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Colorful Organic Chemistry:  Organic Dyes & Pigments Brighten Up the World
By Amy Sillup

The world of color is important to plants and animals and like sunlight passing through a prism these colors unfold as rainbows. Explore the organic chemistry of colors and dyes here and now.
 
 
 
 
Light Spectrum (Credit:NASA)  Prism-generated Rainbow ((Credit:LBL)
Pigments Dyes and Mordants: Definitions and General Information 

Solid dyestuffs are called pigments, while colored liquids are known as dyes.  Organic or “carbon-based” pigments are sometimes combined with inorganic substances like alumina (aluminum hydroxide) to create “lakes”; this combination helps to make the pigment insoluble in water and promotes useful pill and candy coatings that won’t rub off or transfer onto consumers’ hands or skin.  “Mordants” are substances that are absorbed onto cloth.   the dye is applied, it reacts to form a complex with the mordant and the final color obtained depends on the structure and chemistry of both reactants.  

Classifications of Color Chemicals –Organic and Inorganic

Dyes and pigments are often organized according to specific groups of atoms in their molecular structures as are, for example, the “azo dyes.”  Dyes may also be named according to how they work or the way they are employed:  Reactive dyes react with the substance they color by forming covalent bonds with it, whereas direct dyes are applied directly from a bath of sodium salt.    

Colored chemicals may also be simply classified as organic (carbon-based) or inorganic.  Some dyes have an organic portion linked to a metal ion.  Ferrous gluconate, used to color black olives, is composed of iron in the +2 oxidation state plus two molecules of a negatively-charged form of glucose.    

An Overview of Organic Colors:  Structure and Function       
 
Frequently, colorful organic molecules contain aromatic or conjugated systems in their structure with a double bond, single bond, double bond arrangement which spreads out electron density and charge very nicely.  The electrons scurry or “dance” back-and-forth, to-and-fro, along the hydrocarbon chain or whirl dizzyingly around an aromatic ring, forming “resonance structures”. Because of this arrangement of the bonds or resonance, it doesn’t take much light energy to get these electrons even more excited, so lower-energy visible light works just fine to activate the electrons.  
 
Naturally-occuring molecules readily absorb distinct and particular wavelengths and reflect (do not absorb) the opposite color: 
  • beta-Carotene is the biochemical that makes carrots, squash and pumpkins appear orange because these carotene-rich vegetables absorb blue light and reject orange.
  • Lycopene in tomatoes absorbs green wavelengths of light and rejects the red wavelengths.
  • Chlorophyll found in plants absorbs blue and red light, but rejects green light and reflects green wavelengths back to our eyes. Therefore, what color you see is what the organic molecule primarily is not absorbing.
 
Beta-Carotene Molecule. Photo Credit: Chemicalbook.com


Lycopene Molecule. Photo Credit: Chemical Book.com
 
 
Chlorophyll Molecule. Photo credit: DNR, South Carolina
 
Carotene-rich Pumpkins on Chlorophyll-Rich Grass. Photo Credit: State of Maine
 
Lycopene-rich Tomatoes. Photo Credit: Tarrant County.
 
Chlorophyll-Laden Evergreen Trees at Sprague Lake. Photo: NASA
 
Chromophores: Special Chemical Atoms for Color in a Molecule

The part of the molecule that gives a substance its color is known as a chromophore.  Any “helper functional groups” in the molecule that can donate extra electrons to the conjugated system are referred to as auxochromes.  Auxochromes intensify the color of the dye.  For example, the nitrogen atom in an amine group has a lone pair of electrons, and is present in many brightly colored molecules. Here below is a diagram of aniline as an obvious example.

Aniline Molecule. Photo Credit: NIST WebBook
A circle is used in the diagram to symbolize and remind us that the electrons are whizzing about the ring wildly rather than sitting staidly in a pure double-single-double bond formation. 
Some amine-containing molecules, like para-amino benzoic acid (PABA), were removed from products like sunscreen, partly because of their tendency to stain or dye clothing. The ring structure in PABA and related molecules absorbs harmful UV radiation energy and protects the skin cells and DNA within from UV damage.
 
PABA or Para-AminoBenzoic Acid. Phot0 Credit: Chemicalbook.com
 
 
Melanin, the dark brown to black pigment of human skin, is produced by melanocytes and is the best known natural sunscreen and protectant molecule. Melanin is formed in a series of reactions from naturally-occurring tyrosine amino acid molecules. 
 
