**3. Dicarboxylic acids**

328 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

(Charlson and Heintzenberg, 1995).

and Penner, 1993).

*2.5.2. Water soluble organic compounds* 

inorganic salt such as ammonium sulphate.

As for other aerosols, the removal of particulate carbon is likely to occur via two main scavenging processes: the in-cloud process, whereby particles are directly incorporated into cloud droplets; and the below-cloud process, where particles are washed out by precipitation itself. The physico-chemical atmospheric processes which transform young combustion particles, expected to be hydrophobic, into a water soluble aerosol phase remains a major unknown. The atmospheric behaviour of the carbonaceous particles is likely to be dictated by the chemical nature of their surfaces (Cachier et al., 1989). If the surface is hydrophobic, the particle remains inactive. However, if it is coated with hygroscopic substances, it may be activated enough to be incorporated into water droplets

A significant fraction of the particulate organic carbon is water soluble, ranging from 20% to 70% of the total soluble mass, thus making it important to various aerosol-cloud interactions (Decesari et al., 2000; Facchini et al., 2000). Water soluble organic compounds (WSOC) contribute to the ability of the particles to act as cloud condensation nuclei (CCN) (Novokov

WSOC have been postulated to be partially responsible for the water uptake of airbone particulate matter, which can substantially affect the physical and chemical properties of atmospheric aerosols (Yu et al., 2005). Decesari et al. (2001) have suggested that WSOC are composed of higly oxidised species with residual aromatic nuclei and aliphatic chains. The current understanding of atmospheric particles describes their WSOC fraction as a complex mixture of very soluble organic compounds, slightly soluble organic compounds, and some

The composition of WSOC varies among sampling regions. It was found to constitute between 20 and 67% of the total organic carbon present in aerosol samples collected in Tokyo (Sempere and Kawamura, 1994). The percentage is ranged from 65 to 75% in aerosol samples collected in Hungary, Italy and Sweeden (Zappoli et al., 1999). The study also found that the percentage of WSOC species with respect to the total soluble mass was much higher at the background site (Aspvreten, Central Sweeden) (c.a. 50%) compared to the polluted site (San Pietro Copofiume, Po Valley, Italy) (c.a. 25%). A very high fraction (over 70%) of organic compounds in the aerosol consisted of polar species. A study by Wang et al. (2002) showed that most water soluble carbon is total organic carbon (TOC) and range between 20.53 to 35.58 μg m-3 in PM10 and PM 2.5. A further study by (Narukawa et al., 1999) concluded that individual haze particles over Kalimantan of Indonesia were mainly composed of water soluble organic materials and

The ionic organic compounds (including carboxylic, dicarboxylic and ketoacids) were distributed between both sub-micron and super micron mode, indicating origins in both gas-to-particle conversion and heterogeneous reaction on pre-existing particles. WSOC in atmospheric aerosols and droplets can be divided by their functional groups into three classes which are neutral, mono- and dicarboxylic acid and also polycarboxylic acid, which

undetermined macromolecular compounds (MMCs)(Saxena and Hildemann, 1996).

During the past decade, much attention has been paid to the low molecular weight dicarboxylic acids and related polar compounds which are ubiquitous water-soluble organic compounds that have been detected in a variety of environmental samples including atmospheric aerosols, rainwaters, snow packs, ice cores, meteorites, marine sediments, hypersaline brines and freshwaters (Kawamura and Ikushima, 1993; Tedetti et al., 2006). In the atmosphere, dicarboxylic acids originate from incomplete combustion of fossil fuels (Kawamura and Ikushima, 1993; Kawamura and Kaplan, 1987), biomass burning (Narukawa et al., 1999), direct biogenic emission and ozonolysis and photo-oxidation of organic compound (Sempere and Kawamura, 2003).

Low molecular weight (LMW) dicarboxylic acids have also been identified in cloud water samples collected at a high mountain range in central europe (Puxbaum and Limbeck, 2000), in the condensed phase at a semi-urban site in the northeastern US (Khwaja, 1995) and in Arctic aerosol (Kawamura et al., 1996). As a result of their hygroscopic properties, dicarboxylic acids can act as cloud condensation nuclei and have an impact on the radiative forcing at earth's surface (Kerminen et al., 2000). Dicarboxylic acids also participate in many biological processes. They are important intermediates in the tricarboxylic acid and glyoxylate cycles and the catabolism and anabolism of amino acids (Tedetti et al., 2006).

