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THE CHEMISTRY OF COLOUR: Colour in Azo Compounds and How Dyes are Stuck onto Textiles. by empressteemah

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THE CHEMISTRY OF COLOUR: Colour in Azo Compounds and How Dyes are Stuck onto Textiles.
<p class="MsoNormal"><span style="font-size: 1rem;">The first commercially successful azo dye was
Chrysoidine. The azo group, – N=N –, acts as a ‘delocalisation bridge’ between
the two benzene rings to form an extended delocalised system, which is the
chromophore. The two amine functional groups of chrysoidine have lone pairs of
electrons on the nitrogen atoms. These lone pairs interact with the delocalised
system.</span><br></p><p class="MsoNormal"><span lang="">The nature of the functional groups that
interact with the chromophore can dramatically alter the colour of the azo dye
molecule by causing a shift in the electrons of the chromophore. This electron
shift alters the energy required to promote them into an excited state, and so
shifts the wavelength of light absorbed.</span></p><p class="MsoNormal" style="text-align: center; "><img src="https://res.cloudinary.com/drrz8xekm/image/upload/v1578400266/yusxfbjeuvdlhq5l9qlw.jpg" data-filename="yusxfbjeuvdlhq5l9qlw" style="width: 325.526px;"><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal" style="text-align: center; "><a href="https://commons.wikimedia.org/wiki/File:A_bis-azo_compound_depicted_as_eagle.jpg" target="_blank"><sup>A bis-azo compound is depicted as an eagle, and the trans-cis isomerization of the azo groups is compared to the flapping wings. Mass Spectra, CC BY-SA 4.0</sup></a><span lang=""><o:p><br></o:p></span></p><h2><span lang="">How are dyes stuck onto textiles?<o:p></o:p></span></h2><p class="MsoNormal"><span lang="">The dye of blue denim Jeans is indigo and it
is called a vat dye. A vat dye is usually soluble in its reduced form, but when
oxidized becomes insoluble and precipitates in the pores of the denim cotton
fibres. This property means that vat dyes do not wash out of clothes. (To learn
a little more about the history of blue denim and indigo, read the first two posts I
wrote on colour <b><a href="https://www.steemstem.io/#!/@empressteemah/the-chemistry-of-col-1555341343" target="_blank">here</a> and <a href="https://www.steemstem.io/#!/@empressteemah/the-chemistry-of-col-1555590726" target="_blank">here</a></b>)<o:p></o:p></span></p><p class="MsoNormal"><span lang="">Vat dyes are particularly effective for cotton
fibres and other fabrics that contain cellulose. The large number of hydroxyl
groups on the cellulose molecules mean that the fabric readily absorbs water
and hence the water-soluble dye. This is due to the formation of hydrogen bonds
between water molecules and the hydroxyl groups.<o:p></o:p></span></p><p class="MsoNormal"><span lang="">Another type of cotton dye is called a <b>direct</b>
<b>dye</b>. Direct dyes are long planar molecules that can lie alongside the
cellulose polymer chains, and form intermolecular hydrogen bonds and induced
dipole-induced dipole forces. Such large dye molecules are made water-soluble
using SO<sub>3</sub><sup>-</sup> Na<sup>+</sup> groups. Direct dyes are usually
diazo or triazo dyes.<o:p></o:p></span></p><p class="MsoNormal"><span lang="">The intermolecular forces between direct dyes
and cotton are relatively weak, so the water-fastness of the dye is poor and
some of the dye comes out when the material is washed. However, in 1956
chemists at ICI produced a dye that would form covalent bonds with the
cellulose fibres of cotton. The covalent bonds are formed by reactive groups
that first bond to the dye molecules. These groups then react with the hydroxyl
groups on the cellulose fibres. The dye is water-fast and the colour does not
run when the textile is washed because covalent bonds have formed with the
cellulose fibres, Such a dye is called <b>fibre-reactive</b>.<o:p></o:p></span></p><p class="MsoNormal" style="text-align: center; "><img src="https://res.cloudinary.com/drrz8xekm/image/upload/v1578400516/y4284areaupv2kqtjmxw.png" data-filename="y4284areaupv2kqtjmxw" style="width: 320px;"><span lang=""><br></span></p><p class="MsoNormal" style="text-align: center; "><a href="https://commons.wikimedia.org/wiki/File:Indigo_skeletal.svg" target="_blank"><sup>Vat Blue 1. Yikrazuul, Public Domain</sup></a><span lang=""><br></span></p><p class="MsoNormal"><span lang="">Wool and silk contain many amino functional
groups (NH<sub>2</sub>). These groups are basic and react with acid dyes that
contain sulfonic acid groups (SO<sub>3</sub>H) to form ionic bonds.<o:p></o:p></span></p><p class="MsoNormal"><span lang="">Poly(propenenitrile) fibres (acrylics) contain
CO<sub>2</sub>H and SO<sub>3</sub>H groups, which are acidic. These can form
ionic bonds with basic dyes. The last group of dyes I consider are <b>disperse</b>
<b>dyes</b>. All the other dye groups are water-soluble when applied, but
polyester fibres do not form hydrogen bonds with water and are known as <b>hydrophobic</b>
(water-hating). They do not allow water molecules to penetrate them. Disperse
dyes are a fine suspension of dye particles that are absorbed by the fibres and
held there by induced dipole-induced dipole forces and some hydrogen bonding.