Melanin (Eumelanin) Segment of the Polymer Molecule.
Photo credit: WikiMedia Commons, Roland Mattern. 
 
Imines or Schiff Bases, Azo Dyes & Fluorescein Molecules – Chemistry 
 
Certain molecules with a double bond between carbon and nitrogen are called imines or “Schiff’s bases”. Imines give fragrances like Georgio and Tabu their eye-catching colors (please make sure to keep them off your clothes!).  Double bonds between two nitrogen atoms form azo groups and  brilliantly colored azo dyes are used in textiles, plastics, and as food colorings. 
 
 
Azo Groups in Azo Chemical Used in Manufacturing.
Photo Credit: Chemicalbook.com
 
Some chameleon-like molecules actually change color depending on their chemical environment.  These compounds often are good and useful indicators of the pH or hydrogen-ion (H+) concentration. pH indicators can be used to measure the pH in the range of 0 (very acid) to 14 (very basic).  Litmus paper contains dye that turns blue in the presence of a base and red in an acidic environment.  Pigmented water left over from boiling red cabbage, blueberries or grape juice are useful pH indicators because they contain flavenoids called anthocyanins which can lose or accept a proton depending on whether they are exposed to basic or acidic surroundings. 
 
Some dyes make good adsorption indicators, too; or may be used to detect the presence of other substances.  For instance, yellow fluorescein makes a good indicator in the reaction of silver nitrate and sodium chloride. Green diaminofluoresceins react with nitric oxide and can detect nitric oxide down to the parts per trillion range.  The versatile fluoresceins even make pretty makeup colors. Two of the most common dyes in lipstick are red 4,5-dibromofluorescein and 2,4,5,7-tetrabromofluorescein.     

FLUORESCEIN DISODIUM SALT DIHYDRATE. Photo credit: Chemicalbook.com
Natural Animal and Plant Dyes Start the Craze for Color

The above-mentioned litmus paper uses a plant dye derived from a lichen which grows on trees.  The earliest-known dyes were all made from natural animal or plant substances. 

Very expensive Tyrian purple was made in imperial dye-works and only was used for the garments of nobles and royals.  Originally Tyrian purple was extracted from shellfish glands and about 9000 mollusks were sacrificed to obtain one gram of the dye.  Legends say that the hero Hercules first discovered this dye while out walking his dog, who liked to crunch on the tasty snails and ended up dyeing the fur around its mouth! Tyrian purple actually ranges in color from red to blue-black. 
 
Marine life such as sea urchins contain useful red pigments as exemplified by echinochrome. One of the most ubiquitous red colors, the dye known as carmine, comes from an insect.  Carmine is also called cochineal, and contains about 19-22% carminic acid. Carmine has this long International Union of Pure and Applied Chemistry (IUPAC) name: 7-D-glucopyranosyl-3,5,6,8-tetrahydroxy-1-methyl-9-10-dioxoanthracene-2-carboxylic acid, which gives clues to the source of its color:  Anthracene provides the aromatic systems and all those oxygen-containing hydroxy and ketone groups make super auxochromes.
 
 
Carmine Dye Molecular Structure. Photo Credit: Chemicalbook.com

Carmine Dye was used by the ancient Aztecs and brought to Europe by explorer Hernán Cortéz in the 16th century.  Sometimes derisively referred to as “crushed bug dye,” seventy thousand tiny cactus-dwelling beetles are needed to make a pound of carmine.  The female beetles’ shells are dried and dissolved in solvent and then the “insect parts” are filtered out of solution.

Carmine is a lovely magenta at almost neutral pH, orange at pH levels of 3 or less, and purple at pH 7 or above.  It is used in lipsticks, blushes, mascaras, shampoos, and as a food coloring.  Vegans and those who wish to keep kosher have to check labels carefully in order to avoid this animal dye.  In 2012, in response to an angry petition signed by over 6000 customers, the coffee chain Starbucks replaced the carmine in its red and pink drinks and snacks with “bug-cruelty-free” lycopene. Now, that’s a better choice for everyone! 

Popular early plant dyes include indigo, from Indigofera tinctoria and Isatis tinctoria. When fermented under basic conditions and allowed to oxidize in air, these plants produce the blue molecule indigotin.  Indigo production today is usually based on synthetic processes, however.  Indigotin can be dibrominated to produce a form of Tyrian purple, however cheaper synthetics have generally replaced this process as well.   