Photochemical reactions are also an important source of atmospheric dicarboxylic acids. For example, glutaric acids photooxidation is likely the dominant pathway formation, as measured atmospheric concentrations of dicarboxylic acids in Los Angeles far surpasses contributions from direct emissions and seasonal trends suggest that dicarboxylic acids are largely produced in photochemical smog (Puxbaum and Limbeck, 2000; Rogge et al., 1993).

Aliphatic dicarboxylic acids (or diacids) can be described by the following general formula:

The Chemistry of Dicarboxylic Acids in the Atmospheric Aerosols 331

**Table 1.** Physical properties of saturated dicarboxylic acid (Clarke, 1986)

## HOOC-(CH2)n-COOH

According to IUPAC nomenclature, dicarboxylic acids are named by adding the suffix dioic acid to the name of the hydrocarbon with the same number of carbon atoms, e.g., nonanedioic acid for *n* = 7. The older literature often uses another system based on the hydrocarbon for the (CH2)*n* carbon segment and the suffix dicarboxylic acid, e.g., heptanedicarboxylic acid for *n* = 7. However, trivial names are commonly used for the saturated linear aliphatic dicarboxylic acids from *n* = 0 (oxalic acid) to *n* = 8 (sebacic acid) and for the simple unsaturated aliphatic dicarboxylic acids; these names are generally derived from the natural substance in which the acid occurs or from which it was first isolated.

Aliphatic dicarboxylic acids are found in nature both as free acids and as salts. For example, malonic acid is present in small amounts in sugar beet and in the green parts of the wheat plant; oxalic acid occurs in many plants and in some minerals as the calcium salt. However, natural sources are no longer used to recover these acids.

The main industrial process employed for manufacturing dicarboxylic acids is the ringopening oxidation of cyclic compounds.

Oxalic acid is the most important dicarboxylic acid. Adipic, malonic, suberic, azelaic, sebacic, and 1,12-dodecanedioic acids, as well as maleic and fumaric acids, are also manufactured on an industrial scale.

**Physical properties**: Dicarboxylic acids are colorless, odorless crystalline substances at room temperature. Table 1 lists the major physical properties of some saturated aliphatic dicarboxylic acids.

The lower dicarboxylic acids are stronger acids than the corresponding monocarboxylic ones. The first dissociation constant is considerably greater than the second. Density and dissociation constants decrease steadily with increasing chain length. By contrast, melting point and water solubility alternate: Dicarboxylic acids with an even number of carbon atoms have higher melting points than the next higher odd-numbered dicarboxylic acid. In the *n* = 0 – 8 range, dicarboxylic acids with an even number of carbon atoms are slightly soluble in water, while the next higher homologues with an odd number of carbon atoms are more readily soluble. As chain length increases, the influence of the hydrophilic carboxyl groups diminishes; from *n* = 5 (pimelic acid) onward, solubility in water decreases rapidly. The alternating solubility of dicarboxylic acids can be exploited to separate acid mixtures. Most dicarboxylic acids dissolve easily in lower alcohols; at room temperature, the lower dicarboxylic acids are practically insoluble in benzene and other aromatic solvents.



**Table 1.** Physical properties of saturated dicarboxylic acid (Clarke, 1986)

natural sources are no longer used to recover these acids.

opening oxidation of cyclic compounds.

manufactured on an industrial scale.

dicarboxylic acids.

aromatic solvents.

formula:

isolated.

Aliphatic dicarboxylic acids (or diacids) can be described by the following general

HOOC-(CH2)n-COOH According to IUPAC nomenclature, dicarboxylic acids are named by adding the suffix dioic acid to the name of the hydrocarbon with the same number of carbon atoms, e.g., nonanedioic acid for *n* = 7. The older literature often uses another system based on the hydrocarbon for the (CH2)*n* carbon segment and the suffix dicarboxylic acid, e.g., heptanedicarboxylic acid for *n* = 7. However, trivial names are commonly used for the saturated linear aliphatic dicarboxylic acids from *n* = 0 (oxalic acid) to *n* = 8 (sebacic acid) and for the simple unsaturated aliphatic dicarboxylic acids; these names are generally derived from the natural substance in which the acid occurs or from which it was first

Aliphatic dicarboxylic acids are found in nature both as free acids and as salts. For example, malonic acid is present in small amounts in sugar beet and in the green parts of the wheat plant; oxalic acid occurs in many plants and in some minerals as the calcium salt. However,

The main industrial process employed for manufacturing dicarboxylic acids is the ring-

