Читать книгу Creative Chemistry: Descriptive of Recent Achievements in the Chemical Industries - Edwin E. Slosson - Страница 19
COAL-TAR COLORS
ОглавлениеIf you put a bit of soft coal into a test tube (or, if you haven't a test tube, into a clay tobacco pipe and lute it over with clay) and heat it you will find a gas coming out of the end of the tube that will burn with a yellow smoky flame. After all the gas comes off you will find in the bottom of the test tube a chunk of dry, porous coke. These, then, are the two main products of the destructive distillation of coal. But if you are an unusually observant person, that is, if you are a born chemist with an eye to by-products, you will notice along in the middle of the tube where it is neither too hot nor too cold some dirty drops of water and some black sticky stuff. If you are just an ordinary person, you won't pay any attention to this because there is only a little of it and because what you are after is the coke and gas. You regard the nasty, smelly mess that comes in between as merely a nuisance because it clogs up and spoils your nice, clean tube.
Now that is the way the gas-makers and coke-makers—being for the most part ordinary persons and not born chemists—used to regard the water and tar that got into their pipes. They washed it out so as to have the gas clean and then ran it into the creek. But the neighbors—especially those who fished in the stream below the gas-works—made a fuss about spoiling the water, so the gas-men gave away the tar to the boys for use in celebrating the Fourth of July and election night or sold it for roofing.
THE PRODUCTION OF COAL TAR
A battery of Koppers by-product coke-ovens at the plant of the Bethlehem Steel Company, Sparrows Point, Maryland. The coke is being pushed out of one of the ovens into the waiting car. The vapors given off from the coal contain ammonia and the benzene compound used to make dyes and explosives.
IN THESE MIXING VATS AT THE BUFFALO WORKS, ANILINE DYES ARE PREPARED
But this same tar, which for a hundred years was thrown away and nearly half of which is thrown away yet in the United States, turns out to be one of the most useful things in the world. It is one of the strategic points in war and commerce. It wounds and heals. It supplies munitions and medicines. It is like the magic purse of Fortunatus from which anything wished for could be drawn. The chemist puts his hand into the black mass and draws out all the colors of the rainbow. This evil-smelling substance beats the rose in the production of perfume and surpasses the honey-comb in sweetness.
Bishop Berkeley, after having proved that all matter was in your mind, wrote a book to prove that wood tar would cure all diseases. Nobody reads it now. The name is enough to frighten them off: "Siris: A Chain of Philosophical Reflections and Inquiries Concerning the Virtues of Tar Water." He had a sort of mystical idea that tar contained the quintessence of the forest, the purified spirit of the trees, which could somehow revive the spirit of man. People said he was crazy on the subject, and doubtless he was, but the interesting thing about it is that not even his active and ingenious imagination could begin to suggest all of the strange things that can be got out of tar, whether wood or coal.
The reason why tar supplies all sorts of useful material is because it is indeed the quintessence of the forest, of the forests of untold millenniums if it is coal tar. If you are acquainted with a village tinker, one of those all-round mechanics who still survive in this age of specialization and can mend anything from a baby-carriage to an automobile, you will know that he has on the floor of his back shop a heap of broken machinery from which he can get almost anything he wants, a copper wire, a zinc plate, a brass screw or a steel rod. Now coal tar is the scrap-heap of the vegetable kingdom. It contains a little of almost everything that makes up trees. But you must not imagine that all that comes out of coal tar is contained in it. There are only about a dozen primary products extracted from coal tar, but from these the chemist is able to build up hundreds of thousands of new substances. This is true creative chemistry, for most of these compounds are not to be found in plants and never existed before they were made in the laboratory. It used to be thought that organic compounds, the products of vegetable and animal life, could only be produced by organized beings, that they were created out of inorganic matter by the magic touch of some "vital principle." But since the chemist has learned how, he finds it easier to make organic than inorganic substances and he is confident that he can reproduce any compound that he can analyze. He cannot only imitate the manufacturing processes of the plants and animals, but he can often beat them at their own game.
When coal is heated in the open air it is burned up and nothing but the ashes is left. But heat the coal in an enclosed vessel, say a big fireclay retort, and it cannot burn up because the oxygen of the air cannot get to it. So it breaks up. All parts of it that can be volatized at a high heat pass off through the outlet pipe and nothing is left in the retort but coke, that is carbon with the ash it contains. When the escaping vapors reach a cool part of the outlet pipe the oily and tarry matter condenses out. Then the gas is passed up through a tower down which water spray is falling and thus is washed free from ammonia and everything else that is soluble in water.
This process is called "destructive distillation." What products come off depends not only upon the composition of the particular variety of coal used, but upon the heat, pressure and rapidity of distillation. The way you run it depends upon what you are most anxious to have. If you want illuminating gas you will leave in it the benzene. If you are after the greatest yield of tar products, you impoverish the gas by taking out the benzene and get a blue instead of a bright yellow flame. If all you are after is cheap coke, you do not bother about the by-products, but let them escape and burn as they please. The tourist passing across the coal region at night could see through his car window the flames of hundreds of old-fashioned bee-hive coke-ovens and if he were of economical mind he might reflect that this display of fireworks was costing the country $75,000,000 a year besides consuming the irreplaceable fuel supply of the future. But since the gas was not needed outside of the cities and since the coal tar, if it could be sold at all, brought only a cent or two a gallon, how could the coke-makers be expected to throw out their old bee-hive ovens and put in the expensive retorts and towers necessary to the recovery of the by-products? But within the last ten years the by-product ovens have come into use and now nearly half our coke is made in them.
