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PYRONES

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Originally appearing in Volume V22, Page 695 of the 1911 Encyclopedia Britannica.
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PYRONES , in See also:

chemistry, a See also:group of heterocyclic compounds, containing a six-membered See also:ring composed of five See also:carbon atoms and one See also:oxygen See also:atom. Two types are known, namely, the a-pyrones, which may be regarded as the See also: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)OC<CH:CH/O(1) (R) (a) (5) (6) a-Pyrone. y-Pyrone. As a class, the pyrones are rather unstable compounds, the ring being readily broken. When digested with See also:ammonia, the oxygen atom is replaced by the imino (:NH) group, and pyridones or oxypyridines are formed. a-Pyrones.—The coumalic compounds belong to this See also:series, and were first obtained by A. Hantzsch in 1884 (See also:Ann. 222, p. 1) and H. v. Pechmann (Ber., 1884, 17, p. 936). a-Pyrone or coumalin, C1H402, is obtained by distilling the See also:mercury See also:salt of coumalic See also:acid (from malic acid and sulphuric acid) in a current of See also:hydrogen. It is an oily liquid which boils at 206—209° C., and with alkalis it gives formyl crotonic acid, HO2C•CH:CH•See also:CH2•CHO. a'y-Dimethyl-a-pyrone or mesitene lactone, C7H802, is obtained from iso-dehydracetic acid (from aceto-acetic ester and sulphuric acid).

Phenylcoumalin or a'-phenyl-a-pyrone, C5H3(See also:

C6H5)02, is found in coto-bark. When heated with alkalis it yields benzoic acid and See also:acetophenone; reduction by hydriodic acid gives 6-phenyl valeric acid, and when heated with ammonium acetate and ammonia it yields phenylpyridone. It forms an addition product with phenol and with See also:aniline; the latter gives diphenylpyridone when boiled with concentrated hydrochloric acid. Paracotoin, C18H804, which also occurs in coto-rind, appears to be a bisoxymethylene phenylpyrone, C5H302•C6H3(CH202). Various pyronones (keto-dihydropyrones) derived from the See also:compound having See also:formula I. (below) are known, the most important of which is dehydracetic acid, C8H804, first obtained by Geuther (See also:Jena'sche Zeit, 1866, p. 8). It may be prepared by distilling acetoacetic ester alone, by See also:heating it with acetic anhydride to 200° C. or by heating acetyl chloride with See also:pyridine to 200—220° C. J. N. Collie regards it as having formula II., whilst Feist (Ann. 189o, 257, p.

253) favours formula III. OC•CH :CH OC•CH :C•CH2•CO•CHa OC•CH :C•CHa See also:

H2C•CO.O H2C•CO.O See also:CH3•CO•HC•C0.0 (I.) (II.) (III.) It crystallizes in tables which melt at 108—1o9° C., and is a weak acid. Alcoholic potash converts it into aceto-acetic ester, and with concentrated aqueous See also:caustic potash it is completely decomposed into See also:acetone, acetic acid and carbon monoxide. y-Pyrones.—Many of these compounds are found as naturally occurring substances: thus chelidonic acid is found in Chelidonium majus and meconic acid in See also:opium, and the more complex flavone and flavonol derivatives are also found in various See also:plants. The 7-pyrones may be synthesized by eliminating See also:water from the 1.3.5 triketones: CH2•CO•CO2R See also:CHIC—0O2R OC --j OC / '>O 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 See also: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 See also:copper derivative of acetoacetic ester, and by the action of concentrated hydrochloric acid on dehydracetic acid. It forms a See also:barium salt which with an acid yields diacetyl acetone. The most striking See also:property of this compound is that it forms salts with See also:mineral acids (J. N. Collie and Tickle, Journ.

Chem. See also:

Soc., ,1899, p. 710). For example, hydrochloric acid adds on at the oxygen atom, since the salts so formed are relatively unstable and undergo See also:complete See also:hydrolysis in dilute aqueous See also: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 See also:opinion that both oxygen atoms are to be regarded as tetravalent in these salts and gives the second formula below for the See also:molecule: HC•CO•CH HaC•C•O•C•CH3 H Cl Meconic acid, or oxypyrone tricarboxylic acid (3.2.6) C5H02(OH) (See also:CO2H)2, found in opium, crystallizes in prisms and gives a characteristic deep red See also:colour with ferric chloride. On heating to 20o° it gives comenic acid, C5H2O2(OH)(CO2H), and on See also: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 See also:

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 See also:cyanide (Ber., 1892, 25, p. 3563); by the molecular rearrange- ment of the benzal or alkylidene phthalides (S. CH See also:Gabriel, Ber., 1885, 18, p. 2443 ; 1887, 20, p. 2363), /\''''' . and by the action of See also:manganese dioxide and hydrochloric acid on ,B-naphthoquinone. 0 The See also: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 See also:

blue See also: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 See also: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 See also: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 See also: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 See also:sodium ethylate. Chrysin or I.3-dioxyflavone, C15H1004, is a yellow dye, which may be obtained from the buds of different varieties of the See also:poplar. On hydrolysis it yields phloroglucin and benzoic and acetic acids.