Oxalic acid is the most important dicarboxylic acid. Adipic, malonic, suberic, azelaic, sebacic, and 1,12-dodecanedioic acids, as well as maleic and fumaric acids, are also

**Physical properties**: Dicarboxylic acids are colorless, odorless crystalline substances at room temperature. Table 1 lists the major physical properties of some saturated aliphatic

The lower dicarboxylic acids are stronger acids than the corresponding monocarboxylic ones. The first dissociation constant is considerably greater than the second. Density and dissociation constants decrease steadily with increasing chain length. By contrast, melting point and water solubility alternate: Dicarboxylic acids with an even number of carbon atoms have higher melting points than the next higher odd-numbered dicarboxylic acid. In the *n* = 0 – 8 range, dicarboxylic acids with an even number of carbon atoms are slightly soluble in water, while the next higher homologues with an odd number of carbon atoms are more readily soluble. As chain length increases, the influence of the hydrophilic carboxyl groups diminishes; from *n* = 5 (pimelic acid) onward, solubility in water decreases rapidly. The alternating solubility of dicarboxylic acids can be exploited to separate acid mixtures. Most dicarboxylic acids dissolve easily in lower alcohols; at room temperature, the lower dicarboxylic acids are practically insoluble in benzene and other **Chemical properties**: The chemical behavior of dicarboxylic acids is determined principally by the two carboxyl groups. The neighboring methylene groups are activated generally to only a minor degree. Thermal decomposition of dicarboxylic acids gives different products depending on the chain length. Acids with an even number of carbon atoms require higher decarboxylation temperatures than the next higher odd-numbered homologues; lower dicarboxylic acids decompose more easily than higher ones. To avoid undesired decomposition reactions, aliphatic dicarboxylic acids should only be distilled in vacuum. When heated above 190 °C, oxalic acid decomposes to carbon monoxide, carbon dioxide, and water. Malonic acid is decarboxylated to acetic acid at temperatures above 150 C:

The Chemistry of Dicarboxylic Acids in the Atmospheric Aerosols 333

Succinic acid ester reacts with aldehydes or ketones in the presence of sodium ethoxide or potassium *tert*-butoxide to form alkylidenesuccinic acid monoesters (Stobbe condensation). These can subsequently be converted into monocarboxylic acids by hydrolysis,

**Scheme 3.** Production number of straight-chain aliphatic dicarboxylic acids and their derivatives occur

**Production:** A number of straight-chain aliphatic dicarboxylic acids and their derivatives occur in nature. However, isolation from natural substances has no commercial significance. Although many syntheses for the production of aliphatic dicarboxylic acids are known, only a few have found industrial application. This is due partly to the

**Individual saturated dicarboxylic acids**: Dicarboxylic acids are used mainly as intermediates in the manufacture of esters and polyamides. Esters derived from monofunctional alcohols serve as plasticizers or lubricants. Polyesters are obtained by reaction with dialcohols. In addition, dicarboxylic acids are employed in the manufacture of hydraulic fluids, agricultural chemicals, pharmaceuticals, dyes, complexing agents for

Oxalic acid (ethanedioic acid, acidum oxalicum) is the simplest saturated dicarboxylic acid (Clarke, 1986). The compound exists in anhydrous form [144-62-7] or as a dihydrate [6153- 56-6]. The anhydrous acid is not found in nature and must be prepared from the dihydrate even when produced industrially. Oxalic acid is widely distributed in the plant and animal kingdom (nearly always in the form of its salts) and has various industrial applications.

decarboxylation, and hydrogenation (Clarke, 1986).

in nature

shortage of raw materials.

**3.1. Oxalic acid** 

heavy-metal salts, and lubricant additives (as metal salts).

**Scheme 4.** Chemical structure of oxalic acid

#### HOOC-(CH2)n-COOH-CH3COOH + CO2

When malonic acid is heated in the presence of P2O5 at ca. 150 °C, small amounts of carbon suboxide (C3O2) are also formed. Succinic and glutaric acids are converted into cyclic anhydrides on heating:

**Scheme 1.** Succinic and glutaric acids are converted into cyclic anhydrides on heating

When the ammonium salt of succinic acid is distilled rapidly, succinimide is formed, with the release of water and ammonia.

