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Pyrimidines, Metadiazines

PYRIMIDINES, METADIAZINES or MIAZINES, in organic chemistry, a series of heterocyclic compounds containing a ring complex, composed of four carbon atoms and two nitrogen atoms, the nitrogen atoms being in the meta-position. The oxyderivatives of the tetrahydro- and hexahydro-pyrimidines are the uracils and the ureides of malonic acid (see PURIN). The purins themselves may be considered as a combination of the pyrimidine and glyoxaline ring systems. For formulae see below; the numbers about the first ring explain the orientation of pyrimidine derivatives.

The pyrimidines may be obtained by condensing i-3- diketones with the amidines (A. Pinner, Ber., 1893, 26, p. 2125). CH.-CO , NHrC-CeH, _,CH,-C:N=C.C.H..

CH 2 -CO(CH.) HN CH:C(CH,)-N The /3-ketonic esters under like treatment yield oxypyrimidines, whilst if cyanacetic ester be employed then amino- oxypyrimidines are obtained. By using urea, guanidine, thiourea and related compounds instead of amidines, one obtains the uracils. The cyanalkines (aminopyrimidines) were first obtained, although their constitution was not definitely known, by E. Frankland and H. Kolbe (Ann. 1848, 65, p. 269) by heating the nitriles of acids with metallic sodium or with sodium ethylate between 130 C. and 180 C.

3CH 3 CN = C 4 HN 2 (CH,) 2 -NH 2 [2-4-6].

Pyrimidine, C 4 H 4 N 2 , itself is a water-soluble base which melts at 21 C. and possesses a narcotic smell. Its methyl derivatives yield the corresponding carboxylic acids when oxidized by potassium permanganate. The amino derivatives are stable bases which readily yield substitution derivatives when acted upon by the halogen elements. Cyanmethine, C 6 H,N a (dimethyl- aminopyrimidine 2-4-6), melts at l8o-l8lC. The simple oxypyrimidines are obtained by the action of nitrous acid on the amino derivatives, or by heating these latter with concentrated hydrochloric acid to 180 C. They show both basic and phenolic properties and are indifferent to the action of reducing agents. Acid oxidizing agents, however, completely destroy them. By the action of phosphorus pentachloride, the hydroxyl group is replaced by chlorine.

Hydropyrimidines. The dihydro derivatives are most probably those compounds which are formed in the condensation of acidyl derivatives of acetone, with urea, guanidine, etc. Tetrahydropyrimidines are obtained by the action of amidines on trimethylene bromide:

Br(CH 2 ),Br+C,H s C(:NH)-NH 2 =2HBr+C 4 H 7 N 2 (C,H,)[2]. The 2-6-diketo-tetrahydropyrimidines or uracils may be considered as the ureides of 0-aldehydo, and 0-ketonic acids. Uracil and its homologues may be obtained in many cases from the hydrouracils by the action of bromine, and subsequent elimination of the elements of hydrobromic acid; or by the condensation of aceto-acetic ester and related substances with urea, thiourea, guanidine, etc. Uracil, C 4 H4OjN 2> crystallizes in colourless needles, is soluble in hot water and melts with decomposition at 335 C. Hydrouracil, C 4 HeO 2 Nj, is obtained by the action of bromine and caustic alkalis on succinamide (H. Weidel and E. Rpithner, Monats., 1896, 17, p. 172); by the fusion of 0-aminoprppionic acid with urea; by the electrolytic reduction of barbituric acid (J. Tafel, Ber., 1900, 33, p. 3385), and by the condensation of acrylic acid with urea at 210-220 C. (E. Fischer, Ber.,- 1901, 34, p. 3759). It crystallizes in needles and is soluble in water. It melts at 275 C. 4-Methyluracil, CHO 2 N 2 , has long been known, having first been synthesized by R. Behrend (see PURIN). It crystallizes in needles which melt at 320 C. and is soluble in caustic alkalis. On oxidation with potassium permanganate it is converted into acetyl urea, together with other products. 5-Methyluracil (Thymin) is obtained from the corresponding methyl bromhydrouracil (E. Fischer) ; or from 2-4-6-trichlor-5- methylpyrimidine by the action of sodium methylate. This yields a 2-4-< limethoxy-5-methyl-6-chlorpyrimidine, which on reduction and subsequent treatment with hydrochloric acid is converted into thymin (O. Gerngross, Ber., 1905, 38, p. 3394). For methods of preparation and properties of numerous other pyrimidine compounds see T. B. Johnson, Journ. Biol. Chem., 1906, etc. ; Amer. Chem. Journ., 1906, etc.; W. Traube, Ber., 1900, etc. ; O. Isay, ibid., 1906, 39. P- 251- CH:CH-CH Pyrimidine N:C(CHO-N NHi-C:CH-C-CH t Cyanmethine NH-CO-NH CH:CH-CO Uracil PYRITES, a term applied to iron disulphide when crystallized in the cubic system, but used also in a general sense to designate a group of metallic sulphides of which this mineral is the most characteristic example. When employed as a group-name the constituent species are distinguished by prefixes: thus the type is called iron pyrites, whilst other species are known as copper pyrites, arsenical pyrites, etc. The original word pyrites (from Gr. irvp, fire) had reference to the fact that sparks might be elicited on striking the mineral violently, as with flint, so that irupinjs Xt0os meant a stone which struck fire. Hence the name seems to have been applied also to flint, and perhaps to emery and other hard stones. Nodules of pyrites have been found in prehistoric barrows and elsewhere under conditions suggesting their use as a primitive means of producing fire. Even in late historic time it was employed in some of the old wheel-lock guns. Iron-pyrites was formerly called marcasite, a word variously written marcasin, marchasite, marchesite, marquesite, etc. The two names are now applied to distinct mineral species. The compound FeS 2 is dimorphous, and the modern practice is to distinguish the cubic forms as pyrites and the orthorhombic as marcasite (q.v.). Sometimes, however, the term pyrites is loosely applied to both species, and the cubic pyrites is then differentiated by the name " pyrite " a form which brings the last syllable into harmony with the spelling of the names of most minerals.