Indigo Dye Molecular Structure. Photo Credit: Chemicalbook.com
 
Madder plants contain red alizarin; walnuts yield brown juglone; and henna plants produce the orange-red color of lawsone.  Logwood provides violet/blue shades, while barwood furnishes red ones.  Aloe contains aloetin, which is a muddy brown color, while catechu from the Khair tree gives a rusty brown.  Black gallic acid was obtained from gall.  
 
The saffron crocus has been used to make yellow-orange dye since at least 1900 B.C.  Not surprisingly, a portion of the crocetin molecule in saffron contains a 14-carbon conjugated chain exactly like that in beta-carotene.  Harvesting of saffron is very labor-intensive and expensive, so as with indigo plants and many other natural substances, dyes made of it are rare today.  Annatto is a yellow-orange pigment from Bixa orellana seeds; it is used as a cheap replacement for saffron in cheese, butter, and other foods.
 
Saffron Molecule. Photo credit: Chemicalbook.com

Luckily for 19th century citizens, that time period produced plenty of inexpensive synthetic dye molecules to supplant or supplement the colors which Nature so generously supplies.     

Synthetic & Manufactured Dyes: The Nineteenth Century and Beyond

The first artificial dyes were derived from uric acid in bird excrement (makes those little carmine beetles sound a lot better as dye sources, doesn’t it?) and nitric acid.  Uric acid synthesized chemicals, known as murexide derivatives, were first synthesized in Manchester, England and certain areas of France.  They were attractive purple shades, and eventually became known as French purple.

Britain may have lost out on the name “Manchester purple,” but won big by producing the real father of modern industrial dyestuffs – William Henry Perkin. During the 1850s, Britain was importing 75,000 tons of costly natural dyes per year. Synthetic alternatives were sorely needed, and Perkin created one quite by accident.

While he was studying chemistry at London’s Royal College, much against his father’s will, the elder Perkin thought nobody could make a living as a “head in the clouds” science major.  At the age of eighteen Perkin tried to synthesize an inexpensive form of the malaria cure quinine from impure allyl toluidine and potassium dichromate.  Instead of quinine he produced an ugly brown sludge.  Disappointed, Perkin decided to see if this product could at least be used as a test for aromatic bases.  When he reacted it with aniline sulfate, a black precipitate formed.  He dissolved this in ethanol, and noticed its deep purple color.  Curious, Perkin dipped some silk strips into the liquid, thereby discovering mauveine, also known as mauve or  aniline purple, which did not fade as readily as French purple.

By 1859, the factory Perkin promptly opened was producing tons of aniline purple. Just about everybody, including Queen Victoria herself, wore clothes dyed with the exquisitely pale purple. Aniline became popular as a mourning color, and the era eventually earned the moniker the Mauve Decade.  However, the 19th century’s contributions to color were far from over.    

 
Also in 1859, a creative British pharmacist’s assistant named Edward Chambers Nicholson oxidized aniline and made magenta fuchsine, sometimes called roseine or rosaniline and he also synthesized a blue dye from aniline. 
 
  
Rosaniline Molecule. Photo Credit: Chemicalbook.com
 
Soon after this, Germany’s scientists began to add their colorful chemical expertise and ingenuity to the dyestuff industry.  1863 brought the world beautiful amidoazobenzene or aniline yellow, developed at the German company AGFA.  
 
Another German powerhouse, Bayer and Company, was actually founded in 1861 to make aniline dyes. Bayer’s  colors made the firm so successful that by 1900 it had the capital to branch out into producing pharmaceuticals, including that particularly useful molecule acetylsalicylic acid, better known as aspirin.
 
 
Acetylsalicylic Acid, Aspirin, Molecule. Photo Credit: Chemicalbook.com
 
In 1869, Perkin and German chemical company Badische Anilin und Soda Fabrik (BASF) head chemist Heinrich Caro simultaneously and separately oxidized anthracene to anthraquinone, and then acid-hydrolyzed it to alizarin.  Caro’s work was done in collaboration with German academics Carl Graebe and Carl Liebermann.  Both Caro and Perkin rushed to patent the process, but Caro’s patent issued 24 hours earlier, and Perkin had to license the new red synthetic dye from his rival.  By 1900, BASF was producing 2000 tons/year of alizarin.
 
 
Alizarin Molecule. Photo Credit: Chemicalbook.com

A year later, another German company called Hoechst was founded to produce fuchsine.  Hoechst’s chemists came up with another way to make alizarin, and swiftly patented their lucrative process. 
 