Higher dicarboxylic acids from *n* = 4 (adipic acid) to *n* = 6 (suberic acid) split off carbon dioxide and water to form cyclic ketones:

**Scheme 2.** Higher dicarboxylic acids from n = 4 (adipic acid) to n = 6 (suberic acid) split off carbon dioxide and water to form cyclic ketones

The decomposition of still higher dicarboxylic acids leads to complex mixtures. With the exception of oxalic acid, dicarboxylic acids are resistant to oxidation. Oxalic acid is used as a reducing agent for both commercial and analytical purposes. Dicarboxylic acids react with dialcohols to form polyesters and with diamines to form polyamides. They also serve as starting materials for the production of the corresponding diamines. Reaction with monoalcohols yields esters. All of these reactions are commercially important. Several reactions with malonic and glutaric acids are of interest in organic syntheses: the Knoevenagel condensation, Michael addition, and malonic ester synthesis (Clarke, 1986)

Succinic acid ester reacts with aldehydes or ketones in the presence of sodium ethoxide or potassium *tert*-butoxide to form alkylidenesuccinic acid monoesters (Stobbe condensation). These can subsequently be converted into monocarboxylic acids by hydrolysis, decarboxylation, and hydrogenation (Clarke, 1986).

**Scheme 3.** Production number of straight-chain aliphatic dicarboxylic acids and their derivatives occur in nature

**Production:** A number of straight-chain aliphatic dicarboxylic acids and their derivatives occur in nature. However, isolation from natural substances has no commercial significance. Although many syntheses for the production of aliphatic dicarboxylic acids are known, only a few have found industrial application. This is due partly to the shortage of raw materials.

**Individual saturated dicarboxylic acids**: Dicarboxylic acids are used mainly as intermediates in the manufacture of esters and polyamides. Esters derived from monofunctional alcohols serve as plasticizers or lubricants. Polyesters are obtained by reaction with dialcohols. In addition, dicarboxylic acids are employed in the manufacture of hydraulic fluids, agricultural chemicals, pharmaceuticals, dyes, complexing agents for heavy-metal salts, and lubricant additives (as metal salts).

### **3.1. Oxalic acid**

332 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

anhydrides on heating:

the release of water and ammonia.

dioxide and water to form cyclic ketones

dioxide and water to form cyclic ketones:

**Chemical properties**: The chemical behavior of dicarboxylic acids is determined principally by the two carboxyl groups. The neighboring methylene groups are activated generally to only a minor degree. Thermal decomposition of dicarboxylic acids gives different products depending on the chain length. Acids with an even number of carbon atoms require higher decarboxylation temperatures than the next higher odd-numbered homologues; lower dicarboxylic acids decompose more easily than higher ones. To avoid undesired decomposition reactions, aliphatic dicarboxylic acids should only be distilled in vacuum. When heated above 190 °C, oxalic acid decomposes to carbon monoxide, carbon dioxide,

and water. Malonic acid is decarboxylated to acetic acid at temperatures above 150 C:

**Scheme 1.** Succinic and glutaric acids are converted into cyclic anhydrides on heating

HOOC-(CH2)n-COOH-CH3COOH + CO2 When malonic acid is heated in the presence of P2O5 at ca. 150 °C, small amounts of carbon suboxide (C3O2) are also formed. Succinic and glutaric acids are converted into cyclic

When the ammonium salt of succinic acid is distilled rapidly, succinimide is formed, with

Higher dicarboxylic acids from *n* = 4 (adipic acid) to *n* = 6 (suberic acid) split off carbon

**Scheme 2.** Higher dicarboxylic acids from n = 4 (adipic acid) to n = 6 (suberic acid) split off carbon

The decomposition of still higher dicarboxylic acids leads to complex mixtures. With the exception of oxalic acid, dicarboxylic acids are resistant to oxidation. Oxalic acid is used as a reducing agent for both commercial and analytical purposes. Dicarboxylic acids react with dialcohols to form polyesters and with diamines to form polyamides. They also serve as starting materials for the production of the corresponding diamines. Reaction with monoalcohols yields esters. All of these reactions are commercially important. Several reactions with malonic and glutaric acids are of interest in organic syntheses: the Knoevenagel condensation, Michael addition, and malonic ester synthesis (Clarke, 1986)

Oxalic acid (ethanedioic acid, acidum oxalicum) is the simplest saturated dicarboxylic acid (Clarke, 1986). The compound exists in anhydrous form [144-62-7] or as a dihydrate [6153- 56-6]. The anhydrous acid is not found in nature and must be prepared from the dihydrate even when produced industrially. Oxalic acid is widely distributed in the plant and animal kingdom (nearly always in the form of its salts) and has various industrial applications.

