and decomposed by dilute sulphuric acid and the acrylic acid separated by distillation. Glycerin iodo-propionic acid, when treated with alcoholic potash, gives along with a salt of hydracrylic also that of acrylic acid. A better method consists in distilling glycerine iodo-propionic acid over finely powdered plumbic oxide; the hydracrylic acid is then decomposed into water and acrylic acid. In the dry condition acrylic acid is only obtained by decomposing a silver or lead salt by dry SH2, in which case it is a colourless, strongly acid liquid, crystallising in the cold and miscible in all proportions with water. It melts between 7 and 8° and boils at 139°-140°. It combines very slowly with nascent hydrogen, but by long contact with zinc and dilute sulphuric acid in the warm it is completely converted to propionic acid: CH2 CH.CO.OH + 2H = CH3.CH.CO.OH. Most of its salts are easily soluble in water. Its sodium salt can be obtained by evaporation as a white, scarcely crystalline mass, not dissolving to any appreciable extent even in boiling alcohol. The lead salt forms long shiny prisms of the formula (C3H3O2)2Pb. The silver salt, C3H3O,Ag, crystallises in fine needles. The potassic salt, when fused with potash, gives potassic acetate and formate: C2H2O.OK + KOH + OH2 = C2H2O.OK + CHKO2 + H2. Ethylic acrylate is the only ethereal salt of acrylic acid known, being obtained from 3-dibrom-propionic ether by the action of zinc and sulphuric acid: CH,Br.CHBr.00.0C,H. H, CH2Br.CH Br.CO.OC2H, + H2 = 2HBr + CH2:CH.CO.OC2H5. It is a liquid of penetrating odour, boiling at 101°-102°, of sp. gr. at 0° 925. 793. The haloid acids combine with acrylic acid to form the so-called B-halogen propionic acids (§ 744), by means of which it can easily be reconverted into hydracrylic acid. The halogens combine similarly, bromine forming B-dibrom-propionic acid, isomeric with a-dibrom-propionic acid (§ 784). 6-Dibrom-propionic acid, CH,Br.CH Br.CO.OH, also obtained by the oxidation of allyl alcohol dibromide with strong nitric acid; it is colourless and crystalline, melts at 64°-65°, dissolves easily in water, and decomposes on distillation. Its ethylic salt is a fruity-smelling oil, boiling between 211° and 214°. Boiling alcoholic potash converts it into the potassium salt of B-Brom-acrylic acid, CHBr : CH.CO.OH, which latter crystallises in small prisms, fusing between 69° and 70°, and combining directly with hydric bromide to reform B-dibrom-propionic acid. The potassium salt forms beautiful tables. B-Brom-acrylic acid, CH2: CBr.CO.OH, obtained with difficulty from a-dibrom-propionic acid by treatment by alcoholic potash, also melts at 69°-70°. The potassic salt forms rhombic tables. It combines likewise with hydric bromide to form ẞ-dibrompropionic acid, so that by this means propionic can be reduced to acrylic acid. Crotonic Acids, C4H6O2. 794. Three isomeric acids of this formula are known. 1. a-Crotonic acid, CH3.CH: CH.CO.OH, improperly termed crotonic acid, is obtained by the oxidation of its aldehyde (§ 765), by dry distillation of ẞ-oxybutyric acid (§ 747, 2) : CH..CH(OH).CH,.CO.OH = H,O + CH,.CH:CH.CO.OH, by heating ethylic a-brom-butyrate with potassic hydrate: CH,.CH,.CHBr.CO.O.C.H, + 20KH=KBr + OH, + HO.C,H, +CH,CH:CH.CO.OK, 5 and by boiling allylic cyanide with potash. In the latter case, however, a shifting of the hydrogen atoms and the position of the double carbon union takes place. The structural constitution of allylic iodide is undoubtedly CH2:CH.CH2I. This on heating with potassic cyanide should give the nitrile of B-crotonic acid, CH, CH.CH.CN, which by treatment with potassic hydrate would be expected to yield ẞ-crotonic acid, instead of which a-crotonic acid is formed. Owing to the easy convertibility of the 3-acid to the a, the facts are not very surprising; indeed, it is quite a question whether the nitrile contains the nucleus of the a-acid and reacts in this manner: CH,:CH.CH,I+KCN=KI + CHỊCH:CH.CN, or if the nitrile of the B-acid is first formed, and during the replacement of the nitrogen atom by oxygen the hydrocarbon radical changed to that of the a-crotonic acid. a-Crotonic acid crystallises in tables or needles melting at 72°, and distils at 180° without change; it is slowly converted by nascent hydrogen into butyric acid; nitric acid oxidises it to acetic and oxalic acids, chromic acid to acetic and carbonic acids, and by melting with potassic hydrate it is split up into two acetic acid molecules. Hydric iodide and bromide combine with it to form the two possible a- and 3-halogen butyric acids: aCH3.CH: CH.CO.OH + aHI = bCH3.CH2.CHI.CO.OH + (a - b)CH3.CHI.CH2.CO.OH, which by boiling with potash yield a mixture of a- and ẞ-oxybutyric acids. 