Indigo dye

• Indigotin
• 2,2 '-bis (2,3- dihydro -3- oxoindolyliden )
• C. I. Pigment Blue 66
• C. I. 73000

Dark blue, odorless crystalline solid

Fixed

300 ° C.

• Slightly soluble in water
• Insoluble in diethyl ether, alcohols
• Soluble in glacial acetic acid, pyridine, DMSO, DMF

Attention

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Indigo (Greek indikón " the Indian ", after the home East India ) is a deep blue color and its name to its color indigo. Most likely it can be used as the last noticeable shade of blue before it turns into a bluish violet, rewrite. Indigo is the Color Index as C. I. Pigment Blue 66 and C. I. 73000 out.

Occurrence and extraction

Indigo can be obtained from the Indian indigo plant or the alteingebürgerten woad. The recovery from the indigo plant can be demonstrated to ancient times. Can already be found in pre-Christian times Indigofera species in India, East Asia and Egypt. Pliny described the origin of the color from India. In Europe, the dye was rare from the indigo plant until the 12th century, it was in small amounts over Syria and Alexandria imported from India. Also the Woad has been used in the Mediterranean and Western Europe in ancient times for dyeing. In the Middle Ages, people in Europe knew no other blue dye as indigo. From the 12th to the 17th century woad was grown in Thuringia, which, according to estimates approximately 3750 ha were ordered with the plant. The slurry obtained from the woad was dried to Waidkugeln and, mainly dampened demand in the winter months, with urine and fermented after the addition of potash. 300 kg plant material delivered about 1 to 1.5 kg Indigo. The Indigofera plant in India delivered the thirty times the amount of dye in comparison with woad, so that the cultivation became unprofitable in Europe in the 17th century. As a replacement for Indigo by American settlers the bastard indigo ( Amorpha fruticosa ) for the blue staining was used. Today, indigo is still cultivated in Brazil and El Salvador. One uses the dye- rich species Indigofera arrecta and I. sumatrana.

Dyeing

The plants do not contain indigo, but indican, which must first be converted by fermentation into indoxyl. By subsequent oxidation in air arises from the yellow indoxyl the blue indigo, these operations correspond to the vat dyeing.

Synthesis

1870 for the first time succeeded in fully synthetic production of indigo from isatin by the German chemist Adolf von Baeyer. From Baeyer developed 1880-1882 further synthesis routes for Indigo on cinnamic acid and o- nitrobenzaldehyde. A precursor, the o- Nitrophenylpropiolsäure let be reduced directly to the textile fibers to indigo. BASF established as early as 1881 a small production of "Little Indigo". The economic success did not materialize, the production was stopped.

In 1890 the Zurich Professor Karl Heumann developed a new synthetic route via phenylglycine. The chemical company BASF and Hoechst AG patented and developed the method further. The series of experiments with phenylglycine was set by BASF in 1893, again as the indigo yield was very low. However, a good approach was possible with phenylglycine -o -carboxylic acid. BASF was able to produce phthalic acid ( an important precursor ) quite cheap from naphthalene. From 1897 synthetic indigo was produced commercially by the process mentioned by BASF.

In 1901 it was John Pfleger succeeded in Degussa to get from N -phenylglycine using sodium amide and alkali fusion Indigo in high yields.

This process continued first by BASF and Hoechst. In 1904, a much better method on aniline and ethylene chlorohydrin was developed and causes a 2- Anilinoethanol which converts to under base effect and higher temperatures with very good yields indoxyl.

The synthetic production of indigo displaced completely the laborious extraction from plant material. The advantages of synthetic indigo extraction speak for themselves: color more reliable results, an easy dosage, no harvest dependence and no varying color qualities.

Before the First World War, India was the most important British colony, the world's leading producer of indigo: 2.619 million kg, 80 % of world production. Developed in Germany aniline dyes competed successfully:

From the year 1924 based the synthesis of indigo from phenylglycinonitrile, prepared from aniline. In all cases, indoxyl, which is oxidized by atmospheric oxygen to indigo occurs.

Properties

Indigo itself is almost insoluble in water and must be converted prior to staining by the reduction in the water-soluble indigo white ( leuco - indigo). After dyeing, is formed by oxidation again Indigo. This process, which is applied to other textile dyes are called vat dyeing. Previously, the materials were placed in a meadow in the sun for the oxidation of the dye, where the indigo was oxidized by a lawn bleach. In connection with the idea that the dyers have had during this drying process, nothing to do, the thesis has emerged that 's part to the colloquial expression blue for " doing nothing, stay away from his work" had arisen, for the linguistic in the literature but other origin declarations are offered.

In the dyeing of textiles with indigo, you can also achieve green hues. This is achieved by over- dyeing with Reseda ( Färberwau ).

Indigo has a very high melting point ( 300 ° C) and is poorly soluble. This is due to the fact that Indigo forms a hydrogen-bonded polymer in the solid state. X-ray structure analyzes have shown that while each indigo molecule is bonded to four surrounding molecules.

Indigoids dyes

Indigoids dyes are structurally related substances Indigo:

• A red-violet, the indigo dye is structurally isomeric Indirubin
• Indigo carmine (5,5 ' - Indigodisulfonsäure disodium salt )
• Purple ( 6,6 '- dibromoindigo )

A degradation product of Indigo is isatin.

Use

The Maya placed the pigment Maya Blue from the mineral palygorskite and indigo ritually used here.

In 1900 there was a huge market for indigo. It tunics, sailors uniforms and the blue work clothes of the workers were stained with this dye.

During the natural indigo only has a small market share, synthetic indigo is an important dye for invented by Levi Strauss in 1873 jeans. In the textile industry the indigo synthetically produced as a vat dye is widespread. Mostly Indigo is needed for dyeing denim fabrics.

For the technical area of ​​application Indigo can be used in the form of thin organic films for the construction of solar cells. Research has shown that indigo can be used in field effect transistors. A thin film of partially crystalline Indigo is a semiconductor with a band gap of 1.2 eV and thus a potential material for organic electronics .. Indigo thin films have been used in organic electronics as air-stable transistors ..