Colour by Design Chemical Storylines

CD1 Ways of making colour Most colours are due to the way compounds interact with light Pigments and Dyes Pigments –Insoluble –Organic or inorganic –Spread in a layer (e.g. paint) or mixed into the bulk (e.g. plastics) Dyes –Soluble –Organic –Bond to the molecules of the substance they colour – often weak imfs but sometimes covalent or ionic bonds Dyes can be made into pigments by using them to coat particles of an inert solid – these are called lakes CI6.7

Cochineal was first used in Mexico around 1000BC However, to make 1kg of the dye, you need female cochineal beetles! This obviously made dyes expensive Ordinary people’s clothes were dyed with cheap vegetable dyes The colours were therefore drab and faded quickly Many early dyes came from plants (e.g. indigo) or even insects (e.g. cochineal) Ass 1 CD1

In the mid 19 th century, people began heating coal in the absence of air. This produced ‘coal gas’ for things like gas lamps but also coal tar, coke and an ammonia-rich liquid Chemists learned how to take this coal tar and use it to make cheap synthetic dyes The modern colour chemist needs to understand; Why compounds are coloured, Which structures lead to particular colours The structures of the materials they want to colour How we can bind the dye molecules to the fibre or surface Once this is understood it becomes possible to design a coloured molecule for a particular purpose

CD2 Colour by accident Three of the most important blue pigments ever discovered were all the results of mistakes 4000 years ago the ancient Egyptians developed the first synthetic pigment The next important blue pigment, Prussian Blue, was not discovered until 1705; –Heinrich Diesbac used KOH which had been contaminated with cattle blood –The iron present in the haemoglobin reacted –CI 6.9 –Ass 2

Iron phthalocyanine was made in 1928 as a result of iron impurities in the reaction vessel This was then methodically modified and tested until it was found that, if Cu 2+ were in the centre, it was resistant to molten KOH, boiling HCl and high temperatures! Copper phthalocyanine was marketed as Monastral Blue Phthalocyanines are complexes made using a 16-member ring as a ligand These rings are similar to naturally occurring porphyrin rings found in haemoglobin and chlorophyll They have… –4 N-atoms acting as ligands (it is a tetradentate ligand) –Alternating double and single bonds –This results in an extended delocalised electron system

What is a paint? Two main parts; –Pigment –Liquid which carries it and allows it to spread In modern paints the liquid is made of a solvent and a polymer or resin (known as the binder). As it dries the solvent evaporates but the binder remains In oil paints the oil reacts with the air to form a flexible film over the pigment, this makes drying slow When paintings are analysed it is done using a variety of techniques including; –scanning electron microscopy –atomic emission spectroscopy –reflectance spectroscopy

Chrome Yellow In the mid 18 th century yellow pigments used for paints included arsenic(III) sulphide, iron(III) oxide and lead(II) oxide However, none of these were particularly bright Around 1800 a mineral rich in bright yellow lead chromate(VI) was discovered Chemists then discovered that it could be made in the lab by ionic precipitation –Ass 4,5 –CI5.1 (revision) –CD2 Many of these pigments contained toxic substances (e.g. Cd, Pb, CrO 4 2-, As) and are now rarely used Modern paints usually use much less toxic, organic pigments instead of these inorganic ones.

CD3 Chemistry in the art gallery The Incredulity of St Thomas was painted in 1504 by Cima de Conegliano and now hangs in the National Gallery in London It is considered to be a masterpiece During the following 500 years, it had been kept at many different temperatures and humidities and even submerged in salt water It had layers of dirt, the old varnish masked the colours and the paint was flaking off In 1969 it was decided to restore it Analytical chemists played a huge part in this restoration, analysing all the pigments used, the oils and the binding media CI6.8 CI13.6 (Oils and fats) CI7.3 (GLC) CI6.1 (Emission spectroscopy) (revision) Ass 6 CD3.1, 3.3, 3.4

CD4 At the start of the rainbow 19 th century – many new pigments and dyes were developed This was closely linked to the developments in organic chemistry –CI Arene Chemistry –CD In 1856 William Perkin – tried to make quinine… …in his lab at home… …at the age of 18 !! Started with organic amino compounds from coal tar Didn’t actually know structure of quinine! –failed Tried again using phenylamine Made a purple solution which was a very good dye (“mauve”) Still no quinine!