Although the products of destructive distillation vary within wide limits, yet the following table may serve to give an approximate idea of what may be got from a ton of soft coal:
1 ton of coal may give
Gas, 12,000 cubic feet
Liquor (Washings) ammonium sulfate (7–25 pounds)
Tar (120 pounds) benzene (10–20 pounds)
toluene (3 pounds)
xylene (1–½ pounds)
phenol (½ pound)
naphthalene (⅜ pound)
anthracene (¼ pound)
pitch (80 pounds)
Coke (1200–1500 pounds)
When the tar is redistilled we get, among other things, the ten "crudes" which are fundamental material for making dyes. Their names are: benzene, toluene, xylene, phenol, cresol, naphthalene, anthracene, methyl anthracene, phenanthrene and carbazol.
There! I had to introduce you to the whole receiving line, but now that that ceremony is over we are at liberty to do as we do at a reception, meet our old friends, get acquainted with one or two more and turn our backs on the rest. Two of them, I am sure, you've met before, phenol, which is common carbolic acid, and naphthalene, which we use for mothballs. But notice one thing in passing, that not one of them is a dye. They are all colorless liquids or white solids. Also they all have an indescribable odor—all odors that you don't know are indescribable—which gives them and their progeny, even when odorless, the name of "aromatic compounds."
Fig. 8. Diagram of the products obtained from coal and some of their uses.
The most important of the ten because he is the father of the family is benzene, otherwise called benzol, but must not be confused with "benzine" spelled with an i which we used to burn and clean our clothes with. "Benzine" is a kind of gasoline, but benzene alias benzol has quite another constitution, although it looks and burns the same. Now the search for the constitution of benzene is one of the most exciting chapters in chemistry; also one of the most intricate chapters, but, in spite of that, I believe I can make the main point of it clear even to those who have never studied chemistry—provided they retain their childish liking for puzzles. It is really much like putting together the old six-block Chinese puzzle. The chemist can work better if he has a picture of what he is working with. Now his unit is the molecule, which is too small even to analyze with the microscope, no matter how high powered. So he makes up a sort of diagram of the molecule, and since he knows the number of atoms and that they are somehow attached to one another, he represents each atom by the first letter of its name and the points of attachment or bonds by straight lines connecting the atoms of the different elements. Now it is one of the rules of the game that all the bonds must be connected or hooked up with atoms at both ends, that there shall be no free hands reaching out into empty space. Carbon, for instance, has four bonds and hydrogen only one. They unite, therefore, in the proportion of one atom of carbon to four of hydrogen, or CH4, which is methane or marsh gas and obviously the simplest of the hydrocarbons. But we have more complex hydrocarbons such as C6H14, known as hexane. Now if you try to draw the diagrams or structural formulas of these two compounds you will easily get
H H H H H H H
| | | | | | |
H-C-H H-C-C-C-C-C-C-H
| | | | | | |
H H H H H H H
methane hexane
Each carbon atom, you see, has its four hands outstretched and duly grasped by one-handed hydrogen atoms or by neighboring carbon atoms in the chain. We can have such chains as long as you please, thirty or more in a chain; they are all contained in kerosene and paraffin.
So far the chemist found it east to construct diagrams that would satisfy his sense of the fitness of things, but when he found that benzene had the compostion C6H6 he was puzzled. If you try to draw the picture of C6H6 you will get something like this:
| | | | | |
-C-C-C-C-C-C-
| | | | | |
H H H H H H
which is an absurdity because more than half of the carbon hands are waving wildly around asking to be held by something. Benzene, C6H6, evidently is like hexane, C6H14, in having a chain of six carbon atoms, but it has dropped its H's like an Englishman. Eight of the H's are missing.
Now one of the men who was worried over this benzene puzzle was the German chemist, Kekulé. One evening after working over the problem all day he was sitting by the fire trying to rest, but he could not throw it off his mind. The carbon and the hydrogen atoms danced like imps on the carpet and as he watched them through his half-closed eyes he suddenly saw that the chain of six carbon atoms had joined at the ends and formed a ring while the six hydrogen atoms were holding on to the outside hands, in this fashion:
H
|
C
/ \\
H-C C-H
|| |
H-C C-H
\ //
C
|
H
Professor Kekulé saw at once that the demons of his subconscious self had furnished him with a clue to the labyrinth, and so it proved. We need not suppose that the benzene molecule if we could see it would look anything like this diagram of it, but the theory works and that is all the scientist asks of any theory. By its use thousands of new compounds have been constructed which have proved of inestimable value to man. The modern chemist is not a discoverer, he is an inventor. He sits down at his desk and draws a "Kekulé ring" or rather hexagon. Then he rubs out an H and hooks a nitro group (NO2) on to the carbon in place of it; next he rubs out the O2 of the nitro group and puts in H2; then he hitches on such other elements, or carbon chains and rings as he likes. He works like an architect designing a house and when he gets a picture of the proposed compounds to suit him he goes into the laboratory to make it. First he takes down the bottle of benzene and boils up some of this with nitric acid and sulfuric acid. This he puts in the nitro group and makes nitro-benzene, C6H5NO2. He treats this with hydrogen, which displaces the oxygen and gives C6H5NH2 or aniline, which is the basis of so many of these compounds that they are all commonly called "the aniline dyes." But aniline itself is not a dye. It is a colorless or brownish oil.
It is not necessary to follow our chemist any farther now that we have seen how he works, but before we pass on we will just look at one of his products, not one of the most complicated but still complicated enough.