$$\mathbf{^{\bullet}}\mathbf{^{\bullet}} \mathbf{^{\bullet}}\mathbf{^{\bullet}}$$

**Scheme 4.** Chemical structure of oxalic acid

The acidic potassium salt of oxalic acid is found in common sorrel (Latin: oxalis acetosella) and the name oxalic acid is derived from that plant. Table 2 shows examples of plants in which oxalic acid occurs (in the form of potassium, sodium, calcium, magnesium salts, or iron complex salts) are given below (oxalic acid content in milligrams per 100 g dry weight):(Tsu-Ning Tsao G., 1963)

The Chemistry of Dicarboxylic Acids in the Atmospheric Aerosols 335

The solubility in water and the density of these solutions are presented in Table.1. Oxalic acid is readily soluble in polar solvents such as alcohols (although partial esterification occurs), acetone, dioxane, tetrahydrofuran, and furfural. Oxalic acid is sparingly soluble in diethyl ether (1.5 g oxalic acid dihydrate in 100 g ether at 25 °C), and insoluble in benzene, chloroform, and petroleum ether. The ionization constants show that oxalic acid is a strong acid. The value of *K*1 is comparable to that of mineral acids and the value of *K*2 corresponds

In the homologous series of dicarboxylic acids, oxalic acid, the first member, shows unique behavior because of the interaction of the neighboring carboxylate groups. This results in an increase in the value of the dissociation constant and in the ease of decarboxylation: Upon rapid heating to 100 °C oxalic acid decomposes into carbon monoxide, carbon dioxide, and

In aqueous solution decomposition is induced by light and to a much greater extent by *g*- or X-rays (to carbon monoxide, carbon dioxide, formic acid, and occasionally hydrogen). This decomposition is catalyzed by the salts of heavy metals, for example, by uranyl salts. Oxalic acid cannot form an intramolecular anhydride. Upon heating to over 190 °C or warming in concentrated sulfuric or phosphoric acid, oxalic acid decomposes to carbon monoxide,

The reducing properties of oxalic acid (which itself is oxidized to the harmless end products carbon dioxide and water) form the basis for the variety of practical applications. Oxalic acid is also oxidized relatively easily to carbon dioxide by many other oxidizing agents in addition to air, especially in the presence of the salts of heavy metals. Oxalic acid is easily esterified, whereby two types, the acidic mono or neutral diesters can result. These esters are applied as intermediates in chemical syntheses. They react relatively easily with water,

Important chemical characteristics are also demonstrated by the metal salts of oxalic acid. These exist in two types-the acidic and neutral salts. The alkali metal and iron (III) salts are readily soluble in water. All other salts are sparingly soluble in water. The near complete insolubility of the alkaline-earth salts of oxalic acid, especially of calcium oxalate, finds some applications in quantitative analysis. When heated all these metal salts lose carbon monoxide. Other salts which are easier decomposable lose carbon dioxide in addition. The alkali and alkaline-earth salts form carbonates under these conditions. Manganese, zinc, and tin salts form oxides; iron, cadmium, mercury, and copper salts form mixtures of oxides and metals. Nickel, cobalt, and silver salts afford pure metals. Anhydrous fusion of oxalates with

Three-carbon 1,3-dicarboxylic acid derivatives (malonic acid, malonates, cyanoacetic acid, cyanoacetates, and malononitrile) are widely used in industry for the manufacture of pharmaceuticals, agrochemicals, vitamins, dyes, adhesives, and fragrances. The common

to ionization constants of strong organic acids, for example, benzoic acid.

water with formic acid as an isolable intermediate.

carbon dioxide, and water: this decomposition is not exothermic.

ammonia, or amines to afford the corresponding acyl derivatives.

alkali yield carbonates and hydrogen. For a review see Dollimore (1987).

**3.2. Malonic acid**


**Table 2.** Oxalic acid content in milligrams per 100 g dry weight

Oxalic acid is formed in plants through incomplete oxidation of carbohydrates, e.g., by fungi (*Aspergillus niger*) or bacteria (*acetobacter*) and in the animal kingdom through carbohydrate metabolism via the tricarboxylic acid cycle. The urine of humans and of most mammals also contains a small amount of calcium oxalate. In pathological cases, an increased calcium oxalate content in urine leads to the formation of kidney stones (Clarke, 1986). Calcium and iron(II) oxalates are also found as minerals. Both the anhydrous and dihydrated forms of oxalic acid form colorless and odorless crystals.