Iron pyrites, or pyrite, belongs crystallographically to the parallelfaced hemihedral class of the cubic system. Its common forms are FIG. i.

FIG. 2.

FIG. 3.

the cube, the octahedron, and the pentagonal dodecahedron. Fig. I shows P the cube { 100), d the octahedron ( 1 1 1 ), and e the pentagonal dodecahedron w J2io|. In fig. 2 ir |2io[ and finj are associated with /the dyakis-dodecahedron T {321); whilst fig. 3 shows a combination of ir (210) and ir {42 1 j. The faces of the cube are sometimes striated parallel to the edges between P and e (fig. i), the striae on each face being therefore at right angles to those of the adjoining faces, and indicating an oscillatory combination of the cube and pentagonal dodecahedron. Fig. 4 illustrates a characteristic twin, formed by two interpenetrating pentagonal dodecahedra. Such p. supplementary twins, known in Germany as " twins of the Iron Cross," are commonly brown by superficial conversion into limonite.

Pyrites presents a conchoidal fracture, and a very indistinct cubic cleavage. Its hardness is about 6, and its specific gravity 4-9 to 5-2, being rather more than that of marcasite. Moreover, the colour of pyrites is pale brass-yellow, whilst that of marcasite when untarnished may be almost tin-white. From copper-pyrites ( chalcopyrite) iron-pyrites is distinguished by its superior hardness and by its paler colour. On exposure to meteoric influences pyrites commonly becomes brown, by formation of ferric hydrate or limonite, whence the change is called " limonitization." Such a change is very common on the outcrop of mineral veins, forming what miners call " gozzan." Another kind of alteration which pyrites may suffer has been termed " vitriolization," since the products are ferrous sulphate, with free sulphuric acid and sometimes a basic ferric sulphate. It is often said that this saline change is more characteristic of marcasite than of pyrite, but according to H. N. Stokes this statement is incorrect. Contrary, too, to popular belief, he has found a fibrous structure more common in pyrite than in marcasite. In some cases the two forms of iron disulphide occur in intimate association and are difficult to distinguish.

According to the formula FeSj, pyrites contains theoretically 46-67% of iron and 53-33 of sulphur. Practically, however, it frequently contains other metals, such as copper, cobalt and nickel. gold is often present, and in many gold-mining districts the precious metal is obtained mainly from auriferous pyrites. As pyrites, from its brass-yellow colour, is sometimes mistaken for gold, it has been vulgarly called " fool's gold." Traces of thallium, which are present in some pyrites, may be detected in the flues of the furnaces where the metal is roasted. Arsenic is an impurity which may be of serious consequence in some of the purposes to which pyrites is applied. The presence of copper, nickel and arsenic is possibly due in many cases to traces of kindred minerals, like chalcopyrite, pentlandite and mispickel.

