**3.5. Adipic acid**

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

**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 is much less pronounced compared to glutaric or succinic acids.

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, producing carbon dioxide, water, and nitrogen oxides.

**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 diamines such as 1,3-bis(aminomethyl)benzene, 1,4(bisaminomethyl)cyclohexane, and bis(4 aminocyclohexyl)methane are also of commercial interest. Esters of adipic acid with monoand bifunctional alcohols are used as lubricants.

The Chemistry of Dicarboxylic Acids in the Atmospheric Aerosols 339

Urban and Continental Remote marine

Unsaturated fatty acids with a double bond at the C9 position like cis-9-octadecenoic (oleic) acid are oxidized into C9 diacid (azelaic acid) and other products hereafter mainly oxidized into shorter diacids hahah(Kawamura and Ikushima, 1994; Kawamura and Kaplan, 1987; Kawamura et al., 1985). These unsaturated acids which are abundant in marine phytoplankton and terrestrial higher plant leaves are also emitted by anthropogenic sources such as meat cooking (Rogge, 1991; Rogge et al., 1998) and wood burning processes (Rogge

Warneck suggested that in the marine atmosphere clouds generate oxalic acid from glyoxal formed by oxidation of acetylene and glycolaldehyde formed by oxidation of ethane (Warneck, 2000). Note that along these processes glyoxylic acid (CHOCOOH) represents a key intermediate (see figure 3) whereas diacids other than oxalic acid are not produced. The formation of dicarboxylic acids in the continental atmosphere (Ervens et al., 2004a) involves production of glyoxal from toluene and of glycolaldehyde from isoprene as well as aqueous phase reactions of adipic and glutaric acids produced by oxidation of cyclohexene. Recently more literature has become available on the formation of oxalic acid that includes also the oxidation of methylglyoxal, an oxidation product of toluene and isoprene, via intermediate steps involving pyruvic and acetic acids (Lim et al., 2005). Since this diacid production pathway also forms oligomers, the knowledge of the sources of diacids is also of importance

The relative contribution of primary and secondary sources of diacids in the atmosphere remains poorly understood. Even though it is agreed that they are likely to be mainly secondary in origin it is not known in which proportion their precursors come from

**Figure 2.** Comparison of dicarboxylic acids distribution in urban/continental and remote marine based

23456 carbon no. of dicarboxylic acid

for the understanding of secondary organic aerosol formation.

anthropogenic and biogenic sources.

on the data collection on table 3

0

0.5

1

1.5

2

log mol/m -3

2.5

3

3.5

4

et al., 1998).
