Acids polybasic

The acids which have been described so far are monobasic acids as they contain only one carboxyl group, and, therefore, but one hydrogen atom which undergoes ionization and can be replaced by metals. A number of acids of importance are known which contain two or more of these groups and, consequently, resemble the inorganic acids like sulphuric acid and phosphoric acid in their ability to form acid salts. 

The saturated dibasic acids fall into a homologous series having the general formula CnIl2n-204. The following table gives a list of some of the dibasic acids which contain the carbon atoms in a straight chain — 

Name Formula Melting Point Parts by Weight Soluble in 100 Parts of Water  

It is seen from an inspection of the melting points given in the table that the acids which contain an even number of carbon atoms melt at a higher temperature than the acid next in the series which contains an odd number of carbon atoms. This peculiarity, the cause of which is not understood, has already been noted in the case of the saturated monobasic acids. It is also seen that the melting points of the acids with an even 

The acids in this series crystallize well, and those which contain more than three carbon atoms can be distilled under diminished pressure without decomposition. When heated at high temperatures under atmospheric pressure water is given off and anhydrides are formed. The dibasic acids are not volatile with steam and, consequently, can be readily separated from the lower fatty acids. The properties and reactions of a few typical members of the series will now be described in more detail. 

These are acids where there is more than one ionisable proton per molecule of undissociated acid. The simplest examples are the dicarboxylic acids  

For all dibasic acids pKi p/ i. This is because it is more difficult to remove the second proton from the negatively charged anion produced in the first dissociation. However, the difference between the two p/T values decreases as the value of n increases, a consequence of 

For this reason the term end-point rather than equivalence point will be used to describe the regions of more rapid change of pH for polybasic acids. 

Phosphoric acid is a typical tribasic acid showing three dissociation equilibria  

As with the dibasic acids, it becomes increasingly difficult to remove successive protons 

It is usually true with polybasic acids that one hydrogen radical ionizes with greater facility than the remaining ones. Thus phosphoric acid ionizes primarily as a monobasic acid 

When the hydrolysis of the salt of a polybasic acid is considered, the different H radicals must be treated separately. Thus when 1 mole of tertiary sodium phosphate, Na3P04, is dissolved in water, hydrolysis takes place very extensively as follows  

The solution will have a very strong alkaline reaction, since it contains a large fraction of 1 mole each of ionized NaOH and of ionized secondary sodium phosphate, Na2HPC 4. The OH- ions thus formed check the hydrolysis of the secondary sodium phosphate but if solid secondary sodium phosphate is dissolved in water, hydrolysis of this salt ensues to a sufficient extent to make the solution alkaline to litmus. 

When primary sodium phosphate, NaH2P04, is dissolved, a weakly acid solution is obtained, this effect being due to the tendency of the second hydrogen radical of the acid to ionize. 

When the following salts are dissolved in water, decide from a consideration of the degree of ionization of the base and acid concerned in each case whether the solution will be neutral, weakly acidic, strongly acidic, weakly basic, or strongly basic, and give the explanation and an intersecting ionized equation. 

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In view of the difficulty of arranging esters in a chemically logical sequence they have in the following table been arranged in order of increasing b.ps (liquid) and m.ps (solid). Other esters are given in Tables XI and XII (pp. 543-545). The values for esters of polybasic acids refer to the fully esteriiied product. 

Polyesters from polybasic acids and polyhydric alcohols. Alkyd resins. The condensation of polyhydric alcohols and polybasic acids or anhydrides leads to polj esters known as alkyd resins. The most common member of the group is a glycerol - phthahc acid polymer, and this has led to the term glyptal resins being frequently apphed to the whole group. 

The same is true for polybasic acids, with the proviso that all the carboxyl groups must be changed to aldehyde then it is not necessary to introduce affixes. Examples are... 

COMPOSITEMATERIALS - POLYMER-MATRM - THERMOSETS] (Vol 7) Glycidated polybasic acid... 

