P-Diacids

P-Diacids are unstable to heat. They decarboxylate (lose CO2), resulting in cleavage of a carbon-carbon bond and formation of a carboxylic acid. Decarboxylation is not a general reaction of all carboxylic acids. It occurs with P-diacids, however, because CO2 can be eliminated through a cyclic, six-atom transition state. This forms an enol of a carboxylic acid, which in turn tautomerizes to the more stable keto form. 

Hydrolysis and decarboxylation. Heating the diester with aqueous acid hydrolyzes the diester to a P-diacid, which loses CO2 to form a carboxylic acid. 

Diacid (Section 23.9A) A compound containing two carboxylic acids separated by a single carbon atom. 1,3-Diacids are also called p-diacids. 

Nylon A class of synthetic fibres and plastics, polyamides. Manufactured by condensation polymerization of ct, oj-aminomonocarboxylic acids or of aliphatic diamines with aliphatic dicarboxylic acids. Also rormed specifically, e.g. from caprolactam. The different Nylons are identified by reference to the carbon numbers of the diacid and diamine (e.g. Nylon 66 is from hexamethylene diamine and adipic acid). Thermoplastic materials with high m.p., insolubility, toughness, impact resistance, low friction. Used in monofilaments, textiles, cables, insulation and in packing materials. U.S. production 1983 11 megatonnes. 

The diacid components for the manufacture of poly(y -phenyleneisophthalamide) and poly(p-phenyleneterephthalamide) are produced by one of two processes. In the first, the diacid chlorides are produced by the oxidation of / -xylene [108-38-3] or -xylene [106-42-3] followed by the reaction of the diacids with phosgene [75-44-5]. In the second, process m- or -xylene reacts with chlorine initiated by ultraviolet light to form the m- or Nhexachloroxylene. This then reacts with the respective aromatic dicarboxyUc acid to form the diacid chloride. 

In 1973 Du Pont commenced production of another aromatic polytunide fibre, a poly-(p-phenyleneterephthalamide) marketed as Kevlar. It is produced by the fourth method of polyamide production listed in the introductory section of this chapter, namely the reaction of a diamine with a diacid chloride. Specifically, p-phenylenediamine is treated with terephthalyl chloride in a mixture of hexamethylphosphoramide and V-methylpyrrolidone (2 1) at -10°C Figure 18.32). 

It is sparingly soluble in ether or chloroform, readily in alcohol or solutions of caustic alkalis, but not in ammonia, and gives the thalleioquin reaction. Cupreine is a diacidic base the neutral sulphate, Bj. H2SO4, forms colourless anhydrous needles, m.p. 257 , soluble in 813 parts of water at 17 the iwiid sulphate, B. HjSO. HjO, crystallises in prisms and is soluble in... 

Monniera cuneifolia Michx. (Herpestis monniera HB and K. (Scro-phulariaceae). The plant contains alkaloids, of which the chief item, herpestine, has been obtained crystalline. It has the formula C34H4gOjN2, m.p. 116-8°, is diacidic and forms the following salts sulphate, dec. 120°, tartrate, m.p. 209-210°, platinichloride, dec. from 242°, aurichloride, m.p. 210-3° (dec.) (Basu and Walia, Ind. J. Pharm., 1944, 6, 85, 91 Basu and Pabrai, Quart. J. Pharm. Pharmacol., 1947, 20, 137). 

It is also possible to prepare them from amino acids by the self-condensation reaction (3.12). The PAs (AABB) can be prepared from diamines and diacids by hydrolytic polymerization [see (3.12)]. The polyamides can also be prepared from other starting materials, such as esters, acid chlorides, isocyanates, silylated amines, and nitrils. The reactive acid chlorides are employed in the synthesis of wholly aromatic polyamides, such as poly(p-phenyleneterephthalamide) in (3.4). The molecular weight distribution (Mw/Mn) of these polymers follows the classical theory of molecular weight distribution and is nearly always in the region of 2. In some cases, such as PA-6,6, chain branching can take place and then the Mw/Mn ratio is higher. 

Madec, J.-P. and Marechal, E. Kinetics and Mechanisms of Polyesterifications. II. Reactions of Diacids with Diepoxides. Vol. 71, pp. 153-228. 

The cationic pathway allows the conversion of carboxylic acids into ethers, acetals or amides. From a-aminoacids versatile chiral building blocks are accessible. The eliminative decarboxylation of vicinal diacids or P-silyl carboxylic acids, combined with cycloaddition reactions, allows the efficient construction of cyclobutenes or cyclohexadienes. The induction of cationic rearrangements or fragmentations is a potent way to specifically substituted cyclopentanoids and ring extensions by one-or four carbons. In view of these favorable qualities of Kolbe electrolysis, numerous useful applications of this old reaction can be expected in the future. 

The p a values of two systems with two intra-annular COOH groups have been measured. In Cram s study (Bell et al., 1982), the macrocyclic diacid [36] and an open-chain monomeric analogue [37] had almost identical pK values (see Table 18). In contrast, Gennari s compound [38], which contains two intra-annular COOH groups and in addition two"ethyl ester groups, has a different acidity from that of the analogues [39] and [40], as shown in Table 19 (Gennari et al., 1992). But in both systems, the difference Ap a between pK i and p a2 was comparable (ApA a [36] = 1.7, Ap/La[38] = 1.5). The increase from p ai to P a2 may occur for two... 

Aromatic polyesters were efficiently synthesized from aromatic diacid divinyl esters. Lipase CA induced the polymerization of divinyl esters of isoph-thalic acid, terephthalic acid, and p-phenylene diacetic acid with glycols to give polyesters containing aromatic moiety in the main chain. The highest molecular weight (7.2 x 10 ) was attained from a combination of divinyl isophthalate and 1,10-decanediol. Enzymatic polymerization of divinyl esters and aromatic diols also afforded the aromatic polyesters. ... 

Pyruvic acid is not stable at ambient temperature when it is stored for a long period of time. It can only be stored in a refrigerated room. A bottle of this acid was stored in a laboratory at 25°C and detonated, probably because of the overpressure created by the formation of carbon dioxide. Indeed, with diacids and complex acids the decomposition is made by decarboxylation. In this particular case, this decomposition should give rise to acetaldehyde. It could be asked whether, in the exothermic conditions of this decomposition, a polymerisation of this aldehyde (see Aldehydes-ketones on p.310) did not make the situation worse. 

In polar solvents, the structure of the acridine 13 involves some zwitterionic character 13 a [Eq. (7)] and the interior of the cleft becomes an intensely polar microenvironment. On the periphery of the molecule a heavy lipophilic coating is provided by the hydrocarbon skeleton and methyl groups. A third domain, the large, flat aromatic surface is exposed by the acridine spacer unit. This unusual combination of ionic, hydrophobic and stacking opportunities endows these molecules with the ability to interact with the zwitterionic forms of amino acids which exist at neutral pH 24). For example, the acridine diacids can extract zwitterionic phenylalanine from water into chloroform, andNMR evidence indicates the formation of 2 1 complexes 39 such as were previously described for other P-phenyl-ethylammonium salts. Similar behavior is seen with tryptophan 40 and tyrosine methyl ether 41. The structures lacking well-placed aromatics such as leucine or methionine are not extracted to measureable degrees under these conditions. 

---