795. Two molecules of phosphoric chloride act with great energy on one molecule of ethylic aceto-acetate, forming a mixture of the chlorides of two different crotonic acids. Similarly to the reaction between phosphoric chloride and acetone (§ 445), the action is here not exactly confined to that represented in the equation: CH3.CO.CH2.CO.O.C2H, +2PCI, = 2POCI3 + C2H ̧¤1 + CH3.CC12.CH2.CO.Cl; but hydric chloride is also formed under the assumption of difference in 'place value' of the hydrogen atom: aCH3.CC12.CH.CO.ClaHCl + CH3.CCI: CH.CO.Cl HH On contact with water the chlorides decompose into the two isomeric chlor-crotonic acids: CH3.CCI.CH.CO.OH and CH2 : CC1.CH2.CO.OH, which are separable by difference of solubility. Chlor a-crotonic acid, CH3.CC1: CH.CO.OH, crystallises in colourless prisms, melting at 94°, boiling 206°-211°, soluble in 35 parts cold water, easily in hot water. Sodium amalgam gives a-crotonic acid: CH3.CCI: CH.CO.OH + Na2 = NaCl + CH3.CH: CH.CO.ONa. Probably the same crotonic acid results from the action of zinc and HCl on trichlor crotonic acid, С1Н ̧¤1 ̧О2, obtained by the oxidation of trichlor crotonic aldehyde with nitric acid in colourless needles, melting at 44°. Chlor B-crotonic acid, CH, CCI.CH.CO.OH, forms needles difficultly soluble in water, melting at 59.5°, and subliming even at ordinary temperatures and boiling at 195°. Sodium amalgam in aqueous solution gives the salt of 796. 2. B-Crotonic acid, CH2: CH.CH2.CO.OH, isocrotonic or quartenylic acid, a liquid not freezing at -15°, boiling at 172°, but leaving a small residue of a-crotonic acid at each distillation and completely changed into the latter by heating in tube to 180°. B-Crotonic acid, by heating with potassic hydrate, splits up into acetic acid only, whereas from its structure formic and propionic acids were to be expected. ethylic salt from the action of PC1, on the ethylic salt of a-hydroxyisobutyric acid (§ 747, 3): The free acid is liquid at 0°, and is broken up by fused potassic hydrate into formic and propionic acids: CH2:C.CH3 COOK + KOH + OH2 = CHKO2 + H2 + | 2 CH,.CH3 CO.OK A brom substitution product, brom-meth-acrylic acid, results from heating citraconic dibromide with aqueous alkalies (see citra-dibrompyrotartaric acid). Acids of the Formula C5H.02. Four of these acids are known. 797. 1. Methyl crotonic acid, CH3.CH: C(CH3).CO.OH, tiglic acid, or a-ẞ-dimethyl acrylic acid. By action of phosphorus chloride on ethylic eth-meth-oxalate (comp. § 749, 2) ethylic methyl crotonate, CH3.CH: C(CH3).CO.O.C2H5, is obtained as a liquid boiling at 156°. The free acid crystallises in triclinic prisms, melting at 62°, and is decomposed into acetate and propionate by fusion with potassic hydrate. Croton oil (from the seeds of Croton Tiglium) contains, in addition to the glycerine salts of numerous acids of the acetic series, the salt of an acid, tiglic acid, which appears to be identical with methyl crotonic acid; it melts at 64° and boils at 198.5°. This acid is also obtained from oil of camomile (Anthemis nobilis), by saponification of the alkaloids of Veratrum Sabadilla and by heating angelic acid. Hydrobromic acid converts it into a brom-valeric acid. 2. B-Dimeth-acrylic acid, (CH3)2C: CH.CO.OH, is prepared from hydroxy-isopropyl acetic acid, (CH3)2.C(OH).CH.CO.OH, by distillation with sulphuric acid. It forms monoclinic prisms, melting at 70°. 