Aromatics from coal, isolation

In the 1930s, polycyclic aromatic hydrocarbon was isolated from coal tar and demonstrated to be carcinogenic. Despite this evidence, millions of people continue to expose themselves to the soot from tobacco and suffer from the resulting lung cancer. 

Benzene, naphthalene, toluene, and the xylenes are naturally occurring compounds obtained from coal tar. Industrial synthetic methods, called catalytic reforming, utilize alkanes and cycloalkanes isolated from petroleum. Thus, cyclohexane is dehydrogenated (aromatization), and n-hexane(cycli> zation) and methylcyclopentane(isomerization) are converted to benzene. Aromatization is the reverse of catalytic hydrogenation and, in the laboratory, the same catalysts—Pt, Pd, and Ni—can be used. The stability of the aromatic ring favors dehydrogenation. 

Phenol is produced through both natural and anthropogenic processes. It is naturally occurring in some foods, human and animal wastes, and decomposing organic material, and is produced endogenously in the gut from the metabolism of aromatic amino acids. Phenol has been isolated from coal tar, but it is now synthetically manufactured (EPA, 2002). Currently, the largest use of phenol is as an intermediate in the production of phenolic resins, which are used in the plywood, adhesive, construction, automotive, and appliance industries. Phenol is also used in the production of synthetic fibers such as nylon and for epoxy resin precursors such as bisphenol-A. 

FIGURE 15. Gathering the indigo crop, India, around 1900. Aniline was first obtained from indigo, prior to isolation of the aromatic amine from coal tar and synthesis from benzene. Cultivation of the natural product declined following the introduction of synthetic indigo by BASF and Hoechst in 1897. Edelstein Collection... 

Pyridine was first isolated, like pyrrole, from bone pyrolysates the name is constrncted from the Greek for fire, pyr , and the suffix idine , which was at the time being used for all aromatic bases - phenetidine, toluidine, etc. Pyridine and its simple alkyl derivatives were for a long time produced by isolation from coal tar, in which they occur in quantity. In recent years this source has been displaced by synthetic processes pyridine itself, for example, can be produced on a commercial scale in 60-70% yields by the gas-phase high-temperatnre interaction of crotonaldehyde, formaldehyde, steam, air and ammonia over a silica-alumina catalyst. Processes for the manufacture of alkyl-pyridines involve reaction of acetylenes and nitriles over a cobalt catalyst. 

Marvin CH, Lundrigan JA, McCarry BE, et al. 1995. Determination and genotoxicity of high molecular mass polycyclic aromatic hydrocarbons isolated from coal-tar-contaminated sediment. Environ Toxicol Chem 14(12) 2059-2066. 

Peptide chemistry, a subdiscipline of chemistry, developed, therefore, with considerable delay. Its growth is not comparable to the rapid blossoming of the chemistry of aromatic compounds that started with the isolation of benzene from coal gas by Faraday in 1825 and the proposition of its structure by A.v. Kekule in 1865. These discoveries gave birth to the application of aromatic compounds to the production of dyes and medicines, later followed the emergence of petrochemistry. 

Attempts to determine the chemical causes of cancer have evolved from early studies in which the disease was linked to a person s occupation. We now know that a person s lifestyle plays a role as well. In 1775, Dr. Perci-vall Pott, an English physician, first noticed that people employed as chimney sweeps had a higher rate of skin cancer than the general population. It was not until 1933 that benzo(a)pyrene (Gg H g, an aromatic hydrocarbon containing five fused carbon rings) was isolated from coal dust and shown to be metabolized in the body to produce one or more carcinogens. 

Examples of Polynuclear Aromatic Compounds Isolated from Coal... 

Ethylbenzene is produced either by the alkylation of benzene with a C2 component, or by isolation from aromatic fractions derived from coal or petroleum refining. 

Naphthalene was first isolated from coal tar in 1819 by Alexander Garden it represents about 10% of this complex mixture of aromatics. The industrial importance of naphthalene dates from the latter half of the last century, owing mainly to the ease with which it can be converted into sulfonic adds and thence also to the naphthols, for use as dyestuffs intermediates. However, the first synthetic naphthalene-based dye was a nitro-derivative, Martius Yellow (Add Yellow 24), which was patented in 1864 by Carl Alexander Martius. 

Aromatic chemistry is as old as organic chemistry itself. In the beginning most aromatic compotmds were isolated from coal tar and then further functionalized. Early on there were methods developed to assemble aromatic molecules from smaller precursors. The larger the compound the more difficult its synthesis. The... 

The aromatic six-membered ring compound that contains one nitrogen atom is known as pyridine (10) and was first isolated in 1846 from coal tar. Pyridine has been used as a base from time to time in preceding chapters. Because it is aromatic (see Chapter 21, Section 21.7), pyridine is a planar compound with an aromatic n-cloud above and below the plane of the ring. The nitrogen atom is sp2 hybridized, but the lone electron pair on nitrogen is not part of the aromatic... 

---