Acids, aliphatic

Dibasic acids usually have low volatility and hence are converted to esters to increase vapor pressure. Trimethylsilyl esters are often successful. 

1 Aliphatic Acids The molecular ion peak of a straight-chain monocarboxylic acid is weak but usually discernible. The most characteristic (sometimes the base) peak is m/z 60 resulting from the McLafferty rearrangement. Branching at the a carbon enhances this cleavage. 

The heats of neutralization, AHr. for some aliphatic- acids, which are shown in Ta )le 3, are those of the process 

From the pk values f 3r these acids, the free energies, A O, may be calculated, and using the relationship AG AH TAS, the entropies of ionization, AS, may be derived. Such data for some aliphatic acids are shown in TabU 1. 

It is seen that the A G values become more positive along the scries formic, acetic, propionic acid, corresponding to the decrease in acidic charactei . Thus the difference in AG between formic and 

We turn now to the ionization process of a series of methyl mono-substituted ring systems pyridines, anilines, phenols, and substituted benzoic acids. Table 2 shows the free energies, A G, the heats, A H, and entropies of ionization, A S, for a number of these compounds. The compounds fall into two classes, according to the different ionic nature of the ionization process. The pyridine and aniline ionizations are of the type 

Here the heats of ionization are calculated from a measured value of the heat of neutralization BH -r OH — 13 r H2O, together with the known heat of neritralization 11 0 —- OH - 2II2O. 

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Barbier-Wieland degradation A method for the stepwise degradation of aliphatic acids... 

Method (1) is most frequently used for aliphatic acid amides, while Methods (2a), (2b) and (zc) are used most frequently for aromatic acid amides. Of the last three methods, the Acid Chloride Method (zb) is the most rapid and certain. The Ester Method (za) is practicable only when the amide is insoluble in water, and even then is often very slow unless the ester itself is appreciabb soluble in the aqueous ammonia solution. 

Oxamide differs from most aliphatic acid amides in being almost insoluble in water, and therefore can be readily prepared from the diethyl ester by Method 2(a). Place a mixture of 5 ml. of concentrated [d o-88o) ammonia solution and 5 ml. of water in a 25 ml. conical flask, for which a welTfitting cork is available. (The large excess of... 

C3H4(0H)(C00H)3 aliphatic acids mixture turns yellow, no... 

The aliphatic acids are all soluble in cold water. The aromatic acids are very sparingly soluble in cold water, but readily soluble in boiling water. Phthalic acid, having two carboxyl groups, is more soluble than the other aromatic acids in cold water. 

The isolation of an aliphatic acid from its aqueous solution, particularly in the presence of metallic salts, is a tedious operation (cf. p. 56), although a few such acids, e.g., succinic acid, can be extracted with ether. Since, however, a solution of an acid or one of Its salts is admirably suited for most of the tests in this series, the isolation of the free acid is rarely necessary except as a nieans of distinguishing (as in (i)) between aliphatic and aromatic members. 

In general the method is more satisfactory with esters of aromatic acids than with esters of aliphatic acids. Esters of alcohols other than methyl and ethyl are best treated by first converting them into methyl esters thus Heat together under reflux i ml. of the higher ester, 5 ml. of methanol and 0-2 g. of sodium methoxide. [In place of the sodium methoxide, it suffices to add o i g. of metallic sodium to the methanol.] After refluxing, distil off the excess of methanol (b.p, 65 ). The residue is then heated under reflux with benzylamine as described above. 

NOTE. Many esters reduce Fehling s solution on warming. This reduction occurs rapidly with the alkyl esters of many aliphatic acids, but scarcely at all with similar esters of aromatic acids (f.g., ethyl oxalate reduces, but ethyl benzoate does not). Note also that this is a property of the ester itself thus both methyl and ethyl oxalate reduce Fehling s solution very rapidly, whereas neither oxalic acid, nor sodium oxalate, nor a mixture of the alcohol and oxalic acid (or sodium oxalate), reduces the solution. 

The choice of type of derivative should be based on whether the chloride or anhydride is aliphatic or aromatic, because this factoi largely determines the reactivity. Aliphatic acid chlorides are best converted into their anilides, as in 4 above aromatic acid chloride may be similarly converted into their anilides, or they may be converted into their amides by shaking with an excess of ammonia (p, 120). (M.ps., pp. 544-545.) Aliphatic acid anhydrides should be converted into their crystalline anilides, but aromatic acid anhydrides arc best hydrolysed to the acid, which can then be converted into one of the standard derivatives (p. 349). 

If a phenol is not indicated, the solution may contain an aliphatic acid. Transfer to a distilling-flask, make definitely acid with dih H2SO4, and distil the volatile formic and acetic acids if present will distil over. If the distillation gives negative reactions, test the residual solution in the flask for oxalic, succinic, lactic, tartaric and citric acids and glycine, remembering that the solution is strongly acid. 

The molecular weight of many carboxylic acids which arc freely soluble in cold water (i.e., chiefly the aliphatic acids) can readily be obtained by titrating a known weight of the acids in aqueous solution with standard sodium or potassium hydroxide solution, using phenolphthalein as an indicator. To avoid the use of unduly large quantities of the acid, it is advisable to use Mj2 caustic alkali solution, and in order to obtain a sharp end>point, this alkali solution... 

