ORGANIC ACIDS.] CHEMISTRY 569 Classification. By inspecting the formulae given at the commencement of the present section, it will be seen that the number of times COOH occurs in an acid is dependent on the number of times the group CH 2 .OH occurs in the parent alcohol. Now, organic like mineral acids when brought into contact with metallic oxides or hydroxides, give rise to the formation of salts, in which H , H 2 , H 3 , &c., are replaced by M , M 2 or M", M 3 or M" , &c.; but the hydrogen thus replaceable is found to be not that which enters simply into the composition of HO, as with mineral acids, but that which exists in the group COOH. The basicity of an organic acid depends therefore on the number of carboxyl groups it contains acids containing 1, 2, 3, n carboxyl groups being mono-, di-, tri-, rc -basic. Monobasic Acids. Series C.H 2n+1 (CO 2 H), Acetic or Fatty Series. These acids are prepared 1. By the oxidation of the correspond ing primary alcohols and aldehydes (pp. 563 and 567) ; 2. By the action of mineral acids or alkalies on the cyano- derivatives of the paraffins obtained by the methods pre viously given (p. 566 ; see also p. 555) ; 3. By the action of organo-sodium compounds on carbon dioxide : C.H 2n+1 Xa + C0 2 = aH^.COOXa Sodium salt. Isomerism. Just as the alcohols of the methyl series are most conveniently formulated as derivatives of the first term carbinol, so the present acids may be regarded as derived from formic or acetic acids, the two first members of the series : H CH 3 CH 2 (CH 3 ) C n H 2 , +1 CH 2 (C B H 2n+1 ) i i I COOH COOH COOH COOH COOH C 1 H 2 Ji+ 1 acetic acid. Acetic or Propionic or C H Hon+ Formic acid, methyl-formic methacetic formic acid acid. acid. The first kind of isomerism which we have to consider is that depending on the replacement of one C,,H 2-+1 radicle by two or three others, each containing a smaller nunrber of carbon atoms. (Compare with isomerism of alcohols, p. 562.) Thus we may have the following isomerides : CH 2 (C 3 H 7 ) CH(CH 3 )(C 2 H 5 ) C(CH 3 ) COOH Propacetic or valeric acid. COOH Methyl-ethacetic acid. COOH Trimethacetic acid. Acids are thus classified in the same manner as the alcohols, into primary, secondary, and tertiary, according as the carbon atom in combination with carboxyl is combined with one, two, or three other carbon atoms. Another kind of isomerism is dependent on the isomerism of the radicles replacing the H of the CH 3 of the acetic acid ; thus C(C 3 H 7 )H 2 . COOH C[CH(CH 3 ) 2 ]H 2 . COOH Propacetic acid. is isomeric with Pseudopropaeetic acid. Each of the three groups of acids is accordingly sub divided into normal and iso-acids, although the series are far from being complete. The following is a list of the normal primary acids : Names. Formulae. Boiling-points. Names Formulae. Melting-points. _ Formic Ecid H.C0 2 H 100 C Laurie acid C H CO H 44 C. Acetic CH 3 .C0 2 H 117 Myristic ,, Q 11 ^ 8 * ^(^H 54 Propionic C,H 5 .C0 2 H 141 Palmitic , , p l3 iT 27 pr) 2 tr 62" Butyric C 3 H-.C0 2 H 163 Margaric ,, 60 ? Valeric C 4 H 9 .C0 2 H 185 Stearic ,, 69 Caproic C 5 H 11 .CO !1 H 205 Arachidic , , 75 (Enanthylic , 224 Behenic ,, 76 Caprylic C 7 H U .C0 2 H 238 Cerotic ,, . . 78 Pelargonic CgH^.COjH 254 Melissic , , C 29 H 59 .C0 2 H 88 Capric 269 These acids exhibit the general physical properties of all homologous series. Up to pelargonic acid they are limpid liquids, possessing penetrating odours, and of increasing viscidity ; the remaining members are solid fats. The acids of this series are found in nature in the free state (formic acid in auts, cerotic acid in beeswax, &c.), or as glyceric, &c., ethereal salts, in natural fats (mutton fat, glyceric stearate ; spermaceti, cetylic palmitate, &c.) ; hence the term " fatty series." Formic acid can be prepared by certain special methods 1. By passing carbon monoxide over moist potash (CO + KHO = H.COOK) ; 2. By heating dry oxalic acid with glycerine (H 2 C 2 4 = C0 2 + H.COOH). Acetic acid also can be synthesized from ethine and oxygen in presence of potash (C 2 H 2 + + KHO = C H 3 K0 2 ). This acid is obtained on the large scale by the dry distillation of wood, and is likewise contained in vinegar. Butyric acid is formed by the fermentation of sugar in presence of chalk and putrefying cheese or milk. The metallic salts formed by the fatty acids furnish the organic acids unchanged on distillation with a mineral acid. The potassium salts, when distilled with potassium formate, yield the corresponding aldehydes aH^.COOK + HCOOK = C.H 2n+1 .COH + CO(OK) 2 Potassium salt Formate Aldehyde. The aldehydes can be converted into the alcohols by the action of nascent hydrogen, and from the alcohols the haloid ethers can be obtained, which, by methods now known to the reader, can be converted into nitriles, and these again into acids containing one more atom of carbon than the alcohol (p. 555). It will be obvious that by these reactions the homologous series of fatty acids can be ascended term by term. By the action of chlorine, <tc., on the fatty acids substitution-compounds are obtained. Thus acetic acid is converted into CH 2 C1.CO 2 H, CHC1 2 .CO 2 H, and CC1 3 .CO 2 H. Other derivatives can be obtained from these haloid derivatives by double decomposition ; for example, (by the action of KCX), CH 2 (CX).CO.,H, cyan- acetic acid; (by the action of ammonia), CH 2 (XH ).C0 2 H, amidacetic acid or glycocine. Series C,,H 2n (HO)!c0 2 H, Lactic Series. These acids are the hydroxyl derivatives of the fatty acids, from which they can be obtained by treating the mono-haloid substitu tion derivatives with Ag 2 and water 2C B H 2n /7a. C0 2 H + Ag 2 + OH 2 = 2C n H 2n (HO). C0 2 H + 2Ag//a . The isomeric modifications of these acids admit of being grouped under four divisions : 1. Primary Acids, C(C n H sn+1 )H.HO.C0 2 H. These are formed by the above reaction from primary fatty acids, and also by the slow oxidation of glycols of the form CH(C n H 2n - l - 1 )OH.CH 2 .OH. Aldehydes of the acetic series form compounds with HCX, which, on heating with HC1 and water, yield acids of this group : CH 3 . COH + HCN + 20H 2 + HC1 = C(CH 3 )H. HO. C0 2 H + NH 4 C1 . Acetic aldehyde. Ethidene-lactic acid. 2. Secondary Acids, C(C n H 2n - ! - 1 )j,. HO. C0 2 H. Produced by the action of Ag s O and water on mono-haloid derivatives of secondary fatty acids. Also by the action of organo-zinc compounds on ethyl oxalate, C 2 2 (OC 2 H 5 ) 2 , and treatment of the resulting compound
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