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Originally appearing in Volume V22, Page 695 of the 1911 Encyclopedia Britannica.
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PYRONES, in chemistry, a group of heterocyclic compounds, containing a six-membered ring composed of five carbon atoms and one oxygen atom. Two types are known, namely, the a-pyrones, which may be regarded as the lactones of b-oxydiolefine carboxylic acids, and the 7-pyrones, which may be regarded as anhydrides of diolefine dioxyketogqnes: CH :C~H \CH (3) :CH (y)HC•CO/O (4)OCO NCH2•CO•CO2R NCH:C-0O2R Acetone dioxalic ester. —> Chelidonic ester. y-Pyrone or pyrocomane, C5H402, melting at 32° C. and boiling at 210—215° C., is obtained by eliminating carbon dioxide from chelidonic acid (obtained as above), or from comanic acid, obtained by heating chelidonic acid. aa'-Dimethyl-y-pyrone, C5H2(CHa)202, is obtained by the action of hydriodic acid on the ester of the corresponding acid (Feist, Ann., 189o, 257, p. 272); by the action of carbonyl chloride on the copper derivative of acetoacetic ester, and by the action of concentrated hydrochloric acid on dehydracetic acid. It forms a barium salt which with an acid yields diacetyl acetone. The most striking property of this compound is that it forms salts with mineral acids (J. N. Collie and Tickle, Journ. Chem. Soc., ,1899, p. 710). For example, hydrochloric acid adds on at the oxygen atom, since the salts so formed are relatively unstable and undergo complete hydrolysis in dilute aqueous solution. The oxygenatom is probably tetravalent, and the salts are to be regarded as oxonium salts (see OXYGEN). Collie (Journ. Chem. Soc., 1904, 85, p. 971) is of the opinion that both oxygen atoms are to be regarded as tetravalent in these salts and gives the second formula below for the molecule: HC•CO•CH HaC•C•O•C•CH3 H Cl Meconic acid, or oxypyrone tricarboxylic acid (3.2.6) C5H02(OH) (CO2H)2, found in opium, crystallizes in prisms and gives a characteristic deep red colour with ferric chloride. On heating to 20o° it gives comenic acid, C5H2O2(OH)(CO2H), and on distillation pyromeconic acid or /3-oxypyrone. On comenic acid see A. Peratoner, Gazz., 1906, 36 (i.), p. 1. The tetrahydro-y-pyrones may be obtained by the condensation of aldehydes with acetone-dicarboxylic ester in the presence of hydrochloric acid. Benzopyrones. Compounds of this type are known in both the a and y series, the former including the coumarins (q.v.) and isocoumarins, and the latter a number of naturally occurring dyestuffs which may be considered as derivatives of flavone (see under). The isocoumarins (annexed formula) may be prepared by the action of acid chlorides or anhydrides on orthocyanbenzyl cyanide (Ber., 1892, 25, p. 3563); by the molecular rearrange- ment of the benzal or alkylidene phthalides (S. CH Gabriel, Ber., 1885, 18, p. 2443 ; 1887, 20, p. 2363), /\''''' . and by the action of manganese dioxide and hydrochloric acid on ,B-naphthoquinone. 0 The parent substance of the 7-group, namely benzo-y-pyrone (chromone), was obtained in 1900 \/\CO/ by S. Ruhemann (Journ. Chem. Soc., 77, p. 1179) Isocoumarin. by heating its carboxylic acid (formed by the action of concentrated sulphuric acid on phenoxyfumaric acid) in vacuo. It crystallizes in colourless needles, and its solution in concentrated sulphuric acid is yellow with a blue fluorescence. The naturally occurring compounds, chrysin, galanzin, quercetin, apigenine, &c., are considered to be derivatives of flavone (or flavonol), which is a phenyl-2-benzo-7-pyrone (S. Kostanecki, Ber., 1898—1906).. Flavone and flavonol possess the following constitutions, the positions of the substituents being indicated by the numbers: 3/\/ O C— j=am 4~ /\/ 0 _\.—/ 2 LH 6' 5 \/\Co/a. i /\COj 'OH 1 Flavone. Flavonol. Flavone, C15H10O2, is obtained by the action of potassium hydroxide on the acetyl derivative of benzylidene-ortho-oxyacetophenone. It forms colourless needles, which dissolve in concentrated sulphuric acid with a yellow colour and show a faint blue fluorescence. On fusion with caustic alkalis it yields salicylic acid, acetophenone, ortho-oxyacetophenone and benzoic acid, the latter two products being also formed by its hydrolysis. with sodium ethylate. Chrysin or I.3-dioxyflavone, C15H1004, is a yellow dye, which may be obtained from the buds of different varieties of the poplar. On hydrolysis it yields phloroglucin and benzoic and acetic acids. It has been synthesized by heating trimethoxy benzoyl acetophenone (from ethyl benzoate and phloracetophenone trimethyl ether) with hydriodic acid, and also by the action of hydriodic acid on 2.4-dibrom-1.3-dimethoxyflavonone. Galanzin or a'I.3-trioxyflavone or I.3-dioxyflavonol, C16H1005, crystallizes in yellow needles. It has been synthesized from hydroxydimethoxy-chalkone, C6H5•CH:CH•CO[I]•C6H2(OH)(OCH3)2[2.4.6•], the resulting 1.3-dimethoxy-flavanone compound yielding a nitroso-compound from