As the dye is only sparingly soluble it tends to stay in the fibres, and so it
is water-fast.<o:p></o:p></span></p><h2><span lang="">pH INDICATORS<o:p></o:p></span></h2><p class="MsoNormal"><span lang="">Methyl orange is an azo compound that has
different colours depending on the pH of the solution it is in. This property
means that it is used as an indicator in acid-base reactions. Adding or
removing an H<sup>+</sup> ion causes an electron shift in the molecule, and so
alters the wavelength at which methyl orange absorbs. Indicators do change
colour depending on the concentration of H<sup>+</sup>(aq) ions in the
solution. Each indicator changes colour at a specific pH; using the correct
indicator, any neutralisation reaction can be monitored to its end point.
Methyl orange changes colour between pH 3.2 and pH 4.4. It can be used to
determine the neutralisation point of a strong acid and a weak base.<o:p></o:p></span></p><p class="MsoNormal"><span lang="">Phenolphthalein is colourless below pH 8.2,
but above this it changes to purple. This indicator can be used in titrations
with strong alkalis and weak acids.<o:p></o:p></span></p><h2><span lang="">Ultraviolet and visible spectroscopy<o:p></o:p></span></h2><p class="MsoNormal"><span lang="">There are two types of spectroscopy that involve
absorption of electromagnetic radiation by a substance under investigation. In
infrared spectroscopy, light is absorbed in the infrared part of the spectrum
because of the increased vibration of different bonds within a molecule. In
nuclear magnetic resonance spectroscopy, radio waves are absorbed through the
excitation of nuclei within molecules,<o:p></o:p></span></p><p class="MsoNormal"><span lang="">&nbsp;Ultraviolet
and visible spectroscopy are possible because outer electrons of atoms or ions
in compounds absorb in the ultraviolet or visible part of the spectrum when
they are excited. Compounds that absorb only in the ultraviolet part of the spectrum
are colourless. In the spectrometer, a beam of electromagnetic radiation passes
through a monochromator, which selects varying wavelengths. The beam is then
split and one beam passes through a solution of the substance under investigation,
while the other passes through the pure solvent.<o:p></o:p></span></p><p class="MsoNormal"><span lang="">The spectra produced usually have broad
absorption bands, in contrast to atomic absorption spectra in which gaseous atoms
and ions have definite sharp lines. The broad absorption bands occur because in
solution a number of vibrational and rotational energy levels are possible for
each energy level of the electrons.<o:p></o:p></span></p><h2><span lang="">INTERPRETING ULTRAVIOLET-VISIBLE ABSORPTION SPECTRA<o:p></o:p></span></h2><p class="MsoNormal"><span lang="">The horizontal axis of an ultraviolet-visible
absorption spectrum gives the wavelength in nanometres (nm). The shape of the absorption
peak is usually characteristic of a particular compound and so the spectrum can
be used to help identify the compound.<o:p></o:p></span></p><p class="MsoNormal"><span lang="">However, a more common use of this type of
spectroscopy is to measure concentrations accurately from the intensity of absorption
on the y-axis. For example, the uptake of a drug at different sites around the
body can be monitored using ultraviolet-visible spectroscopy by taking samples
from these sites and analysing their solutions.<o:p></o:p></span></p><p class="MsoNormal"><span lang="">In the steel industry, the amount of trace
metal, such as manganese, can be analysed by reacting the metal to form an identifiable
coloured ion and measuring the absorption of its solution. In this case, the
manganese may be oxidised to form the purple MnO<sub>4</sub><sup>-</sup> ion.<o:p></o:p></span></p><p class="MsoNormal"><span lang="">The food industry also uses
ultraviolet-visible spectroscopy to determine how much nitrite has been added
to meat.<o:p></o:p></span></p><h2><span lang="">TRANSITION METAL IONS AND COLOUR<o:p></o:p></span></h2><p class="MsoNormal"><span lang="">A characteristic of transition metals is that
many of their compounds are coloured. Transition metal compounds are
responsible for the colours in gemstones, stained glass windows and pottery
glazes. From reading this topic, you will realize that transition metal ions
