The Chemistry of

DyesDileep Grad student (dil2dilu@yahoo.com)

Dyes : These are coloured, ionising and aromaticorganic compounds which shows an affinity towards the substrate to which it is being applied (textiles, leather, plastic, paper etc). It is generally applied in liquid form.Dyes may also require a mordant to better the fastness of the dye on the material on which it is applied. One characteristic of dye is that the dyes must get completely or at least partially soluble in which it is being put to.

Why dyes are colored?Due to the presence of a group called Chromophorein the dyes Chromophore They have conjugated double bonds ( system), and these structures has delocalised electron systems. The partial structures necessary for color (unsaturated groups that can undergo -* and n-* transitions) were called chromophores: These structures are said to be responsible for the absorption of electromagnetic radiation that has varying wavelengths, based upon the energy of the electron clouds. Chromophore makes the dyes proficient in their ability to absorb radiation. Chromophores acts by making energy changes in the delocalised electron cloud of the dye. This alteration invariably results in the compound absorbing radiation within the visible range of colours and not outside it. Human eyes detects this absorption, and responds to the colours

Basic concepts of of Bandscolor of red, orange, yellow, blue, indigo, green and violet.Visible region 400-700nm

ACHROMATIC colors white (complete reflection of light) Black (Complete absorption) Grey (absorption of a fairly constant fraction throughout the 400-700nm range) CHROMATIC colors show ABSORPTION BANDS in the visible region

The Importance of Conjugation 1-pentene, max= 178 nm isoprene, , max= 222 nm

naphthale ne

Anthracene

Tetracene

Why conjugation cause shift to longer wavelengthExamples of __>* Excitation

Increased conjugation brings the HOMO and LUMO orbitals closer together.

What if conjugation is hampered???Ex: Schiff's reagent: Sulphurous acid reacts with Pararosanilin

colorlesscolored

Early dyesThe preparation and application of dyestuffs is one of the oldest forms of human activities. It was in 2600 BC when earliest written records of the use of dyestuffs were found in China. Natural Dyes: Ex: Indigo - extracted from a plant called woad Tyrian Purple - extracted from sea molluscs Problems associated: - variable supply and quality - inconsistent dyeing - fast fading property

Evolution of Synthtic Dyes:1856 -Real breakthroughs in the history of dyes

William Henry Perkin chanced upon first synthetic dye stuff 'Aniline Mauve' (Mauveine)

To make Quinine ????Knowledge of chemical structures was just emerging (The ring structures were still unknown) completely ignorant of the quinine structure Only molecular formula is known Quinine Mol. Formula C20H24N2O2. 2

(C

10H13N) +

2

O2H

C

20H24N2O2

Oxidizing a compound with formula C10H13N to get a compound of formula C20H24N2O2

Tried with Aniline

The Luck Hid in the Impurities Perkin inferred the presence of toluidine in mauve from its elemental analysis. The aniline sample that he was using was not pure; it contained toluidines in considerable quantities. - aniline reacted withtoluidineimpurities

Aniline

Toulidines:

Mauve itself is not a pure compound.

A

- 2 molecules ofaniline, one ofp-toluidineand one ofo-toluidine aniline, p-toluidine and o-toluidine (one molecule each)

B

In 2007 two other mauveine components were isolated and identified,

What affects the colour quality of the dye? HueThe hue of a dye depends on the wavelength it absorbs. Since the wavelengths) the dye absorbs depends on its structure, we can see that any change which affects thepsystem will affect the hue.A structural change which causes the absorption band to longer wavelengths (i.e. yellow -> orange -> red -> violet -> blue -> green) is called abathochromicshift. The reverse shift, towards shorter wavelengths is known as ahypsochromiceffect.

Chromophores and Auxochromes Chromophore is a group of atoms which control the colour of the dye

Auxochrome was a salt-forming group, which helped to improve the colour of the dye (auxochromes are normally electron-donating) -OH, -OR, -NH2, -NHR, -NR2, -X How a dye molecule's hue be altered: A bathochromic shift may be caused by increasing the electronwithdrawing power of the chromophore (X or Y), increasing the electron-donating power of the auxochrome (Y or X) and by increasing the length of the conjugated system connecting the two

Strength Themolar extinction co-efficientindicates the strength of a dye at low concentrations. It can be calculated using the Beer-Lambert Law A =bc where A is the absorbance of the dye at a particular wavelength,is the molar extinction co-efficient, c is the concentration of the dye and l is the path-length through the cell2. Brightness The brightness is best described by the shape of the absorbance band; if the band is narrow and sharp, then the dye is bright, if broad, then the dye is dull. This is due to other wavelengths absorbed by the dye, other than the one which causes the hue. The superposition of these determines how broad an absorption band is

Fastness

It is the property of a pigment or dye, or the leather, cloth, paper, ink, etc., containing the coloring matter, to retain its original hue, especially without fading, running, or changing. Running Running occurs principally during washing and exposure to detergents and solvents. Due to weak affinity for the material it is attached to OR much stronger affinity for a non-aqueous solvent. Fading Fading is caused by the chemical alteration of unstable dye molecules to a less strongly coloured or colourless form. Caused by the action of sunlight, or by the oxidising action of the atmosphere10. The UV radiation - cause unstable bonds to break or reform. Oxygen and atmospheric water will react with unstable bonds to alter the structure and affect its colour.