 
Fuchsine Molecule. Photo Credit: Chemicalbook.com

I
n 1880, Adolf von Baeyer synthesized indigo from dinitrodiphenylacetylene and other nitrated organic molecules.  Unfortunately, his process utilized seven steps and was prohibitively expensive.  In 1890, Carl Heumann made indigo from anthranilic acid, and his process was simpler and far cheaper.  About 14,000 tons of synthetic indigo are still produced per year to dye blue jeans. 

Dyestuffs remained a mainstay of industrial chemistry.  Even the huge modern chemical conglomerate, IG Farben, can trace its roots back to 1925, when the Syndicate of Dyestuff Industry Corporation (Interessengemeinschaft Farbenindustrie Aktiengesellschaft) was formed to perk up the dye industry after World War I had destroyed the German economy.  

Hair and Hair Dyes: France Produced the First Penetrating Hair Dye

Early synthetic hair dyes included anilines, pyrogallic acid, lead acetate, and silver nitrate.  Some consumers ended up developing nephritis, Bright’s disease, or  severe skin problems and 3-5% of the aniline dye users developed chemical anemia when dye intermediates were absorbed into their blood. Thus, chemistry and humanity intersected with some serious disease consequences for humans.  Early dyes looked terribly fake, and often dyed hair the wrong shade, too. Fans of L. M. Montgomery’s books may remember “Anne of Green Gables” and her attempt to turn her red hair  to raven-black with disastrous (albeit very funny) results.
 
In 1907, French chemist Eugène Schueller synthesized the first relatively safe and effective hair color with para-phenylenediamine, which penetrated the hair shaft instead of simply coating the hair.  He founded a company named after a then-popular “halo” hairstyle called l’auréole; readers know or have probably heard of his cosmetics corporation – “L’Oréal.”  Sales of hair dyes went through the roof during the Great Depression, when a youthful appearance was considered vital to obtaining and keeping employment.  By 2001, one in five Americans admitted to coloring their hair.  Dyes stimulate both confidence and the economy!
 
  

para-Phenylene Diamine Molecule. Photo Credit: Chemicalbook.com

 
Indeed, dyes have always been so profoundly important to the world economy and society as a whole that the ancient Bolos of Mendes, perhaps the first well-known alchemist, listed them as one of his four principal practical testing subjects.  Now, as in times past, many people enjoy these eye-catching, colorful elements, plants, animals and amazing organic synthetics which provide the world so many rich and varied colors. Spectral majesty is everywhere for us to behold and enjoy. Our visual lives affirm these colorful reminders of the beauty of light and chemistry!
 

Sources 

Associated Press. 2012. "Starbucks to Stop Using ‘Crushed Bug’ Dye".  Accessed April 19, 2012 @ foxnews.com/leisure/2012/04/19/starbucks-to-stop-using-crushed-bug-dye  

Brandon, Ruth. 2011. Ugly Beauty. Harper, New York.

Brock, William H. 1992. The Norton History of Chemistry. W.W. Norton & Company, New York.

Daintith, John, ed. 1996. A Dictionary of Chemistry. Third ed., Oxford University Press.

Emsley, John. 1998. Molecules at an Exhibition. Oxford University Press.   
Emsley, John. 2004. Vanity, Vitality, and Virility: The Science Behind the Products You Love to Buy. Oxford University Press.
 
Field, Simon Quellan. 2008. Why There’s Antifreeze in Your Toothpaste:  The Chemistry of Household Ingredients. Chicago Review Press, Chicago, Ill. 

Fenichell, Stephen.1996. Plastic: The Making of a Synthetic Century. Harper Business, New York.

Gray, Theodore. 2009. The Elements. Black Dog & Leventhal Publishers, New York.

Hoffman, Roald. 1995. The Same and Not the Same. Columbia University Press, New York; quote concerning the colors of Tyrian purple from p. 199. 

Le Couteur, Penny and Burreson, Jay. 2003. Napoleon’s Buttons:  17 Molecules That Changed History. Jeremy P. Tarcher/Penguin.
 
Quadbeck-Seeger, Hans-Jürgen. 2007. World of the Elements Elements of the World.  Wiley-VCH, Federal Republic of Germany.
 
Turin, Luca and Sanchez, Tania. 2008 & 2009. Perfumes: the A-Z Guide. Penguin Books, New York.

Bolos of Mendes information taken from Hauck, Dennis William. 2008. The Complete Idiot’s Guide to Alchemy. Alpha, p. 27.