22.2.

Tar happens. It happens in a fireplace; as the wood is heated it begins to break down into charcoal and a kind of greasy steam which condenses in the chimney. That greasy steam is tar. Tar happens in a cigarette; as the tobacco is heated, bejeesical nicotine passes through the filter. The stuff trapped by the filter is tar. Tar happens when the fat from your barbecued chicken drips on the hot coals; the fat decomposes and tar is deposited on the inside lid of the barbecue grill. Tar happens whenever things burn or scorch, releasing vapors which later condense somewhere else. If they condense in your nose, you smell them. Tars make your clothes smell of cigarettes after you leave the bar. Tars alert you that the bacon is burning. Tars make up the aroma of chestnuts roasting on an open fire. Tar is not a single substance; it's just a fancy catch-all name for all the various substances produced by heating things until they decompose. The fancy name for heating things until they decompose is destructive distillation.

Destructive distillation was all the rage in the 1820's. In 1826 Otto Unverdorben destructively distilled indigotin, the blue dye we met in Chapter 12. You probably just got used to molecular formulas like C₁₆H₁₀N₂O₂ and empirical formulas like C₈H₅NO when Chapter 16 went and introduced the molecular model as a way of showing how the atoms in a molecule are hooked together in 3D. Figure 22-1 shows a molecular model for indigotin. Molecular models are great when you have a wooden or plastic model or a computer program to draw them, but when you need a quick 2D sketch of how the atoms are connected, chemists use a structural formula, like the one shown at the bottom of Figure 22-1. To understand structural formulas you need to realize that carbon atoms generally make four bonds. So whenever four lines come together in a structural formula, that point stands for a carbon atom, even if no "C" is shown. A curious feature of indigotin is that it contains two six-member carbon rings with alternating single and double bonds. Those rings are unusually strong on account of the double bonds and so when indigotin decomposes, those rings are likely to survive in the decomposition products. You can imagine lots of ways for the indigotin molecule to fall apart, but the bonds cut by the dotted lines in Figure 22-1 are among the most likely to break during destructive distillation. When that happens, the two hunks containing the carbon rings turn into two molecules of aniline, the stuff Unverdorben recovered in 1826.

What a surprise it must have been when Friedleib Runge isolated this same aniline from the destructive distillation of coal. Granted, most of the coal turns into coke, just as wood turns mostly into charcoal. In addition, several gases are produced—hydrogen, methane, and carbon monoxide. Water and ammonia are also produced. But the distinctive aroma of burning coal comes from the mixture of compounds which constitute coal tar, those volatile components which become gases at elevated temperatures but which condense into liquids and solids as they cool back to room temperature. The strong smell of these coal-tar products has given a name to a whole class of organic compounds: the aromatic compounds. One hundred kg of coal will yield approximately 3 kg of tar; distillation of that tar will produce about 1 kg of aromatic compounds.

You might think that aromatic means smelly, and surely that was the original meaning. But as time when on, it turned out that the aromatic coal tar distillates all shared a common structural feature, that curious six-membered carbon ring. As more and more compounds containing this ring were identified, it turned out that not all of them were smelly. But the name aromatic stuck fast like a spider. The meaning of the term is a little more technical these days, but for our purposes, an aromatic compound is one which contains the six-membered, alternately single and double-bonded carbon ring, the aromatic ring.

Each carbon atom of an aromatic ring can be bonded to one other atom. When all six carbon atoms are bonded to hydrogen atoms, we have a molecule of benzene, shown in Figure 22-2(L). When I was in college benzene was a common laboratory chemical, dry-cleaning solvent, and paint thinner. Benzene is not particularly toxic by ingestion, with an LD₅₀ of 3.8 g/kg in the rat. Because of its low boiling point, inhalation is more likely to be a problem than ingestion, but with adequate ventilation acute exposure to benzene is no more hazardous than exposure to many solvents currently available at hardware stores. But in addition to the risk of acute exposure, there is a danger of chronic exposure; occupational exposure to benzene is associated with an increased risk of cancer. Consequently, benzene has been replaced by toluene for household and many commercial uses.

Toluene, shown in the middle of Figure 22-2, is familiar to most people as the solvent in model airplane glue. You might think of it as a benzene molecule with an extra carbon atom attached. As with benzene, inhalation is the most important hazard but with adequate ventilation it can be handled safely. Unlike benzene, chronic exposure to toluene is not associated with cancer and toluene can still be found in hardware stores among the paint thinners.