Classification: Two practical ways:-

chemical structure azo dyes (-N=N- group) anthraquinones (-C=O) Nitro dyes(-NO2) how it is applied to materials. Acid Basic Direct Disperse Reactive Solvent Vat Mordant

-

AZO DYESAzo compound class accounts for 60-70% of all dyes. All contain an azo group, -N=N-, which links two sp2hybridised carbon atoms. Usually the basic structure Ar-N=N-Ar. Supply more yellows, oranges and reds Azo dyes give bright, high intensity colours fair to good fastnessproperties Cost-effective Synthesis

Acid dyesWater-soluble anionic dyes Contain one or more sulfonic acid substituents (present as sodium sulphonate salts) or other acidic groups Applied from acidic dye baths to nylon, silk, wool, and modified acrylics. Ionic bonding between acid groups on the dye with the amino groups on the fiber. Ex: Acid Yellow 36

Basic dyesWater-soluble and produce colored cations in solution. (-NR3+or =NR2+) Mostly amino and substituted amino compounds soluble in acid Attached to the fibers by formation of salt linkages (ionic bonds) with anionic groups in the fiber. Used to dye paper, polyacrylonitrile, modified nylons, and modified polyesters Ex: Basic brown 1 Triaryl methane or xanthenes.

Direct dyesUsed for the direct dyeing of cotton and regenerated cellulose, paper and leather. Ex: mostly polyazo compounds Their flat shape and their length enable them to lie along-side fibres and maximise the Van-der-Waals, dipole and hydrogen bonds. Lack of fastness during washing They are cheap - popular for items which are less likely to require fastness during washing (for example paper)

Disperse dyesWater-insoluble nonionic dyes Used for application onto synthetic hydrophobic fibers from aqueous dispersions. Adsorbed onto the fibers

Ex:

Mainly used for dyeing polyester

General structure of a polyester Fairly hydrophobic The polymer chains in a sample of polyester are highly crystalline (for a polymer) and quite tightly packed together.

Fiber-reactive dyesForm a covalent bond with the appropriate textile functionality Extremely high wash fastness properties. Cotton, rayon, and some nylons are dyed by this relatively simple dye. Ex:

Cellulose polymer has hydroxy functional groups, - reactive dyes utilize as nucleophiles Under alkali conditions - cellulose-O-groups attack electron-poor regions of the dye; Perform either nucleophilic substitution to aromatics nucleophilic addition to alkenes

Nucleophilic substitution to aromatics

To encourage nucloephilic substitution, groups can be added to the aromatic ring which will decrease the electron density at a position and facilitate attack

Nucleophilic substitution is facilitated by the electron withdrawing properties of the aromatic nitrogens, and the chlorine, and the anionic intermediate is resonance stabilised as well. This resonance means that the negative charge is delocalised onto the electronegative nitrogens

Nucleophilic addition to alkenes Nucleophilic additions occur if there are sufficient electron withdrawing groups are attached to the alkene Ex: Remazol reactive dye (reacts in the presence of a base such as HO-)

*intermediate is resonance stabilised

Vat dyesInsoluble complex polycyclic molecules based on the quinone structure (ketoforms). Reduced with sodium hydrosulfite in a strongly alkaline medium to give soluble leuco forms that have a great affinity for cellulose. After the reduced dye has been absorbed on the fiber, the leuco forms are reoxidized (by hydrogen peroxide, sometimes atmospheric oxygen) to the insoluble keto forms. High wash and light fastness.

Mordant dyesMordants are usually metal salts; used to improve the colour fastness of dyes Some dyes combine with metal salts (mordanting) to form insoluble colored complexes (lakes). The fiber is first treated with an aluminum, chromium and iron salt and then contacted with a lake forming dye (azo and anthraquinone derivatives). Ex: Alizarin

C.I. Mordant Black 1 with and without a chromium (III) ion.

That property of a pigment or dye, or the leather, cloth, paper, ink, etc., containing the coloring matter, to retain its original hue, especially without fading, running, or changing when wetted, washed, cleaned; or stored under normal conditions when exposed to light, heat, or other influences. Essentially, this means that different dyes will have different fastnesses on different materials. Running Running occurs principally during washing and exposure to detergents and solvents- everyone knows what happens if a red sock or blue pants are accidentally put in a white wash.A dye will run if it has a weak affinity for the material it is attached to, or a much stronger affinity for a non-aqueous solvent. Detergents may cause running because they help to stabilise the hydrophobic regions of dye molecules due to their ability to form micelles. Fading Fading is caused by the chemical alteration of unstable dye molecules to a less strongly coloured or colourless form. This is often caused by the action of sunlight, or by the oxidising action of the atmosphere10. The UV radiation in sunlight has enough energy to cause unstable bonds to break or reform. Oxygen and atmospheric water will react with unstable bonds to alter the structure and affect its colour. There is no way to stop an unstable dye from fading.

References: http://www.uniregensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/D -ald_add-e.htm

http://www.dyespigments.com/what-is-dye.html http://www.morechemistry.com/publ/colours_dyes/slide044.html

http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/UVVis/spectrum.htm

A History of Dyes.http://www.straw.com/sigdyehist.html

Chemistry of natural and synthetic indigodyes.http://chriscooksey.demon.co.uk

Dileep