Industrially… Coal tar  benzene  nitrobenzene  phenylamine  “mauve” In Germany in 1860s work centred on a systematic study of organic chemistry as well as on commercial developments At the same time, chemists were discovering… –Periodicity –Atomic mass –Carbon forms 4 bonds –Benzene was formed a ring (Kekule) Dye structures were not known until 1880s Note: Mauve is actually a mixture of compounds, the most important one of which is mauvine Its discovery was pure chance as its formation depends on impurities in the phenylamine

Alizarin Before Perkin's discoveries, dyes had come from plants such as indigo plant for blues and madder root for reds The dye from the madder root is called alizarin; This only sticks to cloth when it has been impregnated with certain metal ions… This called mordanting The specific colour depends on the metal ion used; Al 3+  red Sn 2+  pink Fe 2+  brown

It was discovered that alizarin was derived from anthracene and so methods of synthesising it were investigated… –Ass 7, 8, 9 Route 1 – poor yield and too expensive Route 2 – successful Two groups developed route 2 at the same time. It was agreed that Perkin would have the UK trade while BASF would have Europe and USA In tonnes of madder root were harvested to make 750 tonnes of alizarin 5 years later, Perkin’s company alone made 430 tonnes and the madder fields had gone completely

CD5 Chemists Design Colours Otto Witt was trying to answer two questions; –Why certain structures led to coloured substances –How small changes to the structure led to changes in colour To do this he was studying azo dyes –CI13.10 These are made using a diazonium salt (from phenylamine)… …and reacted it with a coupling agent (an aromatic compound with an O or N joined to it – making it electron–rich) + HCl Coupling agent “phenylamine” Diazonium salt “benzene diazonium chloride” Coupling reaction Electrophilic substitution Electrophile Yellow

Using 1,3,5 triaminobenzene… …he got a brown compound Using 1,3 diaminobenzene he got the ‘in-between’ colour of orange This substance (chrysoidine) was the first commercially useful dye… –Ass10

How does structure affect colour? A dye molecule can be thought of as being made of two parts… The chromophore –largely responsible for its colour –has unsaturated groups forming an extended delocalised electron system Functional groups attached to this chromophore –these will do a number of things… Modify the colour (e.g. things with lone pairs such as –NH 2 or -OH) Improve its solubility in water (e.g. SO 3 - (sulphonate) groups) Make it attach to the fibres of the cloth –CI6.9 –CD5.1 –CD5.2

CD6 Colour for Cotton How do dyes stick to fibres? Need dyes to be fast to light, washing and rubbing Wool and silk are protein-based and so have –COOH and –NH 2 groups These can be ionised and form electrostatic attractions with groups on the dye molecules Cotton however is a cellulose fibre It has no readily active parts

It is made of bundles of glucose-based polymer chains… Alizarin uses a mordant Indigo is a vat dye; it is oxidised and precipitates in the fibres –CI 5.3, –Ass 11

Direct dyes are applied to the cotton in solution They are held to the cotton by hydrogen bonding and instantaneous dipole-induced dipole attractions. Because these are so weak the dyes will only be fast if they are long and straight so they can form lots of imfs For example… Congo Red Sulphonate groups (SO 3 - ) improve solubility Amine groups (NH 2 ) allow H- bonding (and improve solubility) Long straight molecule with large extended delocalised electron system allows lots of i.d.i.d. attractions

A dyemaker’s dream If we could have covalent bondsbetween dye and fibre, we would have dyes that were very fast to washing In the 1950s work was done to modify azo dyes by adding reactive groups The theory was that it would work for wool… It didn’t! Largely because it needed alkaline conditions and these would damage the wool

The dream come true However, it did work for cotton The first fibre reactive dyes were used in 1950s Paradox and problem These are fast because the Cl groups react with OH groups in cotton However these same groups also react with water This destroys their reactivity To overcome this, buffers were developed to control the pH and so control the hydrolysis –Ass 12, 13, 14