SATURATED ALIPHATIC MONOBASIC ACIDS Saturated aliphatic acids may be prepared —... 

The melting points of the derivatives of a number of selected aliphatic acids are collected in Table 111,85. 

The conversion of aliphatic acids Into their acid chlorides is usually accomplished with —... 

REACTIONS AND CHARACTERISATION OF ACID CHLORIDES OF ALIPHATIC ACIDS... 

The above simple experiments illustrate the more important properties of aliphatic acid chlorides. For characterisation, the general procedure is to hydrolyse the acid chloride by warming with dilute alkali solution, neutralise the resulting solution with dilute hydrochloric acid (phenol-phthalein), and evaporate to dryness on a water bath. The mixture of the sodium salt of the acid and sodium chloride thus obtained may be employed for the preparation of solid esters as detailed under Aliphatic Acids, Section 111,85. The anilide or p-toluidide may be prepared directly from the acid chloride (see (iii) above and Section III,85,i). 

With higher aliphatic acids, RCOOH, keten yields first a mixed anhydride CH3COOCOR, which can be distilled under reduced pressure by slow distillation at atmospheric pressure the mixed anhydride undergoes rearrangement into the anhydride of the higher fatty acid and acetic acid, for example ... 

The above simple experiments illustrate the more important properties of the anhydrides of aliphatic acids. For their characterisation, the reaction with aniline or p-toluidine is frequently employed. Alternatively, the anhydride may be hydrolysed with dilute alkali as detailed under Acid Chlorides, Section 111,88, and the resulting acid characterised as in Section 111,85. 

This reaction is applicable to many aliphatic acids and their ammonium salts. 

The use of aliphatic acid anhydrides in place of acid chlorides offers many advantages these include ... 

By dropping au aromatic acid either alone or mixed with an aliphatic acid into a tube containing a thoria catalyst deposited on pumice and heated to 400-450°. This method is generally employed for the preparation of mixed aromatic - aliphatic ketones. Excess of the aUphatic acid is usually present since this leads to by-products which are easily separated and also tends to increase the yield of the desired ketone at the expense of the symmetrical ketone of the aromatic acid. Thus —... 

Plieiiylacetic acid Aliphatic acid Benzyl alkyl ketone... 

With aeetie acid, 2 methylbenzimidazole, m.p. 173-174°, is formed indeed the conversion of aliphatie aeids into 2-alkylbenzimidazoles has been proposed as a method for preparing solid derivatives for the identification of monobasic aliphatic acids. 

When an alkyl aryl ketone is heated with yellow ammonium polysulphide solution at an elevated temperature, an aryl substituted aliphatic acid amide is foimed the product actually isolated is the amide of the ci-aryl carboxylic acid together with a smaller amount of the corresponding ammonium salt of the oarboxylio acid. Thus acetophenone affords phenylacetamide (50 per cent.) and ammonium phenylacetate (13 per cent.) ... 

The formation of acyloins (a-hydroxyketones of the general formula RCH(OH)COR, where R is an aliphatic residue) proceeds best by reaction between finely-divided sodium (2 atoms) and esters of aliphatic acids (1 mol) in anhydrous ether or in anhydrous benzene with exclusion of oxygen salts of enediols are produced, which are converted by hydrolysis into acyloins. The yield of acetoin from ethyl acetate is low (ca. 23 per cent, in ether) owing to the accompanying acetoacetic ester condensation the latter reaction is favoured when the ester is used as the solvent. Ethyl propionate and ethyl ji-butyrate give yields of 52 per cent, of propionoin and 72 per cent, of butyroin respectively in ether. 

Acyl halides are intermediates of the carbonylations of alkenes and organic-halides. Decarbonylation of acyl halides as a reversible process of the carbo-nylation is possible with Pd catalyst. The decarbonylation of aliphatic acid chlorides proceeds with Pd(0) catalyst, such as Pd on carbon or PdC, at around 200 °C[109,753]. The product is a mixture of isomeric internal alkenes. For example, when decanoyl chloride is heated with PdCF at 200 C in a distillation flask, rapid evolution of CO and HCl stops after I h, during which time a mixture of nonene isomers was distilled off in a high yield. The decarbonylation of phenylpropionyl chloride (883) affords styrene (53%). In addition, l,5-diphenyl-l-penten-3-one (884) is obtained as a byproduct (10%). formed by the insertion of styrene into the acyl chloride. Formation of the latter supports the formation of acylpalladium species as an intermediate of the decarbonylation. Decarbonylation of the benzoyl chloride 885 can be carried out in good yields at 360 with Pd on carbon as a catalyst, yielding the aryl chloride 886[754]. 

Lactones formed from aliphatic acids are named by adding -olide to the name of the nonhydrox-ylated hydrocarbon with the same number of carbon atoms. The suffix -olide signifies the change of 2=CH CH, into 2= C C = 0. 

Organic Acid Salts. Slightly soluble or insoluble silver salts are precipitated when mono- and dicarboxylic aliphatic acids or their anions are... 

The hazards of handling branched-chain acids are similar to those encountered with other aliphatic acids of the same molecular weight. Eye and skin contact as well as inhalation of vapors of the shorter-chain acids should be avoided. 

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