Naphthenic acids are a mixture of several cyclopentyl and cyclohexyl carboxylic acids with molecular weight of 120 to well over 700 atomic mass units. The main fraction are carboxylic acids with a carbon backbone of 9 to 20 carbons. McKee et al. claim that "naphthenic acids are primarily cycloaliphatic carboxylic acids with 10 to 16 carbons"Naphthenic acid#cite note-McKee2014-1|, although acids containing up to 50 carbons have been identified in heavy petroleum. The term naphthenic acid has roots in the somewhat archaic term "naphthene" used to classify hydrocarbons. It was originally used to describe the complex mixture of petroleum-based acids when the analytical methods available in the early 1900s could identify only a few naphthene-type components with accuracy. Today "naphthenic" acid is used in a more generic sense to refer to all of the carboxylic acids present in petroleum, whether cyclic, acyclic, or aromatic compounds, and carboxylic acids containing heteroatoms such as N and S. Although commercial naphthenic acids often contain a majority of cycloaliphatic acids, multiple studies have shown they also contain straight chain and branched aliphatic acids and aromatic acids; some naphthenic acids contain >50% combined aliphatic and aromatic acids. Naphthenic acids are represented by a general formula CnH2n-z O2, where n indicates the carbon number and z specifies a homologous series. The z is equal to 0 for saturated, acyclic acids and increases to 2 in monocyclic naphthenic acids, to 4 in bicyclic naphthenic acids, to 6 in tricyclic acids, and to 8 in tetracyclic acids. Salts of naphthenic acids, called naphthenates, are widely used as hydrophobic sources of metal ions in diverse applications.Naphthenic acid#cite note-Ullmann-2|Aluminium salts of naphthenic acid and palmitic acid were combined during World War II to produce napalm. The word "napalm" is derived from the words naphthenic acid and palmitic acid.
Metal naphthenates
Naphthenic acid is removed from petroleum fractions not only to minimize corrosion but also to recover commercially useful products. The greatest current and historical usage of naphthenic acid is in metal naphthenates. Naphthenic acids are recovered from petroleum distillates by alkaline extraction and then regenerated via an acidic neutralization process and then distilled to remove impurities. Naphthenic acids sold commercially are categorized by acid number, impurity level, and color, and used to produce metal naphthenates and other derivatives such as esters and amides. Naphthenates are the salts of naphthenic acids, analogous to the corresponding acetates, which are better defined structurally. Such well-defined species however exhibit low solubility in hydrophobic medias such as paints. They are generally assumed have the formula M2 or are basic oxides with the formula M3O6. The naphthenates have industrial applications including synthetic detergents, lubricants, corrosion inhibitors, fuel and lubricating oil additives, wood preservatives, insecticides, fungicides, acaricides, wetting agents, thickening agent of napalm and oil drying agents used in painting and wood surface treatment. Industrially useful naphthenates include those of aluminium, magnesium, calcium, barium, cobalt, copper, lead, manganese, nickel, vanadium, and zinc.Naphthenic acid#cite note-Ullmann-2| Illustrative is the use of cobalt naphthenate for the oxidation of tetrahydronaphthalene to the hydroperoxide.
Safety
One oft-cited study stated that "naphthenic acids are the most significant environmental contaminants resulting from petroleum extraction from oil sands deposits." However "under worst-case exposure conditions, acute toxicity is unlikely in wild mammals exposed to naphthenic acids in AOS tailings pond water, but repeated exposure may have adverse health effects." Naphthenic acids are present in Athabasca oil sands tailings pond water at an estimated concentration of 81 mg/l. Using Organisation for Economic Co-operation and Development protocols for testing toxicity, McKee et al. argued that based on their studies refined NAs when consumed orally were not acutely genotoxic to mammals. However, damage induced by NAs while transient in acute or discontinuous exposure, may be cumulative in repeated exposure.
