Chapter 2-3 Introduction The Chemical principles of Coloration A single atoms consists of a central core or nucleus which contains numbers of positively charged particles(protons) and uncharged particles(neutrons) together accounting for almost all the mass of the atom. 1 2 Electrolytes Electrolytes Electrolytes are compound made up of ions and are often referred to as ionic compounds. There are many simple electrolytes and the two most commonly used in coloration are - When a fiber is immersed in water a negative electrostatic charge develops on its surface. This charge repels any dye anions present in the solution, so that the fiber can not be dyed satisfactorily. 1) acl : a + + Cl - 2) a 2 S 4 (Glauber s salt) : 2a + + S 4 2- - owever, the dye bath also contains an electrolyte such as acl or a 2 S 4 a diffuse layer of positive sodium ions forms at the fiber surface. Water-soluble dyes are also electrolytes, but the colored part is very large and usually an anion, while the cation(a + ) is very small by comparison. Cellulose Fiber Cellulose Fiber 3 4
The Covalent Bond *interaction of a coordinativelyunsaturated transition metal with a C- bond The Covalent Bond **an electron-deficient chemical bond where three atoms share two electrons Covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding. Covalent bonding includes many kinds of interaction, including σ- bonding, π-bonding, metal-to-metal bonding, *agostic interactions, and **three-center two-electron bonds. The term covalent bond dates from 1939. In the molecule 2, the hydrogen atoms share the two electrons via covalent bonding. 5 6 *Sodium stearate : the most common soap. found in many types of solid deodorants, rubbers, latex paints, and inks. The Covalent Bond The Covalent Bond Carbon chains are often represented in the zig-zag fashion which is a two-dimensional representation of the tetrahedral arrangement of the four bonds of the carbon atoms in space zig-zag arrangement. When ionic groups are added to large hydrocarbon molecules in order to make them soluble, they may form a special kind of solution called a colloidal electrolyte. ; this molecule dissociates in water, but the large ions do not remain separate. * ydrocarbons are chemically unreactive and insoluble in water. 7 8
*an aggregate of surfactant molecules dispersed in a liquid colloid. ne of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lightning I awoke. Friedrich August Kekulé (1829-1896) The Covalent Bond Aromatic Compounds They cluster together to form special loosely knit spherical structures called *micelles, in which the insoluble hydrocarbon chains (the tails ) tend to associate together and form a hydrophobic (water-repelling ) environment with the ionic heads on the outer surface, keeping the micelle dissolved. Water-insoluble matter may dissolve within the micelle, suspended in the solution (ex. etergent act) - A C=C bond is shorter than a C C bond, but benzene is perfectly hexagonal all six carbon-carbon bonds have the same length, intermediate between that of a single and that of a double bond. - A better representation is that of the circular π bond in which the electron density is evenly distributed through a π-bond above and below the ring. 9 10 Aromatic Compounds Aromatic Compounds - All dye molecules contain aromatic ring structures. - The reactions of functional groups such as the carboxyl group or the amino group are noticeably different when they are linked to an aromatic ring and when they form part of an aliphatic molecule. - ne of important reaction in dye synthesis is that carried out using the amino(- 2 ) group attached to a benzene ring in phenyl amine (ex. aniline, diazo component) - iazotization reaction : a colorless aromatic base is diazotized with nitrous acid to form a diazonium salt. - Coupling reaction : iazonium salt react readily with an appropriate coupling component to form a dye molecule. 11 12