which galanzin is obtained by the action of concentrated hydriodic acid. Apigenine or r3.4'-trioxyflavone, C15H10O5, found in woad and in parsley, crystallizes in pale yellow needles. On fusion at moderate temperatures with caustic alkalis it gives phloroglucin and para-oxyacetophenone, whilst at higher temperatures it yields protocatechuic and para-oxybenzoic acids and phloroglucin. It is obtained synthetically by brominating I.3.4'-trimethoxyflavonone, the resulting tribromo-compound by the consecutive reactions of alcoholic potash and hydriodic acid yielding apigenine. Kaempferol or I.3.4'-trioxyflavonol, C15H10O5, is found in the blossoms ofpDelphinium consolida and D. zazil. It is obtained by the action of hydriodic acid on kaempherid, and crystallizes in yellowish needles, which on fusion with caustic alkalis give para-oxybenzoic acid and phloroglucin. It is obtained synthetically from hydro~xcyy-trimethoxychalkone,CH30[4]•C6H4[I]•CH CH•CO•[i]C6H2(OH)(OCHa)2[2.4.6]by a method similar to that used for galanzin. Kaempferid occurs together with galanzin and alpinin in galganta root. It crystallizes in pale yellow needles, which dissolve in the caustic alkalis with an intense yellow colour, and in concentrated sulphuric acid with a HC---C=----CH II CI.O•H H3C• '—0— •CH3 as loose grains in detritus due to the disintegration of the matrix, The grains may be surrounded by a chloritic rind, or by a crust of a fibrous mineral called by A. Schrauf kelyphite (from the Gr, KiXv4,os, a nut-shell), which seems in some cases to be an amphibole. In the serpentine of Zoblitz and of Greifendorf near Leipzig, in Saxony, pyrope is characteristically developed; and the Saxon garnets, found loose in gravels, were referred to by G. Agricola as far back as 1546. Several localities in Bohemia are famous for yielding pyrope, and from its characteristic occurrence here it is often known, even when found elsewhere, as Bohemian garnet. The garnet-bearing district is a tract of about 70 square kilometres in the north of Bohemia, the chief locality being Meronitz near Bilin. It is notable that the pyrope is found at Meronitz in a clayey calcareous tufa or conglomerate, with opal and serpentine, products of the decomposition of a peridotite. It occurs also in sands and gravels near Chrastian, Lobositz, Triblitz, Podseditz, Chodolitz, and at several other localities in the Mittel Gebirge, between Teplitz and Leitmeritz. It is believed that the original pyrope-bearing rocks resulted from the eruptive activity which gave rise to Linhorka Hill, near Starrey. The garnets in the detritus are accompanied by zircon, spinet, corundum, cyanite, tourmaline, olivine, &c. Though generally very small, they are abundant, and are used not only as ornamental stones, but as a counterpoise in delicate weighing and as an abrasive agent. To obtain the stones the detritus is washed, and the garnets picked out by hand and then sized through sieves. The pyrope is generally rose-cut or step-cut, and often mounted with a foil. Beads are faceted all over. Some pyrope is cut en cabochon, forming, like almandine, carbuncle, and if very dark the stone is hollowed at the back so as to form a " garnet-shell." The industry of cutting Bohemian garnets is centred in Turnau on the Iser, near Reichenberg; but there are also works at other localities. Large stones are very rare, but a Bohemian pyrope as large as a hen's egg is preserved in the Imperial treasury at Vienna; and another the size of a pigeon's egg in the Grune Gewolbe of Dresden. Pyrope occurs in many localities in the western part of the United States, especially in Colorado, Arizona and New Mexico, where it is often called `. ruby." It is found loose in sand accompanied by olivine, and has resulted from the alteration of a peridotite. The Navajo Indians of New Mexico collect the garnet from the sands of the ant-hills and scorpion-holes. Very fine pyrope occurs in the diamond-fields of South Africa, having been derived from olivine-bearing rocks. It occurs in the blue-ground and in the detritus of the river-diggings. The Cape garnets have usually a rich colour, but some stones incline to an orange hue. The finest pyrope is often cut as a brilliant, and passes under the misleading name of " Cape ruby." A pyrope-bearing rock, rather like that of South Africa, occurs in Elliott county, Kentucky, U.S.A. ; it is notable, too, that pyrope is found near Elie in Fife, in Scotland, where it occurs in volcanic agglomerates and in basaltic dikes. Sir A. Geikie has pointed out the suggestive resemblance of the occurrence there to that in South Africa. See " Bohemian Garnets," by G. F. Kunz, Trans. Amer. Inst. Mining Eng. (1893), xxi. 241; and " Die bohmischen Granatlagerstatten," by Dr Hans Oehmichen, Zeil. f. prakt. Geol. (1900), viii. I. Both papers contain bibliographical lists. (F. W. R.*)
End of Article: PYRONES
PYROPE (pronounced pirop)

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