appear coloured because they absorb some wavelengths in the visible spectrum,
but transmit or reflect the rest. The electrons absorb photons of a specific
wavelength, become excited and jump to higher energy levels. In compounds of
transition elements, colour results from a difference in energies between d
orbitals.<o:p></o:p></span></p><h3><span lang="">d-d TRANSITIONS<o:p></o:p></span></h3><p class="MsoNormal"><span lang="">You are probably wondering how d orbitals can
be at different energy levels. d orbitals were drawn at the same energy level
(degenerate) on energy level diagrams. However, this is only true for gaseous
transition metal ions. When ligands bond to transition metal ions they cause a
splitting of the energy level of the d orbitals. The energy difference between
the two sets of d orbitals is often such that the wavelength of photons
absorbed is in the coloured part of the spectrum.<o:p></o:p></span></p><p class="MsoNormal"><span lang="">Copper(I) compounds are white because the Cu<sup>+</sup>
ion has a 3d<sup>10</sup> outer electron configuration. Since the 3d subshell
is full, no transition of electrons between d orbitals can occur. The same is true
of scandium(III) compounds which have no electrons in the 3d subshell (3d°).
But not all colour in transition metals results from d-d transitions:
sometimes, electrons can jump from the ligand to the metal. This is called
charge transfer or electron transfer, and it is responsible for the bright colours
of Prussian blue and chrome yellow.</span></p><p class="MsoNormal" style="text-align: center; "><img src="https://res.cloudinary.com/drrz8xekm/image/upload/v1578400792/iba50uqem6pqkof3203b.jpg" data-filename="iba50uqem6pqkof3203b" style="width: 320px;"><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal" style="text-align: center; "><a href="https://commons.wikimedia.org/wiki/File:Coloured-transition-metal-solutions.jpg" target="_blank"><sup>From left to right, aqueous solutions of: Co(NO3)2 (red); K2Cr2O7 (orange); K2CrO4 (yellow); NiCl2 (turquoise); CuSO4 (blue); KMnO4 (purple).  Benjah-bmm27, Public Domain</sup></a><span lang=""><o:p><br></o:p></span></p><h2><span lang="">FACTORS THAT AFFECT d-d SPLITTING AND COLOUR<o:p></o:p></span></h2><p class="MsoNormal"><span lang="">The colour of a transition metal complex
depends chiefly on the central metal cation. Each transition metal is different
and has a different nuclear charge. The larger the nuclear charge, the more
firmly electrons are held in their d orbitals. This affects the energy levels
of the split d orbitals and thus the amount of energy required to excite an
electron from a lower energy d orbital to a higher energy d orbital. This in
turn affects the colour.<o:p></o:p></span></p><p class="MsoNormal"><span lang="">The oxidation state of the metal also affects
the splitting of the d orbitals and, as a result, the colour. This is well
illustrated by vanadium complexes in aqueous solution.<o:p></o:p></span></p><p class="MsoNormal"><span lang="">The nature of the ligand also has an effect on
d-d splitting. Different ligands cause different separations of energy between
d orbitals. The <b>spectrochemical</b> <b>series</b> is a list of ligands
arranged in order of their ability to cause d-d splitting:<o:p></o:p></span></p><p class="MsoNormal"><span lang="">I<sup>-</sup> &lt; Br<sup>-</sup> &lt; Cl<sup>-</sup>
&lt; F<sup>-</sup> &lt; OH<sup>-</sup> &lt; H<sub>2</sub>O &lt; (CO<sub>2</sub>)<sup>2-</sup>
&lt; NH<sub>3</sub> &lt; en &lt; CN<sup>-</sup><o:p></o:p></span></p><p class="MsoNormal"><span lang="">smallest splitting to the greatest splitting<o:p></o:p></span></p><p class="MsoNormal"><span lang="">smallest energy gap to the greatest energy gap<o:p></o:p></span></p><p class="MsoNormal"><span lang="">longest wavelength to the shortest wavelength<o:p></o:p></span></p><p class="MsoNormal"><span lang="">[Cu(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup>
absorbs at the red end of the spectrum, which makes an aqueous solution of copper(II)
ions appear pale blue. However, by substituting H<sub>2</sub>O with NH<sub>3</sub>
ligands to form the ion [Cu(NH<sub>3</sub>)<sub>4</sub>(H<sub>2</sub>O)<sub>2</sub>]<sup>2+</sup>,
the difference in energy of the d orbital split increases.<o:p></o:p></span></p><p class="MsoNormal"><span lang="">This shifts the absorption into the yellow