3. Angelic acid occurs in the root of Angelica archangelica and in Eurysangium sumbul, camomile oil and the roots of various Umbelliferæ, from which it may be obtained by boiling the roots with milk of lime and decomposing the filtered solution with sulphuric acid, and distilling in a current of steam, the angelic acid passing over, and from laserpitin and peucedanin by the action of potassic hydrate. The acid forms large monoclinic prisms, melting at 45° and boiling at 185°, a portion being converted into an isomeric modification during the process. It is insoluble in cold but easily soluble in hot water, and also in alcohol and ether. Its salts are mostly soluble in water. Nascent hydrogen has no effect on the acid either in acid or alkaline solution; with HI it is reduced to a valeric acid. Bromine forms a dibrom-valeric acid, melting at 76° and giving an oily brom-butylene on boiling with caustic potash : CH,Br2.CO.OK + 2KOH = C1Н‚Bг + K2CO2 + KBr + OH2. 7 3 The exact constitution of angelic acid is not yet known, and indeed its isomerism with a-ß-dimethyl acrylic acid cannot be represented by the notation at present in use. 4. Allyl acetic acid, C3H5.CH.CO.OH, is a colourless oil of boiling point 182°, obtained by the decomposition of ethylic allyl-acetoacetate. Acids of the Formula C6H1002. 798. 1. Ethyl crotonic acid, a-methyl B-eth-acrylic acid: CH3.CH: C(C2H).CO.OH, is obtained as ethylic salt on treating ethylic dieth-oxalate (§ 750, 2) with PC1, and then with water. It forms square prisms melting at 41.5°, sublimes at ordinary temperatures, and gives butyrate and acetate on fusion with potash. 2. Hydro-sorbic acid, a-propyl acrylic acid, CH9.CO.OH, is obtained from sobric acid by the action of sodium amalgam. It is only slightly soluble in water, is liquid at -18°, boils at 204°, and at 19° has a sp. gr. of 969; fusion with potassic hydrate splits it up into acetic and butyric acids. 3. Pyroterebic acid, a-isopropyl acrylic acid: (CH3)2.CH2.CH: CH.CO.OH, is obtained by the dry distillation of terebic acid, C,H1004. It is an oil boiling at 210°; fusing potash gives isobutyric and acetic salts. Higher Members of the CnH2n-202 Series. 799. These bodies are very imperfectly known, there being still many gaps in the series. Cimicic acid, C15H2802, occurs in a free state in a beetle (Rhaphigaster punctipennis), from which it may be extracted by ether. melts at 44°. It Hypogaic acid, C16H300, exists in earth-nut oil (Arachis hypogae) along with arachic acid (§ 598), and also in the sperm oil of Physeter macrocephalus. It forms colourless needles, melting at 33° and soluble in alcohol and ether. It oxidises in the air and combines with bromine to form dibrom-palmitic acid, C16H30Br2O2, melting point 29°, which, on treatment with potash, gives monobrom-hypogaic acid, C6H29BrO2. 16 Nitrous acid converts hypogæic acid into an isomeric gæidic acid, of melting point 39°, which does not oxidise in air or decompose on distillation. 800. Oleic acid, C18H34O2, is found as a glycerine salt in nearly all fats, C3H5(O.C18H330)3. Almond, olive, and whale oils are especially rich in this salt. The acid is obtained by saponification with plumbic oxide and finally purified by means of its barium salt. It is a colourless, tasteless oil, solidifying at 0°, and afterwards melting at 14°. It distils in superheated steam at about 250°. When distilled alone it splits up into a mixture of sebacic, caproic, caprylic acids, and other bodies. It rapidly oxidises in the air, and gives salts with the alkalies which are true soaps. It combines with bromine to form C18H3,Br202, from which brom-oleic and stearolic acids may be obtained by the action of potassic hydrate. Oleic acid is converted by nitrous acid into the isomeric elaïdic acid, forming leafy crystals of 45° melting point. This acid also forms a dibromide from which nascent hydrogen reduces the original acid. Oleic acid, when fused with alkalies, gives acetic and palmitic acids. Ricinoleic acid, or hydroxy-oleic acid, C18H3403, stands in near relationship to oleic acid. Its glycerine salt is found in linseed oil. It also gives an isomeric crystalline modification, melting at 50°, on treatment with nitrous acid. It does not oxidise on exposure to air, and gives on dry distillation œnanthol, and with potash yields normal secondary octyl alcohol along with methyl-hexyl ketone and sebacic acid: C18H33KO3+ KOH + OH2 = C10H16K20, + CH180+ H2 Potassic sebate. Octyl alcohol. C18H3303K+ KOH + OH2 = C10H16K2O4 + C8H160 + 2H2 Methyl-hexyl ketone. (comp. § 453). |