Page:EB1911 - Volume 22.djvu/710

Rh Pyromorphite has resulted from the alteration of galena in the oxidized portions of metalliferous veins, and is frequently met with in the upper levels of lead mines. Finely crystallized specimens have been found at Braubach and Ems in Nassau, Wheal Alfred in Cornwall, Roughten Gill in Cumberland, Leadhills in Scotland, Phoenixville in Pennsylvania, Huelgoat in Finistère, Brittany, &c. At the last-named locality, as well as at Wheal Hope, near Truro in Cornwall, there were formerly found curious pseudomorphs of galena after pyromorphite, known as “ blue lead ore.” (L. J. S.)

 PYRONES, in chemistry, a group of hetero cyclic 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 6-oxyd1olefine carboxylic acids, and the 'y-pyrones, which may be regarded as anhydrides of diolefine dioxyketones:- HC, a.>.a.O OC<ai.e.aOm

7 CH-CO/ 4 CH:CH/

(B) (11) (5) (6)

o.-Pyrone. 'y-Pyrone.

As a class, the pyrones are rather unstable compounds, the ring being readily broken. When digested with 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 series, and were first obtained by A. Hantzsch in 1884 (Ann. 222, p. 1) and H. v. Pechmann (Ber., 1884, 17, p. 936). a-Pyrone or coumalin, C5H 402, is obtained by distilling the mercury salt of coumalic acid (from malic acid and sulphuric acid) in a current of hydrogen. It is an oily liquid which boils at 206-209° C., and with alkalis it gives formyl crotonic acid, HO2C~CH:CH-CH¢-CHO. a/'y-Dimethyl-a-pyfone or mesitene lactone, C, HgO¢, is obtained from iso-dehydracetic acid (from aceto-acetic ester and sulphuric acid). Phenylcoumalin or a'-phenyl-o.-pyrone, C, H3(C, ,H5)O2, is found in coto-bark. When heated with alkalis it yields benzoic acid and ace top hen one; reduction by hydriodic acid gives 5-phenyl valeric acid., and when heated with ammonium acetate and ammonia it yields phenylpyridone. It forms an addition product with phenol and with aniline; the latter gives diphenylpyridone when boiled with concentrated hydrochloric acid. Paracotoin, CHHSO4, which also occurs in coto-rind, appears to be a bisoxymethylene phenylpyrone, Chl-1302-C, ,H3(CH2O2).

Various pyronones (keto-dihydropyrones) derived from the compound having formula I. (below) are known, the most important of which is dehydracetic acid, C8H8O4, first obtained by Geuther (]ena'sche Zeit, 1866, p. 8). It may be prepared by distilling acetoacetic ester alone, by heating it with acetic an hydride to 200° C. or by heating acetyl chloride with pyridine to 200-220° C. I. N. Collie regards it as having formula II., whilst Feist (Ann. 1890, 257, p. 253) favours formula III.

QC-CH:CH QC~CH:C-CH2~CO-CH3 QC-CH:C-CH;

H2C-CO-O HQC-CO-O CH3~CO-HC~CO-O

<1.> ni.) nu.)

It crystallizes in tables which melt at 108-109° C., and is a weak acid. Alcoholic potash converts it into aceto-acetic ester, and with concentrated aqueous caustic potash it is completely decomposed into 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 opium, and the more complex fiavone and flavonol derivatives are also found in various plants. The 'y-pyrones may be synthesized by eliminating water from the 1 ~3- 5 triketonesz-CHQ-CO~CO2R CH:C*"CO2R

oc< e oc(>o

CHQ-CO-COQR CH:C CO2R

Acetone dioxalic ester. 9 Chelidonic ester. -7-Pyrone or pyrocomane, C5H4O2, 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, C5H¢(CH3), O2, is obtained by the action of hydriodic acid on the ester of the corresponding acid (Feist, Ann., 1890, 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 'I'ickle, Joum. 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 hydrol ysis in dilute aqueous solution. The oxygen atom is probably tetravalent, and the salts are to be regarded as oxonium salts (see OXYGEN). Collie (Joum. 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:CO1CH HC-C=CH

H3C-C-O-C-CH; J{C1..H

H/C, HEC- '-o= -CH.