The primary and secondary alcohol functionahties have different reactivities, as exemplified by the slower reaction rate for secondary hydroxyls in the formation of esters from acids and alcohols (8). 1,2-Propylene glycol undergoes most of the typical alcohol reactions, such as reaction with a free acid, acyl hahde, or acid anhydride to form an ester reaction with alkaU metal hydroxide to form metal salts and reaction with aldehydes or ketones to form acetals and ketals (9,10). The most important commercial appHcation of propylene glycol is in the manufacture of polyesters by reaction with a dibasic or polybasic acid. 

Esters. Neopentyl glycol diesters are usually Hquids or low melting soflds. Polyesters of neopentyl glycol, and in particular unsaturated polyesters, are prepared by reaction with polybasic acids at atmospheric pressure. High molecular weight linear polyesters (qv) are prepared by the reaction of neopentyl glycol and the ester (usually the methyl ester) of a dibasic acid through transesterification (37—38). The reaction is usually performed at elevated temperatures, in vacuo, in the presence of a metallic catalyst. 

This system of nomenclature has withstood the impact of later experimental discoveries and theoretical developments that have since the time of Guyton de Morveau and Lavoisier greatiy altered the character of chemical thought, eg, atomic theory (Dalton, 1802), the hydrogen theory of acids (Davy, 1809), the duahstic theory (Berzehus, 1811), polybasic acids (Liebig, 1834), Periodic Table (Mendeleev and Meyer, 1869), electrolytic dissociation theory (Arrhenius, 1887), and electronic theory and modem knowledge of molecular stmcture. 

Alkyd resins are produced by reaction of a polybasic acid, such as phthaUc or maleic anhydride, with a polyhydric alcohol, such as glycerol, pentaerythritol, or glycol, in the presence of an oil or fatty acid. The resulting polymeric material can be further modified with other polymers and chemicals such as acryhcs, siUcones, and natural oils. On account of the broad selection of various polybasic acids, polyhydric alcohols, oils and fatty acids, and other modifying ingredients, many different types of alkyd resins can be produced that have a wide range of coating properties (see Alkyd resins). 

The manufacture of alkyd resins (qv), which are obtained by the reactions of polybasic acids or anhydrides, polyhydric alcohols, and fatty oils and acids, consumes about 17% of the phthahc anhydride demand. While materials such as maleic anhydride, isophthahc acid, and fumaric acid can also be used, phthahc anhydride is the most important. The resin provides a binder for coatings that are apphed for either protection or decoration. Ak quahty concerns have put alkyd resins under pressure from water-based coatings which do not emit organic vapors upon drying. 

All d Ingredients Eor each of the three principal components of alkyd reskis, the polybasic acids, the polyols, and the monobasic acids, there is a large variety to be chosen from. The selection of each of these kigredients affects the properties of the reski and may affect the choice of manufacturkig processes. Thus, to both the reski manufacturers and the users, the selection of the proper kigredients is a significant decision. 

Polybasic Acids and Anhydrides. The principal polybasic acids used ki alkyd preparation are Hsted ki Table 2. 

Table 2. Polybasic Acids Used in Alkyd Resin Preparation...

Polybasic acid, (COOH) Polyol, (OH) " Functionahty, Esterification at gel point, %... 

When the alcoholysis step is complete, the polybasic acid(s) and the balance of polyol, if any, are added. The batch is reheated and maintained at about 250°C to carry out the polycondensation step to the desired endpoint, usually a combination of the acid value and viscosity of the resin. 

If the total alkyds consumed in recent years is classified by their dibasic acid component, about 50% belongs to the unmodified phthaHc type, about 28% modified phthaHc type, about 13% based on isophthaHc, and the balance based on polybasic acids other than phthaHc or isophthaHc. The top alkyd resin manufacturers in the U.S. are Cargill, Reichhold, a subsidiary of Dainippon Ink Chemicals, Inc., and Spencer KeUog, now a part of NL Industries, Inc. The median price (52,53), of general types of alkyd resin, soHds base, was 1.98/kg in Nov. 1990 compared to about 440 in 1955, about 660 in 1975, and about 1.54 in 1983, reflecting the increases in raw material cost. 