Aromatic Compounds Color of yes It is more usual in dye molecules to find fused rings, such as naphthalene, or anthraquinone, on which many vat and disperse dyes are based. - The physiological sensation of color arises when an object does not reflect all the incident white light falling on it. - Some of the light energy is absorbed and the remainder is reflected and perceived color. Absorb Blue and appears Yellow-range (complementary color : Blue range ) Violet: 400-420 nm Indigo: 420-440 nm Blue: 440-490 nm Green: 490-570 nm Yellow: 570-585 nm range: 585-620 nm Red: 620-780 nm Violet avy Blue Green 13 14 Molecular rbital(m) Theory E = hc / λ Color of yes Color of yes LUM - the molecular orbital picture for the hydrogen molecule consists of one bonding σ M, and a higher energy antibonding σ* M. - When the molecule is in the ground state, both electrons are paired in the lower-energy bonding orbital this is the ighest ccupied Molecular rbital (M). The antibonding σ* orbital, in turn, is the Lowest Unoccupied Molecular rbital (LUM). M 15 - If the molecule is exposed to light of a wavelength with energy equal to E, the M-LUM energy gap, this wavelength will be absorbed and the energy used to bump one of the electrons from the M to the LUM in other words, from the σ to the σ* orbital. - This is referred to as a σ - σ* transition. E for this electronic transition is 258 kcal/mol, corresponding to light with a wavelength of 111 nm. 16
Color of yes estructive interference Constructive interference Color of yes - Where UV-vis spectroscopy becomes useful to most organic and biological chemists is in the study of molecules with conjugated pi systems. 3 nodes, 0 constructive interactions node - When a double-bonded molecule such as ethylene absorbs light, it undergoes a π - π* transition. - Because π- π* energy gaps are narrower than σ - σ* gaps, ethylene absorbs light at 165 nm - a longer wavelength than molecular hydrogen 17 The four atomic (2p z ) orbitals have combined to form four π molecular orbitals. node node node 2 nodes, 1 constructive interactions 1 nodes, 2 constructive interactions 0 nodes, 3 constructive interactions 18 Color of yes E = hc / λ - In these groups, the energy gap for π -π* transitions is smaller than for isolated double bonds, and thus the wavelength absorbed is longer. Color of yes - As conjugated pi systems become larger, the energy gap for a π - π* transition becomes increasingly narrow, and the wavelength of light absorbed correspondingly becomes longer. - Comparing this M picture to that of ethylene(isolated pi-bond example), M-LUM energy gap is indeed smaller for the conjugated system. 1,3-butadiene absorbs UV light with a wavelength of 217 nm. - The absorbance due to the π - π* transition in 1,3,5-hexatriene, for example, occurs at 258 nm, corresponding to a E of 111 kcal/mol. - Molecules or parts of molecules that absorb light strongly in the UV-vis region are called chromophores 19 - In molecules with extended pi systems, the M-LUM energy gap becomes so small that absorption occurs in the visible rather then the UV region of the electromagnetic spectrum. 20
Color of yes Color of yes - The conjugated pi system in 4-methyl-3-penten-2-one gives rise to a strong UV absorbance at 236 nm due to a π - π* transition. - owever, this molecule also absorbs at 314 nm. This second absorbance is due to the transition of a nonbonding (lone pair) electron on the oxygen up to a π* antibonding M: n - π* transition. - β-carotene, the compound responsible for the orange color of carrots, has an extended system of 11 conjugated π bonds. - β-carotene absorbs light with wavelengths in the blue region of the visible spectrum while allowing other visible wavelengths mainly those in the red-yellow region - to be transmitted. This is why carrots are orange. - The nonbonding (n) M s are higher in energy than the highest bonding p orbitals, so the energy gap for an n - π* transition is smaller that that of a π - π* transition and thus the n - π* peak is at a longer wavelength. - In general, n - π* transitions are weaker (less light absorbed) than those due to π - π* transitions. 21 22 E = hc / λ Color of yes Color of yes - The sigma to sigma* transition requires an absorption of a photon with a wavelength which does not fall in the UV-vis range. - Thus, only pi to pi* and n to pi* transitions occur in the UVvis region are observed E = hc / λ - The absorption of light energy by colorant causes an electron to jump into a higher energy level, thus bringing the dye molecule into an excited state. - It is easier for an electron to jump into an excited state from a double bond, where it exists as π electron. - Less energy still is required for the transition if alternate single and double bonds exist in the same molecule. Consequently, as the excitation of an electron becomes easier, the required spectral energy moves from the invisible ultraviolet into the longer wavelengths of the visible spectrum. 23 24