Meconic acid, or oxypyrone tricarboxylic acid (3-2-6) C5HO2(OH) (CO2H)2, found in opium, crystallizes in prisms and gives a characteristic deep red colour with erric chloride. On heating to 200° it gives comenic acid, C 5H2O2 (OH)(CO2H), and on distillation pyromeconic acid or B-oxypyrone. On comenic acid see A. Peratoner, Gazz., 1906, 36 (i.), p. 1.,

The tetra hydro—y-pyrones may be obtained by the condensation of aldehyde's with acetone-dicarboxylic ester in the presence of hydrochloric acid.

Benzopyrones.

Compounds of this type are known in both the a and fy 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 isacoumafins (annexed formula) may be prepared by the action of acid chlorides or anhydrides on orthocyanbenzyl cyanide (Ben, 1892, 25, p. 3563); by the molecular rearrangement o the benzal or alkylidene phthalides (S. CH Gabriel, Bef., 1885, 18, p. 2443; 1887, 20, p. 2363), and by the action of manganese dioxide and CH hydrochloric acid on /3-naphthoquinone. I

The parent substance of the -y-group, namely O benzo-7-pyrone (chromone), was obtained in 1900 CO/ by S. Ruhemann (fourn. Chem. Soc., 77, p. 1179) Isocoumal-in 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 fiavone (or flavonol), which is a phenyl-2-benzo-fy-pyrone (S. Kostanecki, Bef., 1898-1906).. Flavone and fiavonol possess the following constitutions, the positions of the substituents being indicated by the numbers:-

4 O 2/ 1 O

3// C /T:, I / Ei/ >

2| I dH gT-4 CCH

/C0/.1. 5 co/

I Flavgne, 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, ace top hen one, ortho-oxyacetophenone and benzoic acid, the latter two products being also formed by its hydrolysis. with sodium ethylate. Chrysm or I -3-dioxyflavone, C151-l,0O4, is a yellow dye, which may be obtained from the buds of different varieties of the poplar. Qn hydrolysis it yields phloroglucin and benzoic and acetic acids. It has been synthesized by heating trimethoxy benzoyl ace top hen one (from ethyl benzoate and phloracetophenone trimethy ether) with hydriodic acid, and also by the action of hydr1odic ac1d on 2'4-dibrom-1'3-dimethoxyflavonone. Galanzin or a'I'3-tr1oxyiiavone or 1-3-dioxyflavonol, CMHNO5, crystallizes in yellow needles. It has been synthesized from hydroxydimethoxy-chalkone, CGHVCH:CH-CO[1]-C6H2(OH)(OCHs)2[2°4~6'], the resulting I'?-d1methoxy-fiavanone compound ielding a nitroso-compound rom

which galanzin is obtained by the action of concentrated hydriodic acid. Apigenine or 1'3'4'-trioxyfiavone, CUHWOG, 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 ara-oxybenzoic acids and phloroglucin. It is obtained synthetic all; by brominating 1'3'4'-trimethoxyfiavonone, the resulting tribromo-compound by the consecutive reactions of alcoholic potash and hydriodic acid yielding apigenine. Kaemtpferol or 1°3'4'-trioxyflavonol, C,5H10O5, is found in the blossoms o Delphinium 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 ara-oxybenzoic acid and phloroglucin. It is obtained synthetically from hydro1E/y-trimethoxychalkone, CH;O[4]'C, H4[1]'CH:CH-CO'[1]C, ~, H, (OH)(O Hi)2 [2'4'6]by a method similar to that used for galanzin. Kaemlpferzd occurs together with galanzin and alpinin in galganta root. t crystallizes in pale yellow needles, which dissolve in the caustic alkalis with an intense yellow colour, and in concentrated sulphuric acid with a