## **Anhydrous oxalic acid**

Anhydrous oxalic acid [144-62-7] exists as rhombic crystals in the *a*-form and as monoclinic crystals in the *b*-form (West, 1980). These forms differ mainly in their melting points. The slightly stable *b*-form changes into the *a*-form at 97 °C and 0.2 barr. Anhydrous oxalic acid is prepared by dehydration of the dihydrate through careful heating to 100 °C. It is then sublimated in a dry air stream. The sublimation is fast at 125 °C and can be carried out at temperatures up to 157 °C without decomposition. The dehydration can also be accomplished by azeotropic distillation with benzene or toluene. Anhydrous oxalic acid is slightly hygroscopic; it absorbs water from moist air ("weathers") to form the dihydrate again. The hydration occurs very slowly because of surface caking.

## **Oxalic acid dihydrate**

Oxalic acid dihydrate [6153-56-6], HOOC–COOH · 2 H2O is the industrially produced and usual commercial form of oxalic acid. The compound forms colorless and odorless prisms or granules that contain 71.42 wt % oxalic acid and 28.58 wt % water. Oxalic acid dihydrate is stable at room temperature and under normal storage conditions. The most important physical properties are as follows:

The solubility in water and the density of these solutions are presented in Table.1. Oxalic acid is readily soluble in polar solvents such as alcohols (although partial esterification occurs), acetone, dioxane, tetrahydrofuran, and furfural. Oxalic acid is sparingly soluble in diethyl ether (1.5 g oxalic acid dihydrate in 100 g ether at 25 °C), and insoluble in benzene, chloroform, and petroleum ether. The ionization constants show that oxalic acid is a strong acid. The value of *K*1 is comparable to that of mineral acids and the value of *K*2 corresponds to ionization constants of strong organic acids, for example, benzoic acid.

In the homologous series of dicarboxylic acids, oxalic acid, the first member, shows unique behavior because of the interaction of the neighboring carboxylate groups. This results in an increase in the value of the dissociation constant and in the ease of decarboxylation: Upon rapid heating to 100 °C oxalic acid decomposes into carbon monoxide, carbon dioxide, and water with formic acid as an isolable intermediate.

In aqueous solution decomposition is induced by light and to a much greater extent by *g*- or X-rays (to carbon monoxide, carbon dioxide, formic acid, and occasionally hydrogen). This decomposition is catalyzed by the salts of heavy metals, for example, by uranyl salts. Oxalic acid cannot form an intramolecular anhydride. Upon heating to over 190 °C or warming in concentrated sulfuric or phosphoric acid, oxalic acid decomposes to carbon monoxide, carbon dioxide, and water: this decomposition is not exothermic.

The reducing properties of oxalic acid (which itself is oxidized to the harmless end products carbon dioxide and water) form the basis for the variety of practical applications. Oxalic acid is also oxidized relatively easily to carbon dioxide by many other oxidizing agents in addition to air, especially in the presence of the salts of heavy metals. Oxalic acid is easily esterified, whereby two types, the acidic mono or neutral diesters can result. These esters are applied as intermediates in chemical syntheses. They react relatively easily with water, ammonia, or amines to afford the corresponding acyl derivatives.

Important chemical characteristics are also demonstrated by the metal salts of oxalic acid. These exist in two types-the acidic and neutral salts. The alkali metal and iron (III) salts are readily soluble in water. All other salts are sparingly soluble in water. The near complete insolubility of the alkaline-earth salts of oxalic acid, especially of calcium oxalate, finds some applications in quantitative analysis. When heated all these metal salts lose carbon monoxide. Other salts which are easier decomposable lose carbon dioxide in addition. The alkali and alkaline-earth salts form carbonates under these conditions. Manganese, zinc, and tin salts form oxides; iron, cadmium, mercury, and copper salts form mixtures of oxides and metals. Nickel, cobalt, and silver salts afford pure metals. Anhydrous fusion of oxalates with alkali yield carbonates and hydrogen. For a review see Dollimore (1987).

## **3.2. Malonic acid**

334 Atmospheric Aerosols – Regional Characteristics – Chemistry and Physics

**Table 2.** Oxalic acid content in milligrams per 100 g dry weight

oxalic acid form colorless and odorless crystals.

**Anhydrous oxalic acid** 

surface caking.