It has been synthesized by heating trimethoxy benzoyl acetophenone (from See also:

ethyl benzoate and phloracetophenone trimethyl See also: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 See also:woad and in See also:parsley, crystallizes in See also:pale yellow needles. On fusion at moderate temperatures with caustic alkalis it gives phloroglucin and See also: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]•See also: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 See also: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 See also:H3C• '—0— •CH3 as loose grains in detritus due to the disintegration of the See also: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 See also:nut-See also:shell), which seems in some cases to be an See also:amphibole.

In the See also:

serpentine of Zoblitz and of Greifendorf near See also:Leipzig, in See also:Saxony, See also:pyrope is characteristically See also:developed; and the Saxon garnets, found loose in gravels, were referred to by G. See also: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 See also:garnet. The garnet-bearing See also:district is a See also:tract of about 70 square kilometres in the See also:north of Bohemia, the See also:chief locality being Meronitz near See also:Bilin. It is notable that the pyrope is found at Meronitz in a clayey calcareous tufa or See also:conglomerate, with See also:opal and serpentine, products of the decomposition of a See also:peridotite. It occurs also in sands and gravels near Chrastian, Lobositz, Triblitz, Podseditz, Chodolitz, and at several other localities in the Mittel Gebirge, between See also:Teplitz and See also:Leitmeritz. It is believed that the See also:original pyrope-bearing rocks resulted from the eruptive activity which gave rise to Linhorka See also:Hill, near Starrey. The garnets in the detritus are accompanied by See also:zircon, See also:spinet, See also:corundum, See also:cyanite, See also:tourmaline, See also: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 See also:agent. To obtain the stones the detritus is washed, and the garnets picked out by See also:hand and then sized through See also:sieves. The pyrope is generally See also:rose-cut or step-cut, and often mounted with a See also:foil. Beads are faceted all over.

Some pyrope is cut en cabochon, forming, like See also:

almandine, See also:carbuncle, and if very dark the See also:stone is hollowed at the back so as to See also:form a " garnet-shell." The See also:industry of cutting Bohemian garnets is centred in Turnau on the Iser, near See also:Reichenberg; but there are also See also:works at other localities. Large stones are very rare, but a Bohemian pyrope as large as a See also:hen's See also:egg is preserved in the Imperial See also:treasury at See also:Vienna; and another the See also:size of a See also:pigeon's egg in the Grune Gewolbe of See also:Dresden. Pyrope occurs in many localities in the western See also:part of the See also:United States, especially in See also:Colorado, See also:Arizona and New See also:Mexico, where it is often called `. See also:ruby." It is found loose in See also:sand accompanied by olivine, and has resulted from the alteration of a peridotite. The Navajo See also:Indians of New Mexico collect the garnet from the sands of the See also:ant-hills and See also:scorpion-holes. Very See also:fine pyrope occurs in the See also:diamond-See also:fields of See also:South See also:Africa, having been derived from olivine-bearing rocks. It occurs in the blue-ground and in the detritus of the See also:river-diggings. The Cape garnets have usually a See also:rich colour, but some stones incline to an See also:orange See also:hue. The finest pyrope is often cut as a brilliant, and passes under the misleading name of " Cape ruby." A pyrope-bearing See also:rock, rather like that of South Africa, occurs in See also:Elliott See also:county, See also:Kentucky, U.S.A. ; it is notable, too, that pyrope is found near See also:Elie in See also:Fife, in See also:Scotland, where it occurs in volcanic agglomerates and in basaltic dikes. See also:Sir A. See also: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. See also:

Mining Eng. (1893), xxi. 241; and " See also:Die bohmischen Granatlagerstatten," by Dr Hans Oehmichen, Zeil. f. prakt. Geol. (1900), viii. I. Both papers contain See also:bibliographical lists. (F.

W.

End of Article: PYRONES

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