Aniline, shown on the right of Figure 22-2, is the stuff originally produced by the destructive distillation of indigotin. You might think of it as a benzene molecule with an ammonia molecule attached. Aniline is an oily brown liquid only slightly soluble in water. Though aniline is the compound needed for the synthesis of mauveine, indigo is too expensive to be used as a commercial source and coal tar doesn't contain enough of it to be useful. Since benzene is the most abundant coal-tar aromatic it is natural to look for a way of making aniline from benzene. This synthesis occurs in two steps; first benzene is oxidized by nitric acid:

C₆H₆ + HNO₃ = C₆H₅NO₂ + H₂O

Second, the nitrobenzene produced in the first reaction is reduced by iron filings:

C₆H₅NO₂ + H₂O + 2 Fe = C₆H₅NH₂ + Fe₂O₃

Similar reactions produce toluidine, C₇H₈NH₂, from toluene. There is a hitch, however, in that the NH₂ might appear in any of three positions on the ring. Consequently, there are three "flavors," or isomers of toluidine: ortho-, meta-, and para-toluidine, shown in Figure 22-3. Each of these isomers has its own boiling point and other physical properties. Oxidation of toluene with nitric acid and subsequent reduction with iron yield predominantly ortho-toluidine and para-toluidine. Early coal-tar distillation did not completely separate benzene from toluene and consequently commercial "aniline" contained significant quantities of the toluidines. Just as organic molecules which contain the OH group are classified as alcohols, those containing the NH₂ group are classified as amines. The aromatic amines, aniline and the toluidines, are the raw materials for the synthesis of the dye, mauveine.

When you oxidize pure aniline in an acidic solution, the product is the dye pseudomauveine:

4 C₆H₅NH₂ = C₂₄H₁₉N₄⁺ + 9 H⁺ + 10 e^-

The remnants of the four original aniline molecules are evident in the structure of pseudomauveine, as shown in Figure 22-4. The "R's" in the structure are simply hydrogen atoms, for the moment. Note that pseudomauveine is a cation and so there must be an anion in the solution to balance it. If the reaction took place in acetic acid, the result is pseudomauveine acetate; if it took place in hydrochloric acid, the result is pseudomauveine chloride, etc. No matter which acid was used to make it, pseudomauveine is a brown dye, not very soluble in water but quite soluble in ethanol. Brown dyes are neither very rare nor much sought after and if Perkin had used pure aniline in his attempted synthesis of quinine, he would not have been much impressed with the results.

Perkin's aniline was not pure, however; it contained o-toluidine and p-toluidine. With these other amines in the mix, the "R's" in Figure 22-4 might turn out to be either hydrogen atoms or CH₃ groups, depending on the mix of aniline and the toluidines. With CH₃ groups at Ra and Rc, two slightly different purple dyes result, one (mauveine A) with Rb = H and the other (mauveine B) with Rb = CH₃. Like pseudomauveine, the mauveines are not very soluble in water but are quite soluble in ethanol. Unlike pseudomauveine, the mauveines are purple, a color rare among natural dyes and prized as a status symbol. If you had found a way to make such a color in a world of browns and blues and yellows and tans, you would very likely have dropped out of school as Perkin did. And that color would have made you as wealthy as the folks already wearing purple clothes.

Material Safety

Locate MSDS's for ethanol (CAS 64-17-5), aniline (CAS 62-53-3), o-toluidine (CAS 95-53-4), p-toluidine (CAS 106-49-0), acetic acid (CAS 64-19-7), and sodium hypochlorite (CAS 7681-52-9). Summarize the hazardous properties in your notebook, including the identity of the company which produced each MSDS and the NFPA diamond for each material.[1] In the course of this project you will combine vinegar, aniline, and laundry bleach to produce a solution of dye in ethanol. By themselves, vinegar and bleach would release chlorine gas, but you will be using only a few drops of bleach and the liberated chlorine will immediately oxidize the aniline; you are more likely to smell chlorine from the bleach itself than from your dye solution. Be aware that sodium hypochlorite will bleach clothing. You should wear safety glasses and rubber gloves while working on this project. All activities should be performed in a fume hood or with adequate ventilation. Leftover aniline and toluidine should be converted to dye as described below. Leftover dye should be washed down the drain with plenty of water unless prohibited by law.

Research and Development

Before you get started, you should know this stuff. You better know all the words that are important enough to be indexified and glossarated. You should know all of the Research and Development stuff from Chapter 18 and Chapter 19. You should know the structures and properties of the coal-tar distillates: benzene, toluene, and aniline, as shown in Figure 22-2. You should know the structures of aniline and o-, m-, and p-toluidine as shown in Figure 22-3. You should be able to recognize the remnants of aniline and toluidine in mauveine and pseudomauveine, as shown in Figure 22-4. You should know the hazardous properties of ethanol, acetic acid, aniline, o-toluidine, p-toluidine, and sodium hypochlorite. You should know who started the synthetic dye industry, when he did so, and which modern companies were founded as dye companies.