Pyrites is a mineral of very wide distribution, occurring under varied conditions and probably originating in various ways. It is common in mineral-veins, usually associated with quartz, and is often known to miners as " mundic." It occurs crystallized, commonly in cubes, in schistose and slaty rocks, and less abundantly in the younger sedimentary deposits. In coal it not infrequently forms bands and nodules known as " brasses," and may also be finely disseminated through the coal as "black pyrites"; but much of the so-called pyrites of coal is really marcasite. Films of pyrites sometimes coat the joint-planes of coal. It is believed that the bluish colour of many clays and limestones is referable to the presence of finely divided pyrites, and it is known that certain deposits of blue mud now forming around continental shores owe their colour, in part, to disseminated iron sulphide. Pyritous shales have been largely used in the manufacture of alum, and are therefore known as " alum-shales." Many fossils are mineralized with pyrites, which has evidently been reduced by the action of decomposing organic matter on a solution of ferrous sulphate, or perhaps less directly on ferrous carbonate dissolved in water containing carbonic acid, in the presence of certain sulphates. A similar action probably explains the origin of pyrites and marcasite in coal and lignite, in clay and shales, and in limestone like chalk.

Pyrites is largely worked for sake of the sulphur which it contains, and in many cases it has displaced brimstone in the manufacture of sulphuric acid. For this purpose its value depends on the proportion of sulphur present. Pyrites low in sulphur is incapable of sustaining its own combustion without the aid of an external source of heat, and 45 % of sulphur is, for economic reasons, usually regarded as the lowest admissible for sulphuric acid manufacture. It is also important for this purpose that the ore should be as free as possible from arsenic (see SULPHURIC ACID).

An extremely important variety of pyrites is that which is more or less cupriferous, and is commonly known commercially as " copper-pyrites " (q.v.), though distinct mineralogically from that mineral. It consists, indeed, mainly of iron-pyrites, with a notable but variable proportion of copper, sometimes with silver and gold, and not infrequently associated with lead and zinc sulphides. The copper probably exists as disseminated chalcopyrite. Deposits of such cupriferous pyrites are widely distributed and are often of great magnitude. They are generally of lenticular form, and usually occur in or near the contact of eruptive rocks with schists or slates; the presence of the igneous rock being probably connected genetically with their origin. Among the best-known deposits of this character are those in the Huelva district, in the south-west of Spain, including the mines of Rio Tinto, Tharsis, Calanas, etc.; with those of San Domingos in Portugal. At Rio Tinto the ore is divided into three classes:

(i) The poorest, containing an average of about lj% of copper, which is treated locally by leaching with water and liquor containing ferric sulphate, whereby the copper is dissolved out and afterwards precipitated by pig-iron, whilst the residue is exported as ordinary iron-pyrites. (2) Export ore, with from 2 to 5 % of copper, in which the sulphur, copper and precious metals are utilized, and the residual iron oxide then sold as " purple ore " for use in iron manufacture. (3) Smelting ore, which averages about 6 % of copper, and is treated metallurgically as described under COPPER.

The world's annual production of iron-pyrites is about 1,700,000 tons. The largest producer is Spain, with upwards of 350,000 tons, including the cupriferous pyrites. France yields about 300,000 tons, largely from the Sain Bel mines, department of the Rh6ne. Then follows Portugal, with its important output of cupreous pyrites. In the United States the production of pyrites now reaches more than 200,000 tons per annum. The state of Virginia is the chief producer, followed successively by Georgia, North Carolina, Colorado, Massachusetts, California, Missouri, New York, etc. From Indiana and Ohio a quantity of pyrites is obtained as a by-product in coalmining. Newfoundland yields cupreous pyrites," worked at I'ilk-y's Island, whilst the nickeliferous pyrites of Sudbury in Ontario is partly magnetic (see PYRRHOTITE). Magnetic pyrites of commercial importance occurs also in Virginia and Tennessee. The United Kingdom yields but little pyrites, the annual output being not more than about 10,000 tons. Large quantities of " sulphur ore " were, however, formerly worked in the Vale of Avoca, Co. Wicklow, Ireland. Finely crystallized specimens of pyrite are obtained from many other localities, especially from Cornwall, Elba and Traversella, near Ivrea. in Piedmont.

See, for the early history of pyrites, J. F. Henckel's Pyritologia, oder Kieshistorie (Leipzig, 1725) ; of which an English translation appeared in 175^, entitled Pyritologia; or a History of the Pyrites, the Principal Body in the Mineral Kingdom. For a modern description of the deposit of pyrites of economic importance reference may be made to A Treatise on Ore Deposits, by J. A. Phillips (2nd ed. by H. Louis, 1896). For chemical means of distinguishing pyrite from marcasite consult H. N. Stokes, " On Pyrite and Marcasite," Bull. U. S. Geol. .Mo. 186 (1901). (F. W. R.*)

Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)

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