CHAPTER ONE

1.1     Introduction

A dye is a coloured substance that has an affinity to the substrate to which it is being applied.

They are usually strongly coloured compounds which can be intensely yellow red, orange, blue or even green, depending on the exact structure of the molecule. Also as a result of their colour, azo compounds are tremendously importance as dyes and also as pigments for a long time (Ebenso et al, 2008) in fact about half of the dyes in industrial use today are azo dyes, which are mostly prepared from diadorium salts (Robert, 2011; ZOllinger, 2003). The azo group (N=N) and nitroso group (-N=O) confer colours on the dyes.

Azo dyes have also been studied widely because of their excellent thermal and optical properties in applications such as optical recording medium, toner, ink-jet printing and oil-soluble light fast dyes.

Recently azo compounds as organic dyes have also attracted attention due to their interesting electronic features in connections with their application for molecular memory storage, non-linear optical elements and organic photoconductors (yildiz and Boztese, 2002).  

History of Dyes

The most common and valuable dyes were madder- a red mauve, indigo-a deep blue and saffron yellow.

Madder was distilled from Rubua tructorium a plant found in Turkey and the British west indices. Its main colouring component was alizarian and it was used to dye cellulose products. It had excellence feature when combined with an alum-taunin, alim mordant.

Indigo is a blue colourant of vegetable origin that was obtained from ingigofera tinctoria in India. The plant contains indoxyl, which oxides with the air when it is fermented to produce the dye. Indigo is insoluble in water, which made foe excellent fastness.

Saffron was first used in ancient Minoan civilisations as early as 1900B.C. They obtained the saffron lilly from the hillsides of crete. When the stamens were simmered, they produced an orange broth which dyed wool bright yellow. An alum mordant as used to seal the colour features into the clothes.

In 1826 unverdoberis discovered aniline through distillation, aniline is an important dye intermediate of indigo that can be reproduced artificially from coal tar. However in 1856 William Henry Perkins who attended the royal college of chemistry England, discovered a purple dye almost by accident. He had experimented on the dichromate oxidation of aniline and found that it formed a black solid. From this soled he isolated a dye that coloured silk purple.

Aniline span an entire new angle of research; triphenylmethane dyes.

In 1856 natason created magenta. Vergain managed to produced the magenta by oxidising crude aniline with stannic chloride, a tanning solution. In 1859 Hoffman determined that the principal components of magenta were humorous anline, rosanline and toluidine Hofman later developed a violet in 1853, which was followed by Girard Violet Imperial in 1860. The next milestone in dye research was attained by Nicholson and Gibeein 1862 when they sulfonated N-phenyl derivatives of magenta to produce dyes with better solubility which became the first acid dyes.

Azo, a non-triphenymathane dye was discovere d by martins in 1863 by coupling diazonium ions azo dye lead to an entirely new class of azo dyes which were developed by Greiss.

Friedrich August Kekule Von Stradonite, a German Chemist formulated the cyclohexatiene form of Benzene, which explained the aromatic character of benzene in terms of oscillating bonds. This revelation gave scientists everywhere a greater understanding of dye structure.

Madder, the natural mauve dye was created synthetically in the form of alizarin by Graebe and Liebenmann in 1868. Unfortunately, their method of synthesis included a high cost process making their dye economically unstable. Luckily in 1869 Perkins and his partner, Heinrich Caro developed a favourable method of synthesis of Mauve by using substances that had been fused with sulphur.

Synthetic dyes launched a new age of fast-paced commercialisd production that continues even now, driving lurentore, researchers and scientists to create, learn and discover new innovations.

1.2     What Makes Dyes Coloured

This is a very common question that occurs in everybody’s mind. The answer is explained by the consequences of the presence of chromophore in dyes. Since by definition, dyes are aromatic compounds. Their structure includes aryl rings which have delocalised electron system. These structures are responsible for the absorption of electromagnetic radiation of varying wavelength, depending on the energy of the electron clouds. For this reason, chloromoprhores do not makes dyes coloured, in the sense that they confer on them the ability to absorb radiation. Rather chromophore function by altering the energy in the delocalised electron cloud of the dye and this altercation from within the visible range. Instead of outside it. Our eyes detect that absorption and responds to the lack of a complete range of wavelengths by seeing colours.

