthalene differs since there is no hydroxy group present). Learn and remember the structures of naphthalene and naphthalen-2-ol.

Also note that in this reaction Sodium Hydroxide is needed.

NaOH

+ HCl

All coupling compunds contain a benzene ring that performs the coupling because it stablises the delocalised electron structure. Without a coupled benzene ring the compound would remain unstable and break down at over 5^oC like benzenediazonium chloride.

The Chemistry of Colour

Coloured substances absorb radiation in the visible region of the spectrum. The energy absorbed causes changes in electronic energy. Electrons are promoted from a ground state to an excited state. The electrons excited in this case are the outer bonding electrons or lone pairs.

Not all electronic transitions are brought about by visible light as some require ultra-violet radiation. Those which absorb uv radiation appear colourless (unless they fluoresce). The energy needed top excite an electron in a coloured compound and in a colourless compound is shown below:

Coloured Organic Compounds

A dye molecule is built up from a group of atoms called a chromophore, which is largely responsible for its colour. Chromophores contain unsaturated groups such as C=O and -N=N-, which are often part of an extended delocalised electron system involving arene rings. Chrysoidine is a basic dye shown below:

Attached to the chromophore are two NH₂ groups which interact with the chromophore to produce the orange colour.

Other functional groups may be added which can:

modify or enhance the colour of the dye
make the dye more soluble in water
attach the dye molecule to the fibres of the cloth.

All azo dyes contain the R-N=N-R' arrangement.

How do dyes stick to fibres?

This depends on the dye and the fibre to which the dye is attached.

Protein-based fibres such as wool and silk have free ionisable CO₂H and NH₂ groups on the protein chains which can form an electrostatic attraction to parts of the dye molecule. For example the sulphonate group, SO₃^-, on a dye molecule can interact with a NH₃⁺ group on the protein chain.

Cotton is a polymer with a string of glucose units joined together. Indigo which is used to dye denim jeans is a vat dye. Indigo is insoluble in water. The reduced form of indigo is soluble. Cotton is soaked in a colourless solution of the reduced form. This is then oxidised to the blue form of Indigo which precipitates in the fibres.

Direct dyes are applied to the cotton in solution and are held to the fibres by hydrogen bonds and instantaneous dipole-induced dipole forces. These are weak compared with covalent bonds hence these dyes are only fast if the molecules are long and straight.

Fibre reactive dyes actually form covalent bonds with fibre molecules and are therefore extremely colour fast. A dye molecule is reacted with the molecule trichlorotriazine:

Trichlorotriazine can react with either OH groups (present in cotton) or NH groups (present in wool and nylons), thus effectively bonding the dye to the fabric.