part of the spectrum, which makes the solution deep blue.<o:p></o:p></span></p><p>









































































</p><p class="MsoNormal"><span lang="">Another factor that has an effect on d-d
splitting is the number of each type of ligand in the complex: [Cr(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup>
is violet, [Cr(H<sub>2</sub>O)<sub>5</sub>Cl]<sup>2+</sup> is green and [Cr(H<sub>2</sub>O)<sub>4</sub>Cl<sub>2</sub>]<sup>+</sup>
is dark green. The arrangement of ligands around the central metal cation can
also have an effect on the colour of a complex.&nbsp;</span></p><p class="MsoNormal"><br></p><h2><span lang=""><o:p>REFERENCES</o:p></span></h2><p class="MsoNormal"><a href="https://en.wikipedia.org/wiki/Azo_dye" target="_blank">https://en.wikipedia.org/wiki/Azo_dye</a><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><a href="https://en.wikipedia.org/wiki/Azo_compound" target="_blank">https://en.wikipedia.org/wiki/Azo_compound</a><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><a href="https://www.sciencedirect.com/topics/chemistry/azo-compound" target="_blank">https://www.sciencedirect.com/topics/chemistry/azo-compound</a><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><a href="https://www.britannica.com/topic/textile/Dyeing-and-printing" target="_blank">https://www.britannica.com/topic/textile/Dyeing-and-printing</a><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><a href="https://www.quora.com/What-makes-clothing-dye-stay-in-fabrics" target="_blank">https://www.quora.com/What-makes-clothing-dye-stay-in-fabrics</a><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><a href="https://www.moneycrashers.com/how-to-dye-fabric-clothes-make-natural-dyes/" target="_blank">https://www.moneycrashers.com/how-to-dye-fabric-clothes-make-natural-dyes/</a><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><a href="https://en.wikipedia.org/wiki/Disperse_dye" target="_blank">https://en.wikipedia.org/wiki/Disperse_dye</a><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><a href="https://www.chemicalbook.com/ProductCatalog_EN/161113.htm" target="_blank">https://www.chemicalbook.com/ProductCatalog_EN/161113.htm</a><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><a href="https://www.thesprucecrafts.com/fiber-reactive-dye-1106363" target="_blank">https://www.thesprucecrafts.com/fiber-reactive-dye-1106363</a><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><a href="https://www.britannica.com/technology/direct-dye" target="_blank">https://www.britannica.com/technology/direct-dye</a><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><a href="https://textilelearner.blogspot.com/2011/02/defination-classification-application_2111.html" target="_blank">https://textilelearner.blogspot.com/2011/02/defination-classification-application_2111.html</a><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><a href="https://en.wikipedia.org/wiki/Vat_dye" target="_blank">https://en.wikipedia.org/wiki/Vat_dye</a><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><a href="https://www.compoundchem.com/2014/04/04/the-colours-chemistry-of-ph-indicators/" target="_blank">https://www.compoundchem.com/2014/04/04/the-colours-chemistry-of-ph-indicators/</a><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><a href="https://www.thoughtco.com/definition-of-ph-indicator-605499" target="_blank">https://www.thoughtco.com/definition-of-ph-indicator-605499</a><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><a href="https://en.wikipedia.org/wiki/PH_indicator" target="_blank">https://en.wikipedia.org/wiki/PH_indicator</a><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><a href="https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Acids_and_Bases/Acid_and_Base_Indicators/PH_Indicators" target="_blank">https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Acids_and_Bases/Acid_and_Base_Indicators/PH_Indicators</a><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><a href="https://www.jove.com/science-education/10204/ultraviolet-visible-uv-vis-spectroscopy" target="_blank">https://www.jove.com/science-education/10204/ultraviolet-visible-uv-vis-spectroscopy</a><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><br></p>
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