**Oxalic acid dihydrate** 

physical properties are as follows:

weight):(Tsu-Ning Tsao G., 1963)

The acidic potassium salt of oxalic acid is found in common sorrel (Latin: oxalis acetosella) and the name oxalic acid is derived from that plant. Table 2 shows examples of plants in which oxalic acid occurs (in the form of potassium, sodium, calcium, magnesium salts, or iron complex salts) are given below (oxalic acid content in milligrams per 100 g dry

> Spinach 460 – 3200 Rhubarb 500 – 2400 Chard 690 Parsley 190 Beets 340 Cocoa 4500 Tea 3700 Beet leaves up to 12 000

Oxalic acid is formed in plants through incomplete oxidation of carbohydrates, e.g., by fungi (*Aspergillus niger*) or bacteria (*acetobacter*) and in the animal kingdom through carbohydrate metabolism via the tricarboxylic acid cycle. The urine of humans and of most mammals also contains a small amount of calcium oxalate. In pathological cases, an increased calcium oxalate content in urine leads to the formation of kidney stones (Clarke, 1986). Calcium and iron(II) oxalates are also found as minerals. Both the anhydrous and dihydrated forms of

Anhydrous oxalic acid [144-62-7] exists as rhombic crystals in the *a*-form and as monoclinic crystals in the *b*-form (West, 1980). These forms differ mainly in their melting points. The slightly stable *b*-form changes into the *a*-form at 97 °C and 0.2 barr. Anhydrous oxalic acid is prepared by dehydration of the dihydrate through careful heating to 100 °C. It is then sublimated in a dry air stream. The sublimation is fast at 125 °C and can be carried out at temperatures up to 157 °C without decomposition. The dehydration can also be accomplished by azeotropic distillation with benzene or toluene. Anhydrous oxalic acid is slightly hygroscopic; it absorbs water from moist air ("weathers") to form the dihydrate again. The hydration occurs very slowly because of

Oxalic acid dihydrate [6153-56-6], HOOC–COOH · 2 H2O is the industrially produced and usual commercial form of oxalic acid. The compound forms colorless and odorless prisms or granules that contain 71.42 wt % oxalic acid and 28.58 wt % water. Oxalic acid dihydrate is stable at room temperature and under normal storage conditions. The most important

Three-carbon 1,3-dicarboxylic acid derivatives (malonic acid, malonates, cyanoacetic acid, cyanoacetates, and malononitrile) are widely used in industry for the manufacture of pharmaceuticals, agrochemicals, vitamins, dyes, adhesives, and fragrances. The common feature of malonic acid and its derivatives is the high reactivity of the central methylene group. Due to the increasingly electron-withdrawing character of the substituents, the acidity of the hydrogen atoms in the 2-position increases in the order malonates < cyanoacetates < malononitrile. Therefore, all these compounds undergo reactions typical of 1,3-dicarbonyl compounds. For example they are easily alkylated or arylated, undergo aldol and Knoevenagel condensations, and they can be used for the synthesis of pyrimidines and other nitrogen heterocycles.

The Chemistry of Dicarboxylic Acids in the Atmospheric Aerosols 337

**Uses***:* Succinic acid is used as a starting material in the manufacture of alkyd resins, dyes, pharmaceuticals, and pesticides. Reaction with glycols gives polyesters; esters formed by

Glutaric acid occurs in washings from fleece and, together with malonic acid, in the juice of

**Production:** Glutaric acid is obtained from cyclopentane by oxidation with oxygen and cobalt (III) catalysts or by ozonolysis; and from cyclopentanol – cyclopentanone by oxidation with oxygen and Co(CH3CO2)2, with potassium peroxide in benzene, or with N2O4 or nitric acid. Like succinic acid, glutaric acid is formed as a byproduct during oxidation of cyclohexanol – cyclohexanone. Other production methods include reaction of malonic ester with acrylic acid ester, carbonylation of *Υ*-butyrolactone, oxidation of 1,5-pentanediol with

**Uses:** The applications of glutaric acid, e.g., as an intermediate, are limited. Its use as a

Adipic acid, hexanedioic acid, 1,4-butanedicarboxylic acid, C6H10O4, *M*r 146.14, HOOCCH2CH2CH2CH2COOH [124-04-9], is the most commercially important aliphatic dicarboxylic acid. It appears only sparingly in nature but is manufactured worldwide on a large scale. The historical development of adipic acid was reviewed in 1997 (Luedeke, 1997)