Removal of electrons may cause the remaining electrons to revert to local orbits. A good example is schiffs reagent. In conclusion chromophore are atomic configuration which can alter the energy in delocalised electron systems. Generally they are composed of carbon, nitrogen, oxygen and sulphur. They can have alternate single and double bonds.

1.3     Colour Alteration of Dyes (Modifiers)

The colour alteration of dyes is caused by methyl or ethyl groups alter the colour of dyes by altering the energy in the delocalised electrons. It has been found that adding more of a particular modifiers results in a progressive alteration of colour. A very good example is seen with the methyl violet series.

1.4     Classification of Dyes

There are several ways of classification of dyes. It should be noted that each class of dyes has a very unique chemistry, structure and particular way of bonding while substrate forming strong bonds in the process, others can be held by physical forces. Some of the prominent ways of classification is given here under:

Organic/inorganic

Natural/synthetic

By are and method of application

Chemical Classification: based on the nature of their respective chromophore.

By the nature of electronic excitation (i.e energy transfer colorants, absorption colorants and fluorescent colorants).

ü    According to dyeing methods

ü    Anionic (for protein fibre)

ü    Direct (cellulose)

ü    Disperse (polyamide fibre)

However the most popular classification is the one that is advocated by the United States international Trade Commission. This system classifies dyes into these types.

Acids Dyes: these are water soluble anionic dyes that are applied to fibres, such as silk, wool, nylon and modified acrylic fibres using neutral to acid dye baths. Attachment to the fibres is attributed partly to salt formation between anionic groups in dyes and cationic groups in the fibres.

Acid dyes are most sustentative to celluslosic fibre. Most synthetic food colours fall in this category.

Basic Dyes: these are water soluble cationic dyes that are mainly applied to acrylic fibres. But find some use for wool and silk usually acetic acid is added to the dye bath to help the uptake of the dye onto the fibre.

Direct Dyes: these are used on cotton, paper, leather, wool, silk and nylon. They are also used as PH indicators and biological stains.

Mordant Dyes: three required a mordant, which improves the fastness of the dye against water, light and perspiration.

Vat Dyes: these are essentially insoluble in water and incapable of dyeing fibres directly. However, reduction in alkaline liquor produces the water soluble alkaline metal salt affinity for the textile fibre.

Reactive Dyes: these utilizes a chromophore attached to a substituent that is capable of directly reacting with the fibre subtract. The covalent bonds that attach reactive dye to natural fibres make them among the most permanent of dyes.

Disperse Dyes: these were originally developed for the dyeing of cellulose acetate and are water insoluble. The dyes are finely ground in the presence of a dispersing agent and sold as a paste, or spray-dried and sold as a powered. Their main use is to dye polyester but they can also be used to dye nylon, cellulose triacetate and acrylic fibres.

Azoic Dyeing: this is a technique in which an insoluble azo dye is produced directly onto or within the fibre. This is achieved by treating a fibre both diazoic and coupling components.

Sulphur Dyes: these are two part developed dyes used to dye cotton with dark colours. The initial bath imparts a yellow or pale.

Chartreuse Colour: this is after treated with a sulphur compound in place to produce the dark black, we are familiar with in socks for instance, sulphur black 1 is the largest selling dye by volume.

1.5     Objectives of this Study

The objectives of this work are as follows:

1. To prepare an azo dye from 6-methyl-2-thiouracil
2. To characterise the dyes using uv-visible spectrophotometer, IR and NMS analysis.

1.6     Relevance of Study

This project work takes a critical look at the ways of exploring synthetic dyes for scientific and technological advancement by synthesising an azo dyes form 6-methyl-z-thiouracil. It also looks at various ways of identifying the composition of azo dyes and their characterization.

1.7     Limitation of the Study

The major factors that limits the synthesis of azo dyes are the high cost of raw materials (chemicals). Another major setback is the difficulty associated with purification and the high cost of analysis, especially NMR.