Color of yes Color of yes - The simplest Azo molecule is Phenylazobenzene which has weak yellow color. - Phenylazobenzene is called the chromospheres group of Azo dyes - and the color of the molecule may be modified and increased in intensity of color by introducing a variety of smaller groups into the molecule is called Auxochrome (X,Y,Z & R 1,R 2,R 3 ). X Phenylazobenzene Y Z R 3 Azo dye R 1 R 2 25 26 2 C S 3a C Color of yes anthracene-9,10-dione (anthraquinone) Vander Waals forces and hydrophobic bonding ye group (C 12 25) S 3a Aliphatic Chain Solubilizing group - Anthraquinone contains two carbonyl groups in a conjugated system - It absorbs ultraviolet light weakly and appears cream in color. - Replacement of hydrogen atoms in the outer rings by suitable auxochromes groups based on nitrogen, oxygen or sulfur brings about the development of strong color. C.I. isperse Blue 56 - The large size of dye molecules contributes to the general attractive forces that they exert on surrounding molecules: van der Waals forces. - ydrophobic groups in molecules tend to associate together and escape from an aqueous environment from a dye bath to a fiber. Bond Type Relative Strength Van der Waals 1.0 ydrogen 3.0 Salt link 7.0 Covalent 30.0 - Such compounds are the basis of many vat and disperse dyes. Indigo 27 28
a 3S S 3a a 3S S 3a 3C C Vander Waals forces and hydrophobic bonding ydrogen Bonds 2C 2C 2C 2C 2C - van der Waals forces are individually weak but their collective effect in large organic molecules is considerable. - They originate as weak interactions between the nuclei of the constituent atoms of one molecule with the electrons of another. - They operate only when dye and fiber molecules are in close proximity to each other, when they can become the dominant force of attraction. - A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine that comes from another molecule or chemical group. - Most dye and fiber molecules possess groups with hydrogen bonding capability. Consequently hydrogen bonds are involved in dye fiber attractions. 29 30 ye aggregation Covalent bonding Cell R Cell Cl R Cell Cl R Cell Individual dye molecules are also attracted to each other through van der Waals forces and hydrogen bonds with the result that many dyes exist in solution as large molecular clusters called dye aggregates. ( colloidal electrolytes) Covalent bonding between dye and celluosic fibers is achieved by the incorporation in the dye molecule of special reactive groups, linked to the rest of the molecule through a bridging group. 1) The large size of the dye aggregates can lead to a drastic reduction in the rate of fiber penetration, or in some cases to the precipitation of a dye from solution after prolonged storage. 2) The addition of electrolyte to a dye bath can increase the dye aggregation but the rise in temperature decrease the dye aggregation. 3) A certain degree of aggregation can be beneficial since it tends to increase the attraction of the dye for the fiber. S C C S a Cell S C C 2 Cell S C C + a 2S 4 + 2 S C C Cell S C C Cell 31 32
Acid and Basic groups in dyes and fibers p Values Cl + Cl -a + a - The sulfonic acid group in a dye molecule will react with sodium hydroxide to form the sodium salt of dye (-S 3 a). - a basic group in a large molecule, Such as a fiber molecule, will react with an acidic group and form a salt. (- 3+ ) a 3 S a 3 S = = CI Acid Yellow 23 Pyrazolone (igh light fastness) Ca S 3 a CC 3 a 3 S S 3 a = CI Acid Red 138 S 3 3 33 = ye 34 S S Insoluble Alkali reduction Acid oxidation < Sulfur dyeing > a + + S S a + Soluble and substantive Redox reactions Redox reactions xidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion. Insoluble pigment a Alkali reduction Acid oxidation Solubilized sodium leuco vat Reduction is the gain of electrons or a decrease in oxidation state by a molecule, atom, or ion. When a compound is oxidized it gains oxygen and reduction is a substance can gaining hydrogen atoms. - Vat dyes such as indigo and compounds derived from anthraquinone are applied after the temporary reduction of two carbonyl groups in a conjugated chain; this converts the dye into a colorless water-soluble form. - The conversion is carried out using a strong reducing agent, and in the reaction the two oxygen atoms become reduced to - called leuco vat acid, and is applied from an alkaline solution. 35 - nce on the fiber it can be re-oxidized back to the insoluble carbonyl form by air or by the use of an oxidizing agent. 36