**Physical properties**: Adipic acid is isolated as colorless, odorless crystals having an acidic taste. It is very soluble in methanol and ethanol, soluble in water and acetone, and very slightly soluble in cyclohexane and benzene. Adipic acid crystallizes as monoclinic prisms

**Chemical properties**: Adipic acid is stable in air under most conditions, but heating of the molten acid above 230 – 250 °C results in some decarboxylation to give cyclopentanone [120-92-3], *bp* 131 °C. The reaction is markedly catalyzed by salts of metals, including iron, calcium, and barium. The tendency of adipic acid to form a cyclic anhydride by loss of water

Adipic acid readily reacts at one or both carboxylic acid groups to form salts, esters, amides, nitriles, etc. The acid is quite stable to most oxidizing agents, as evidenced by its production in nitric acid. However, nitric acid will attack adipic acid autocatalytically above 180 °C,

**Use:** Adipic acid has been used in the manufacture of mono- and diesters as well as polyamides. Nylon 6,8 is obtained by reaction of suberic acid with hexamethylenediamine, and nylon 8,8 by reaction with octamethylenediamine. Polyamides of adipic acid with

starting material in the manufacture of maleic acid has no commercial importance.

reaction with monoalcohols are important plasticizers and lubricants (Bolton, 1995).

**3.4. Glutaric acid** 

unripened sugar beet.

**3.5. Adipic acid** 

N2O4, and oxidative cleavage of *Υ*-caprolactone.

from water, ethyl acetate, or acetone/petroleum ether.

producing carbon dioxide, water, and nitrogen oxides.

is much less pronounced compared to glutaric or succinic acids.

**Physical Properties:** Important physical properties of malonic acid (propanedioic acid, methanedicarboxylic acid) are listed in Table 1. Its p*K*a values are 2.83 and 5.70. Malonic acid forms a colorless hygroscopic solid which sublimes in vacuum with some decomposition. It's really soluble in the water; but slightly soluble in ethanol and diethyl ether, and is completely insoluble in benzene.

**Chemical Properties:** Malonic acid is found in small amounts in sugar beet and green wheat, being formed by oxidative degradation of malic acid. Reaction with sulfuryl chloride or bromine gives mono- and dihalogenated malonic acid, whereas treatment with thionyl chloride or phosphorus pentachloride leads to mono- or diacyl chloride. When heated with phosphorus pentoxide, malonic acid does not form an anhydride but rather carbon suboxide, a toxic gas that reacts violently with water to reform malonic acid. On heating the free acid above 130 °C, or an aqueous solution above 70 °C, decomposition to acetic acid and carbon dioxide takes place. The mono- and dianion of malonic acid are more stable. In aqueous solution the monosodium salt decomposes above 90 °C and the disodium salt above 130 °C (Bolton, 1995).

## **3.3. Succinic acid**

Succinic acid is found in amber, in numerous plants (e.g., algae, lichens, rhubarb, and tomatoes), and in many lignites.

**Production***:* A large number of syntheses are used to manufacture succinic acid. Hydrogenation of maleic acid, maleic anhydride, or fumaric acid produces good yields of succinic acid; the standard catalysts are Raney nickel, Cu, NiO, or CuZnCr, Pd – Al2O3, Pd – CaCO3, or Ni – diatomite. 1,4-Butanediol can be oxidized to succinic acid in several ways: (1) with O2 in an aqueous solution of an alkaline-earth hydroxide at 90 – 110 °C in the presence of Pd – C; (2) by ozonolysis in aqueous acetic acid; or (3) by reaction with N2O4 at low temperature. Succinic acid or its esters are also obtained by Reppe carbonylation of ethylene glycol, catalyzed with RhCl3 – pentachlorothiophenol; Pd-catalyzed methoxycarbonylation of ethylene; and carbonylation of acetylene, acrylic acid, dioxane, or β- propiolactone (Bolton, 1995).

Acid mixtures containing succinic acid are obtained in various oxidation processes. Examples include the manufacture of adipic acid; the oxidation of enanthic acid and the ozonolysis of palmitic acid. Succinic acid can also be obtained by phase-transfer-catalyzed reaction of 2-haloacetates, electrolytic dimerization of bromoacetic acid or ester, oxidation of 3-cyanopropanal, and fermentation of *n*-alkanes.

**Uses***:* Succinic acid is used as a starting material in the manufacture of alkyd resins, dyes, pharmaceuticals, and pesticides. Reaction with glycols gives polyesters; esters formed by reaction with monoalcohols are important plasticizers and lubricants (Bolton, 1995).