Chelating agents that are polybasic acids give two or more hydrogen ions per molecule. The four stages of dissociation of EDTA, eg, are represented by the equations... 

Alkyds. Alkyd resins (qv) are polyesters formed by the reaction of polybasic acids, unsaturated fatty acids, and polyhydric alcohols (see Alcohols, POLYHYDRic). Modified alkyds are made when epoxy, sUicone, urethane, or vinyl resins take part in this reaction. The resins cross-link by reaction with oxygen in the air, and carboxylate salts of cobalt, chromium, manganese, zinc, or zirconium are included in the formulation to catalyze drying. 

Fig. 1. Structures of commercial epoxy resins (a) phenolic novolac epoxy resin, (b) glycidated polybasic acid, (c) glycidated polyamine (A,A,A, A -tetraglycidyl-4,4 -diaminodiphenylmethane [28768-32-3] (TGMDA)), and (d) glycidated bisphenol A.

Possible uses for these polyhydroxy compounds include the preparation of alkyd-type resins with polybasic acids, the formation of ester plasticizers, and the preparation of surface-active agents. 

Reaction of polyhydroxy compounds with polybasic acids gives rise to condensation polymers containing ester (—COO—) groups. Because of the presence of these groups such polycondensates are known as polyesters and find use in such diverse applications as fibres, surface coatings, plasticisers, rubbers and laminating resins. These materials are discussed in detail in Chapter 25. 

Condensation of polyhydroxy compounds with polybasic acids, e.g. a glycol with a dicarboxylic acid ... 

These resins are produeed by reacting a polyhydric alcohol, usually glycerol, with a polybasic acid, usually phthalic acid and the fatty acids of various oils such as linseed oil, soya bean oil and tung oil. These oils are triglycerides of the type shown in Figure 25.30. R], R2 and R3 usually contain unsaturated groupings. The alkyd resins would thus have structural units, such as is shown in Figure 25.31. 

Paraplex. A trademark for a group of alkyd type polymeric materials known as polyester resins. These resins are primarily long chain polybasic acids esterified with polyhydric alcohols such as glycol sebacate, glycerol, or ethylene glycol. Some are oil-modified while others are unmodified polyesters... 

Carboxylic acids with one acid group are known as monobasic acids while those with two acid groups are dibasic acids. All acids with more than one acid group are in the class of polybasic acids. The simplest organic acid, formic acid, is responsible for the irritation of bee and ant stings. Vinegar is a 5% solution of acetic acid in water. The acetic acid is responsible for the characteristic sour taste. Citric acid, found in citrus fruits and used in soft drinks, is a tribasic acid with three carboxylic acid groups. The dibasic acid, adipic acid, is a major component of nylon. 

Polyesters may be used [27-30,223] instead of a fatty acid modifier for imidazoline. Thus a corrosion inhibitor with film-forming and film-persistency characteristics can be produced by first reacting, in a condensation reaction, a polybasic acid with a polyalcohol to form a partial ester. The partial ester is reacted with imidazoline or fatty diamines to result in a salt of the ester. Oil-soluble, highly water-dispersible corrosion inhibitor or oil-dispersible. 

J. A. Alford, P. G. Boyd, and E. R. Fischer. Polybasic acid esters as oil field corrosion inhibitors (esters d acides polybasiques utilises comme inhibiteurs de corrosion dans les champs petroliferes). Patent FR 2681597,1993. 

Thus, the cation acts as a polybasic acid. Another case of consecutive complex formation occurs if the solution contains a complexing agent, X,... 

This conclusion is based on a high char yield and similar rates of mass loss at 300°C for untreated and boric acid treated samples. Boric acid samples also had much higher AH+ s and, consequently, higher Ea s. Our results suggest that certain thermally-stable, weak polybasic acids which can complex with polysaccharides may provide fire-resistant properties to lignocellulosics. The results and conclusions were strongly influenced by the technique used to analyze the TGA data. 

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